Patent Description:
An electrophotographic image forming apparatus (hereinafter also referred to as "electrophotographic apparatus") which adopts an electrophotographic system mainly includes an electrophotographic photosensitive member (hereinafter also referred to as "photosensitive member"), a charging apparatus, an exposure apparatus, a developing apparatus, a transfer apparatus and a fixing apparatus.

Members having a roller shape, a blade shape or the like are suitably used for the charging apparatus, the developing apparatus and the transfer apparatus. In these apparatuses, an apparatus which is particularly preferably used is a roller-shaped member (hereinafter also referred to as "electrophotographic roller").

An outer-diameter shape of the electrophotographic roller to be used in the electrophotographic apparatus is required to be highly accurate in both the circumferential direction and the longitudinal direction.

Here, the shape will be described that is required in an electrophotographic roller (hereinafter also referred to as "charging roller") which is used for the charging apparatus. When the charging roller is brought into contact with the photosensitive member, it is general to apply a predetermined force to both end portions of a shaft of the charging roller, and press the charging roller against the photosensitive member. In this case, a "crown shape" is known as a shape of the charging roller which is adopted for the purpose of uniformizing a "nip width" in the longitudinal direction (refer to <CIT> and <CIT>). Note that the "nip width" refers to the width at which the charging roller and the photosensitive drum come into contact with each other. The "crown shape" refers to a shape in which the outer diameter of the charging roller decreases from the central portion to both end portions, specifically, a shape in which the diameter decreases from the central position of the length in a direction orthogonal to the circumferential direction of the charging roller (hereinafter also referred to as "longitudinal direction") toward both ends.

Further electrophotographic rollers according to the prior art are shown in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

However, according to studies by the present inventors, there has been a case where the charging roller having the crown shape causes blots on the surface of the end region due to the sticking of the toner or an external additive of the toner, along with being used. In particular, this phenomenon has been more conspicuous in recent electrophotographic apparatuses that have been speeded up.

It is the object of the present invention to provide an electrophotographic roller that can suppress the adhesion of blots in an end region of the electrophotographic roller, even in a high-speed electrophotographic apparatus.

The object of the present invention is achieved by an electrophotographic roller having the features of claim <NUM>.

In addition, an advantage of the present invention is directed to providing a process cartridge that contributes to stable formation of a high-quality electrophotographic image.

Furthermore, another advantage of the present invention is directed to providing an electrophotographic apparatus that can stably form a high-quality electrophotographic image.

Further features, advantages and effects of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

Preferred embodiments of the present disclosure will now be described in detail in accordance with the accompanying drawings.

The present inventors presume the cause of the above adhesion of the blot to the end region of the charging roller having the crown shape, as follows. Specifically, the charging roller generally comes into contact with the photosensitive member and follows the driving thereof. At this time, in the charging roller having the crown shape, a difference in a peripheral speed occurs between the charging roller and the photosensitive member depending on the position in the longitudinal direction. Then, the difference in the peripheral speed becomes large in the end region, and the toner and the external additive of the toner on the photosensitive member are rubbed against the surface of the charging roller. It is considered that it becomes thereby easier for the end region to be more blotted.

Then, the present inventors have repeatedly studied to prevent the adhesion of the blot due to the difference in the peripheral speed in the end region of the electrophotographic roller having the crown shape. As a result, the present inventors have found that the electrophotographic roller having the specific crown shape as described above contributes to the achievement of the above object.

Embodiments according to the present disclosure will be described below in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements and the like of the constituent components described in the embodiments should be appropriately changed according to the configuration, various conditions and the like of an apparatus to which the disclosure is applied, and the scope of this disclosure is not intended to be limited to the following embodiments.

<FIG> illustrates a shape of the electrophotographic roller according to the present disclosure. <FIG> illustrates an enlarged view from the center of the electrophotographic roller in the longitudinal direction to one end portion of the mandrel <NUM>.

An electrophotographic roller according to one aspect of the present disclosure includes a mandrel and an elastic layer on the mandrel; and has a crown shape in which the diameter decreases from the central portion toward both ends in a longitudinal direction of the mandrel. In addition, when a position of an end portion in a longitudinal direction of the elastic layer is defined as X2, a position between a central position O and X2 is defined as X1, a distance between the central position O and X1 is defined as L1, and a distance between the X1 and X2 is defined as L2, L1 = <NUM> × (L1 + L2), and.

An electrophotographic roller according to one aspect of the present disclosure will be described below in detail.

Firstly, a crown amount will be described. A charging roller which is commonly used is brought into contact with a photosensitive member so that a predetermined pressure is applied to both end portions of a mandrel <NUM>. Because of this, when the outer diameter of the elastic layer of the charging roller is constant from the central position in the longitudinal direction of the mandrel to the end portions of the elastic layer, the pressing force at the end portion becomes larger than the pressing force at the central portion. In order to reduce the unevenness of the pressure distribution in the longitudinal direction, the charging roller has such a crown shape that the diameter decreases from the center in the longitudinal direction of the mandrel toward both ends, as is illustrated in <FIG>.

The crown amount in the present disclosure is defined as (Do-DX2)/<NUM>. Here, Do and DX2 represent average values of diameters at respective arbitrary points.

In order to uniformly bring a main body of the roller into contact with the photosensitive member, when a length of the elastic body in the longitudinal direction is <NUM> or longer and <NUM> or shorter, the crown amount is preferably <NUM> or larger and <NUM> or smaller.

In addition, a suitable crown amount changes depending on a pressing load applied to both ends of the mandrel. For example, when <NUM> or more and <NUM> or less of pressure is applied to each of both the ends of the mandrel, the crown amount is preferably <NUM> or larger and <NUM> or smaller, and is more preferably <NUM> or larger and <NUM> or smaller, in order to enhance the uniformity of the pressure.

When the position of the end portion in the longitudinal direction of the elastic body is defined as X2, a position between the central position O and the X2 is defined as X1, a distance between the central position O and the X1 is defined as L1, and a distance between the X1 and the X2 is defined as L2, the L1 is defined as L1 = <NUM> × (L1 + L2). This means that a range from the central position O to the position X1 is a central region of the roller, and a range from the position X1 to the position X2 is an end region of the roller. Furthermore, the outer diameters at O, X1 and X2 are defined as Do, DX1 and DX2, respectively, and the following is defined: <MAT> and <MAT>.

The Z1 is a decrement of a "radius at position X1" relative to a "radius at central position O" of the mandrel.

The Z2 is a decrement of a "radius at position X2" relative to a "radius at position X1".

Each of the Z1 and the Z2 is determined to be an arithmetic mean value of values calculated at <NUM> positions at a pitch of <NUM> degree in a circumferential direction of the roller.

An arithmetic mean value of Z2 is calculated in the same manner.

The electrophotographic roller in the present disclosure has a shape in which Z1, Z2, L1 and L2 satisfy the following calculation expression (<NUM>): <MAT> wherein α is <NUM>.

The constant α represents a change rate of curvature in the central region and the end region of the roller.

<FIG> illustrates roller shapes in which the respective curvatures of the end regions are different. The shapes of rollers B and C according to one aspect of the present disclosure are indicated by a broken line and an alternate long and short dash line, respectively. In addition, the shape of a roller A having a general crown shape illustrated in <FIG> is illustrated by a solid line. Table <NUM> shows each of numerical values of Z1, L1, Z2 and L2 of the rollers A to C, and values of β, which are calculated from these numerical values with the use of the following calculation expression.

The lengths in the longitudinal direction of elastic layers <NUM> of the rollers have been set at <NUM>, and diameters thereof at the central position O have been set at <NUM>. <FIG> illustrates diameters at positions in the longitudinal direction of the rollers, respectively.

For information, when Y represents the diameter of the roller, and X represents a position in the longitudinal direction of the roller, a shape of a contour line of the elastic layer of the roller A which is indicated by the solid line in <FIG> is expressed as the following quadratic function of the X. <MAT> (wherein a, b and c are arbitrary constants).

In the roller A which has the general crown shape, the diameters in the end region (between the X1 and the X2) also gradually decrease, and the difference in the peripheral speed between the photosensitive member and the roller has been large in the end region. When an electrophotographic roller having the general crown shape such as the roller A is used as the charging roller, a large difference in the peripheral speed occurs between the photosensitive member and the electrophotographic roller (charging roller) in the end region. Then, due to the difference in the peripheral speed, the toner and the external additive which interpose between the photosensitive member and the electrophotographic roller are rubbed against the outer surface of the electrophotographic roller and stick thereto. Such a sticking matter causes abnormal discharge from the electrophotographic roller to the photosensitive member, and as a result, a spot (hereinafter, also referred to as "white spot") is formed in some cases, which should not be originally formed in an electrophotographic image. This phenomenon remarkably occurs, in particular, in such a low-temperature and low-humidity environment that the temperature is <NUM> and the relative humidity is <NUM>%, for example.

Similarly, when the electrophotographic roller having the general crown shape like the roller A is used as the developing roller, a large difference in the peripheral speed occurs between the photosensitive member and the electrophotographic roller (developing roller), in the end region. Then, due to the difference in the peripheral speed, the toner which interposes between the photosensitive member and the electrophotographic roller is rubbed against the surface of the electrophotographic roller, and sticks thereto. Such a sticking matter reduces such a capability of the electrophotographic roller as to give an electric charge to the toner. As a result, the toner cannot be sufficiently charged on the developing roller. The toner having an insufficient amount of the electrostatic charge is transferred onto the surface of the photosensitive member, onto which the toner should not be transferred originally, and causes fogging in an electrophotographic image. This phenomenon occurs particularly remarkably in a high-temperature and high-humidity environment in which the temperature is <NUM> and the relative humidity is <NUM>%, for example.

In contrast, in the roller B and the roller C according to the present disclosure, the curvature of the outer diameter of the end region is smaller than that of the central region. In other words, the radius of curvature is large. Thereby, in the central region, the nip width is more uniform in the longitudinal direction. In addition, in the end region, the difference in the peripheral speed from the photosensitive member is smaller than that of the roller A. Because of this, even when the roller B or the roller C is used as the charging roller and is applied to an image forming process which has been speeded up, the occurrence of the blot in the end region is slight compared to that of the roller A. In addition, even under a low-temperature and low-humidity environment, the occurrence of the white spot at the position corresponding to the end region of the electrophotographic image can be suppressed. In addition, also when the electrophotographic roller has been used as the developing roller, the electrophotographic roller can reduce or prevent the occurrence of the blot in the end region and suppress the occurrence of the fogging at a position corresponding to the end region of the electrophotographic image.

In <FIG>, in the case of any β, the curvature of the central region is the same, and the contact stability of the central region is ensured. As previously described, the smaller the difference in the outer diameter in the longitudinal direction is, the smaller the difference in the peripheral speed in the end region is. Because of this, the roller C and the roller B are advantageous in this order (roller C is the most advantageous). On the other hand, when the β value is small, there is a case where the contact pressure of the end portion becomes high, and thereby the blot adheres, though the outer diameter of the end portion becomes large and thereby the occurrence of the blot is improved which originates in the difference in the peripheral speed. Therefore, the constant β needs to be controlled to be smaller than <NUM>, for the purpose of uniformization of the contact pressure with respect to the photosensitive member, in addition to suppression for the blot which originates in the difference in the peripheral speed in the end region. In other words, it is necessary to control [(Z2/L2)/(Z1/L1)] to smaller than <NUM>. In addition, it is preferably <NUM><[(Z2/L2)/(Z1/L1)]<<NUM>. In addition, though an example has been shown in the above, in which the Z2 is changed while the Z1 is almost constant, the Z1 may be changed while the Z2 is almost constant; and even in this case, as long as the relationship of Expression (<NUM>) is satisfied, the blot in the end region, which originates in the difference in the peripheral speed, can be suppressed.

A magnitude of a vibration at the time when the electrophotographic roller rotates while following the photosensitive member is proportional to a magnitude of the difference in the peripheral speed of the electrophotographic roller with respect to the photosensitive member. By utilizing this property, the magnitude of the vibration is evaluated at the time when the electrophotographic roller is driven by the photosensitive member, and thereby the magnitude of the difference in the peripheral speed is evaluated.

The magnitude (amplitude) of the vibration of the electrophotographic roller can be measured with a laser Doppler vibrometer (trade name: LV-<NUM>, manufactured by Ono Sokki Co. The measurement position is set at a position close to the center by <NUM> from the end portion of the elastic portion of the electrophotographic roller, and is a position opposite to the contact position with the photosensitive member (opposite side by <NUM> degree). The electrophotographic roller is incorporated into the electrophotographic process cartridge, and the vibration is measured at the time when the electrophotographic apparatus is operated. The amplitude is evaluated by subjecting the vibration of the electrophotographic apparatus to frequency analysis, and evaluating the frequency with the largest amplitude. The amplitude and the difference in the peripheral speed are proportional to each other, and accordingly, a smaller amplitude means a smaller difference in the peripheral speed; but, from the viewpoint of preventing the blot originating in the difference in the peripheral speed, the amplitude is preferably <NUM> or less.

The amount of fogging by toner after endurance at the time when electrophotographic roller is incorporated as a developing roller can be measured by a reflection densitometer (trade name: TC-6DS/A; manufactured by Tokyo Denshoku Co. An electrophotographic roller is incorporated into an electrophotographic process cartridge as a developing roller, and <NUM> sheets of paper are continuously passed. A reflectance R1 at a position <NUM> distant from the end portion in the 1000th image is measured by the reflection densitometer. In addition, a reflectance R0 of unprinted paper is also measured with the reflection densitometer.

Then, the amount of decrease in the reflectance "R0 - R1" (%) with reference to unprinted paper is evaluated as a fogging value.

When the fogging value is <NUM>% or smaller, it is determined that the influence of the lowering of the capability of giving the electric charge to the developing roller is small and the fogging does not occur.

A non-contact type laser length measuring instrument can be used for the measurement of the outer diameter of the electrophotographic roller. As the non-contact type laser length measuring instrument, for example, there is a laser scan type of dimension/outer diameter measuring instrument "LS-<NUM>" (trade name) manufactured by Keyence Corporation.

<FIG> illustrate a method of measuring an outer diameter with the use of the above laser scan type of dimension/outer diameter measuring instrument. <FIG> illustrates a perspective view of a state in which an electrophotographic roller <NUM> which is a measurement object and a reference roller <NUM> are placed on the measuring instrument, and <FIG> illustrates a side view thereof. A light receiving portion <NUM> receives a laser beam <NUM> (hatched portion) emitted from a laser light emitting portion <NUM>. Firstly, the reference roller <NUM> is placed on the measuring instrument so that its axis and the laser beam <NUM> are orthogonal to each other. Next, the electrophotographic roller <NUM> as the measurement object is placed on the measuring instrument so as to be parallel to the axial line of the reference roller <NUM>. In this state, a width <NUM> of the laser beam is measured which has been emitted from the laser emitting portion <NUM> and has transmitted between the reference roller <NUM> and the electrophotographic roller <NUM>. The measurement pitch is preferably <NUM> or smaller in the longitudinal direction, and <NUM> degrees or smaller in the rotational direction, and is more preferably <NUM> pitch in the longitudinal direction and <NUM> degree pitch in the rotational direction. As illustrated in <FIG>, at the time of specifying the position in the longitudinal direction of the measured shape data, when the central position of the electrophotographic roller <NUM> is denoted by O, the position of one end portion of the elastic portion is denoted by X2, and the distance from O to the X2 is denoted by L, the distance L1 from O to the X1 is determined to be L1=<NUM>×L. The distance between the X1 and the X2 is denoted by L2.

The diameter of the electrophotographic roller <NUM> is measured as a width of the laser which has been interrupted by the electroconductive base layer roller <NUM>. Specifically, the diameter is measured every time the electrophotographic roller <NUM> is rotated, and the diameter of one circumference of the electrophotographic roller <NUM> is determined in one measurement cross section. The diameter in the present disclosure is defined as an average value of diameters of one circumference in each cross section. Next, the electrophotographic roller <NUM> is moved in the longitudinal direction, and a diameter is measured at another position in the longitudinal direction. In addition, the diameters at the central position O of the electrophotographic roller <NUM>, X1 and X2 described previously are denoted by Do, DX1 and DX2, respectively, and Z1 and Z2 are calculated with the use of the following expressions: <MAT> and <MAT>.

Subsequently, the elastic deformation power (hereinafter, also referred to as "ηIT") will be described.

The ηIT can be calculated from a physical quantity that can be measured by measurement by the nanoindentation method (international standard: ISO14577). Detailed measurement conditions for the electrophotographic roller will be described in Example <NUM>, and accordingly will be omitted here. <FIG> illustrates one example of a load-displacement curve required in a process of deriving the elastic deformation power. A load application curve <NUM> (curve from point A to point B) profiles a behavior of displacement with respect to a load, at the time when the load is applied to an object. A load-unloading curve <NUM> (curve from point B to point C) profiles a behavior of displacement with respect to the load, at the time when the load is unloaded from the object.

A work amount at the time when the load is applied (area of region <NUM> surrounded by load application curve <NUM>, load-unloading curve <NUM>, and straight line from point A to point C) is denoted by Wp (plastic deformation work amount). A work amount at the time when the load is unloaded (area of region <NUM> surrounded by load-unloading curve <NUM>, straight line from point B to point D, and straight line from point C to point D) is denoted by We (elastic deformation work amount). The total work amount is denoted by Wt (total deformation work amount). The ηIT can be calculated with the use of We, Wt and the following expression: <MAT> and <MAT>.

In the case where the ηIT of the electrophotographic roller is high, it means that the electrophotographic roller has a high property of returning to an original state even if having been deformed, in other words, has a high elastic recovery rate. To the roller surface having high ηIT, even though a stress is applied thereto because of a pressure or the difference in the peripheral speed, in a state in which a blot such as the toner or the external additive such as a toner shell or silica adheres to the roller surface, the toner or the external additive resists sticking, because the elastic recovery rate of the roller is high. Because of this, the roller surface having the high ηIT is more advantageous for preventing the adhesion of the blot to the end region.

<FIG> schematically illustrate a state at the time when an electrophotographic roller is incorporated as a charging roller into an electrophotographic apparatus, the endurance of <NUM>,<NUM> sheets has been evaluated, then the electrophotographic roller has been removed and cut, and the cross section has been observed. <FIG> illustrates a case where the ηIT is <NUM>%, and <FIG> illustrates a case where the ηIT is <NUM>%. Detailed endurance conditions will be described in Example <NUM>.

As is illustrated in <FIG>, when the ηIT is <NUM>, the shell of the toner and the blot <NUM> of the external additive are buried in the elastic layer <NUM>. On the other hand, as illustrated in <FIG>, when the ηIT is <NUM>, the shell of the toner and the blot <NUM> of the external additive are not buried in the elastic layer <NUM>, and resist causing an image defect compared to the case where the ηIT is <NUM>. In the present disclosure, the ηIT is set to <NUM>% or larger, which is effective for suppressing the burying of the blot of the external additive into the charging roller, but is preferably <NUM>% or larger for the purpose of more effectively suppressing the burying, and is more preferably <NUM>% or larger from the same viewpoint.

In order to enhance the ηIT of the charging roller, it is necessary to promote crosslinking only in the vicinity of the surface of the elastic layer, while the hardness of the inside of the elastic layer is kept at an appropriate crosslinking density. As a specific method for promoting the crosslinking in the vicinity of the surface of the elastic layer, there is electron beam irradiation (hereinafter also referred to as "EB treatment"), for example. By the EB treatment being performed, only the outermost surface of the charging roller can be crosslinked while the crosslinking density inside the elastic layer is kept. Further, the EB treatment can easily enhance the ηIT by changing the intensity of the electron beam irradiation. In the above, the units for enhancing the ηIT have been described, but the units for increasing the ηIT are not limited to these techniques.

<FIG> schematically illustrates an external view of the electrophotographic roller. The electrophotographic roller has a structure having an elastic layer <NUM> on the outer circumference of a mandrel <NUM>, in which both ends of the mandrel <NUM> are exposed without being covered with the elastic layer <NUM>. The charging roller is provided in an electrophotographic image forming apparatus, as a charging unit that electrically charges the photosensitive member. Specifically, the charging roller is brought into contact with the photosensitive drum, is driven by the photosensitive member of the photosensitive drum, and is charged by friction at a contact portion between the photosensitive drum and the charging roller. In addition, the developing roller is provided as a unit for stably supplying the toner to the photosensitive member.

The mandrel <NUM> that is used in the elastic roller of the present disclosure has electroconductivity, and has a function of supporting the elastic layer or the like, which is provided on the outer circumference thereof. Examples of the material include metals such as iron, copper, stainless steel, aluminum and nickel, and alloys thereof. In addition, these materials may be subjected to plating treatment or the like, in order that these surfaces are imparted with scratch resistance. Furthermore, as the mandrel, a mandrel can also be used that is a substrate formed from a resin, of which the surface is coated with a metal or the like and imparted with surface electroconductivity, or a mandrel can also be used that is produced from an electroconductive resin composition.

In addition, an adhesive layer (unillustrated) may be provided between the mandrel <NUM> and the elastic layer <NUM>. In this case, it is preferable that the adhesive is electroconductive. In order to impart the electroconductivity, appropriate agents are selected from known electroconductivity imparting agent (for example, ion conductivity imparting agent and electron conductivity imparting agent), and can be used alone or in combination of two or more thereof, for the adhesive.

Examples of the binder of the adhesive include thermosetting resins and thermoplastic resins, and known binders can be used such as urethane-based, acrylic-based, polyester-based, polyether-based and epoxy-based binders. Examples of the adhesive include METALOC N33 (epoxy-based adhesive having electroconductivity, produced by Toyokagaku Kenkyusho Co. As a coating method of the adhesive, known methods can be used such as a roll coater, sponge coating and spray coating.

The adhesive layer between the mandrel <NUM> and the elastic layer <NUM> may be provided over the whole face at which the mandrel <NUM> and the elastic layer <NUM> come in contact with each other, or may be provided only in the range of from <NUM> to <NUM> by width at both end portions of the face at which the mandrel <NUM> and the elastic layer <NUM> come in contact with each other. The thickness of the adhesive layer is preferably <NUM> to <NUM>.

The elastic layer <NUM> provided on the outer circumferential surface of the mandrel <NUM> may be a solid body or a foamed body, and may be a single layer or may be formed of a plurality of layers.

It is preferable that a hardness of the elastic layer <NUM> is <NUM> degrees or higher and <NUM> degrees or lower by an Asker C hardness. When the Asker C hardness of the elastic layer <NUM> is <NUM> degrees or higher, exudation of an oil component from a rubber material constituting the elastic layer <NUM> can be easily suppressed, and contamination of a contact member such as a photosensitive drum can be suppressed. In addition, when the Asker C hardness of the elastic layer <NUM> is <NUM> degrees or lower, the contact of the elastic roller with the contact member becomes stable, and deterioration in an image quality of the output image can be suppressed. Here, the Asker C hardness can be defined by a measurement value that is measured by an Asker Rubber hardness meter (manufactured by Kobunshi Keiki Co. ) with the use of a test piece which has been separately prepared according to the standard specification Asker C type SRIS (The society of Rubber Industry, Japan standard) <NUM>.

The material for forming the elastic layer contains a binder resin and an electroconductive particle.

Examples of the binder resin include the following. Rubbers and thermoplastic elastomers including: natural rubber, butadiene rubber, styrene-butadiene rubber (SBR), nitrile rubber, ethylene propylene rubber (EPDM), chloroprene rubber (CR), acrylonitrile butadiene rubber (NBR), epichlorohydrin rubber, butyl rubber, silicone rubber, urethane rubber, fluorine rubber and chlorine rubber. Among the substances, NBR can be suitably used from the viewpoint of compression permanent set and the hardness. Furthermore, among the NBRs, an NBR is preferable which has a nitrile content of <NUM>% or more and less than <NUM>%, and is referred to as a so-called medium-high nitrile. As such a medium-high nitrile, a nitrile commercially available as "N230SV" (trade name, manufactured by JSR Corporation) can be used, for example.

As the electroconductive particle, electroconductive carbons can be used such as Ketjen Black EC, acetylene black, carbon for rubber, carbon for color (ink) subjected to oxidation treatment, and pyrolytic carbon. Specific examples of the carbon for rubber include the following carbons for rubber. Super Abrasion Furnace (SAF: super abrasion resistance), Intermediate Super Abrasion Furnace (ISAF: intermediate super abrasion resistance), High Abrasion Furnace (HAF: high abrasion resistance), Fast Extruding Furnace (FEF: adequate extrudability), General Purpose Furnace (GPF: general purpose), Semi Reinforcing Furnace (SRF: medium reinforcement), Fine Thermal (FT: fine particle thermal decomposition), and Medium Thermal (MT: medium particle thermal decomposition). In addition, graphite can also be used such as natural graphite and artificial graphite.

Examples of the usable electroconductive particle include: particles of metallic oxides such as TiO<NUM>, SnO<NUM> and ZnO; particles of composite oxides such as solid solutions of ZnO and Al<NUM>O<NUM>; powders of metals such as Cu and Ag; and various known particles. The particles may be used alone, or a blend of two or more types thereof may be used.

To the material for forming the elastic layer, a spherical particle may be added which has a particle diameter, for example, in a range of <NUM> to <NUM>. Examples of the spherical particle include at least one spherical particle selected from the following particles:
a phenol resin particle, a silicone resin particle, a polyacrylonitrile resin particle, a polystyrene resin particle, a polyurethane resin particle, a nylon resin particle, a polyethylene resin particle, a polypropylene resin particle, an acrylic resin particle, a silica particle, and an alumina particle. Due to the use of such a rubber composition, a charging roller can be produced in which the outer surface of the elastic layer has projecting portions derived from the spherical particle.

In addition, if necessary, a vulcanizing agent, a vulcanization accelerator, an electroconductivity imparting agent, a charge control agent, a plasticizer, an anti-aging agent and the like can be appropriately added. In addition, an antistatic agent, an ultraviolet absorber, a reinforcing agent, a filler, a lubricant, a mold release agent, a pigment, a dye, a flame retardant and the like can be appropriately added, as needed.

As a method for manufacturing the electrophotographic roller according to one aspect of the present disclosure, there is a method of coating the mandrel with the material for forming the elastic layer, with the use of an extrusion machine having a crosshead, for example.

<FIG> is an explanatory view of a process of forming a layer of the material for forming the elastic layer on the outer circumferential surface of the mandrel, by coextruding the mandrel and the material for forming the elastic layer with the use of an extrusion molding machine <NUM> provided with a crosshead <NUM>.

The extrusion molding machine <NUM> is provided with the crosshead <NUM> into which the mandrel <NUM> and the material <NUM> for forming the elastic layer are fed, a feed roller <NUM> which feeds the mandrel <NUM> into the crosshead <NUM>, and a cylinder <NUM> which feeds the material <NUM> for forming the elastic layer into the crosshead <NUM>.

The material <NUM> for forming the elastic layer is fed into the crosshead <NUM> by a screw <NUM> in a cylinder <NUM>, and at the same time, the mandrel <NUM> is fed into the crosshead <NUM>; and thereby, the mandrel <NUM> having the layer of the material for forming the elastic layer is discharged from the crosshead, which is formed on the outer circumferential surface. Hereinafter, the mandrel which has the layer of the material for forming the elastic layer formed on the outer circumferential surface is also referred to as a "coated roller <NUM>".

The outer diameter of the electrophotographic roller according to one aspect of the present disclosure can be controlled by changing a relative ratio between the feed speed of the mandrel <NUM> to the crosshead by the feed roller <NUM> and the feed speed of the material for forming the elastic layer from the cylinder <NUM>. For example, the thickness of the elastic layer in the longitudinal direction of the mandrel can be adjusted by keeping the feed speed of the material for forming the elastic layer from the cylinder to the crosshead constant, and changing the feed speed of the mandrel <NUM> to the crosshead. Specifically, by enhancing the feed speed of the mandrel, the thickness of the elastic layer can be relatively thinned, in other words, the diameter can be reduced. Here, in order to adjust the thickness of the elastic layer more accurately in the longitudinal direction of the mandrel, in other words, the diameter of the electrophotographic roller, it is preferable to use a manufacturing method having the following four steps.

(First step) A step of obtaining a relationship between the feed speed of the mandrel to the crosshead and the thickness of the layer of the material for forming the elastic layer, which is formed on the mandrel, by performing preforming of keeping the feed speed of the material for forming the elastic layer to the crosshead constant, and changing the feed speed of the mandrel to the crosshead;.

The third and fourth steps will be specifically described below.

When it is determined that the measured value of the thickness of the layer of the material for forming the elastic layer is large at a predetermined position in the longitudinal direction of the mandrel, compared with the target crown shape, the rotation speed of the feed roller <NUM> is increased to increase the feed speed of the mandrel, and thereby the outer diameter becomes small. On the other hand, when it is determined that the thickness is small, the rotation speed of the feed roller <NUM> is decreased to decrease the feed speed of the mandrel, and thereby the outer diameter becomes large. This feedback control is performed at intervals of, for example, <NUM> seconds, and thereby the outer shape of the elastic layer can be controlled to the target crown shape with high accuracy, without grinding.

For information, when the crown-shaped electrophotographic roller according to the present disclosure is obtained by the above method, it is preferable to determine the target shape at the time of extrusion molding, considering that there is a case where the elastic layer shrinks in the subsequent vulcanizing step.

In addition to the above method, there is a method of using a mold having a cavity shape corresponding to the crown shape according to one aspect of the present disclosure, filling the cavity around the mandrel arranged in the mold with the material for forming the elastic layer, and curing the material.

In addition, when the crosshead is used, the electrophotographic roller can also be manufactured by operations of: producing a coated roller having an outer diameter equal to the outer diameter of the thickest central portion of the target crown shape or larger than the outer diameter of the thickest central portion, while keeping the feed speed of the mandrel to the crosshead and the feed speed of the material for forming the elastic layer constant; and grinding the outer circumferential surface of the rubber layer obtained by curing the layer of the material for forming the elastic layer, with the use of a grindstone or the like.

The molded electrophotographic roller precursor is heated and vulcanized by units of a hot air oven, a vulcanizing can, a hot plate, far-near infrared rays, induction heating and the like. The heating temperature varies depending on the material for forming the elastic layer, but is preferably <NUM> to <NUM>, and is more preferably <NUM> to <NUM>; and the heating time is preferably <NUM> to <NUM> minutes, and is more preferably <NUM> to <NUM> minutes.

A vulcanized rubber composition at both end portions of the vulcanized rubber roller is removed in a later separate step, and the electrophotographic roller is completed. Accordingly, both end portions of the core metal are exposed in the completed electrophotographic roller.

The surface layer may be subjected to surface treatment such as ultraviolet irradiation for the purpose of reducing a friction coefficient of the surface, or electron beam irradiation for the purpose of enhancing the ηIT.

<FIG> illustrates one example of a process cartridge for electrophotography, which is provided with a charging roller according to one embodiment of the present disclosure. The process cartridge <NUM> illustrated in <FIG> is configured so that a developing apparatus and a charging apparatus are integrated, and so as to be detachably attachable to a main body of an electrophotographic apparatus.

The developing apparatus is formed by integrating at least a developing roller <NUM>, a toner container <NUM>, and a toner <NUM>. The photosensitive drum <NUM> is one example of an electrophotographic photosensitive member. The developing apparatus may include a toner supply roller <NUM>, a developing blade <NUM>, and a stirring vane <NUM>, as needed.

The charging apparatus is formed by integrating at least the photosensitive drum <NUM> and a charging roller <NUM>. The charging roller <NUM> is arranged at a position at which the photosensitive drum101 can be electrically charged. The charging apparatus may be provided with a cleaning blade <NUM> and a waste toner container <NUM>. The charging roller <NUM>, the developing roller <NUM>, the toner supply roller <NUM> and the developing blade <NUM> are configured so that voltages are applied thereto, respectively.

<FIG> illustrates a view illustrating one example of the electrophotographic apparatus that uses the charging roller according to one embodiment of the present disclosure. The electrophotographic apparatus <NUM> illustrated in <FIG> is configured so that four process cartridges <NUM> are detachably attached thereto. Each process cartridge <NUM> corresponds to each color of black, magenta, yellow and cyan, and toners corresponding to the respective colors are used. Each process cartridge <NUM> has the same configuration except that the color of the toner to be used is different.

Each process cartridge <NUM> has basically the same configuration as that illustrated in <FIG>. The process cartridge <NUM> includes a photosensitive drum <NUM>, a charging roller <NUM>, a developing roller <NUM>, a toner supply roller <NUM>, a cleaning blade <NUM>, a toner container <NUM>, a waste toner storage container <NUM>, a developing blade <NUM>, a toner <NUM>, and a stirring vane <NUM>.

The photosensitive drum <NUM> rotates in an arrow direction (clockwise direction), and is uniformly charged by the charging roller <NUM> to which a voltage is applied from a charging bias power source. When the surface of the photosensitive drum <NUM> is irradiated with an exposure light <NUM>, an electrostatic latent image is formed on the surface.

On the other hand, the toner <NUM> stored in the toner container <NUM> is supplied to the toner supply roller <NUM> by the stirring vane <NUM>. The toner supply roller <NUM> supplies the toner <NUM> to the developing roller <NUM>. The surface of the developing roller <NUM> is uniformly coated with the toner <NUM> by the developing blade <NUM> which is arranged in contact with the developing roller <NUM>, and an electric charge is given to the toner <NUM> by frictional charging.

The above electrostatic latent image is developed by being imparted with the toner <NUM> that is conveyed by the developing roller <NUM> which is arranged in contact with the photosensitive drum <NUM>, and is visualized as a toner image.

The visualized toner image on the photosensitive drum is transferred onto an intermediate transfer belt <NUM> by a primary transfer roller <NUM> to which a voltage is applied by a primary transfer bias power source. The intermediate transfer belt <NUM> is supported and driven by a tension roller <NUM> and an intermediate transfer belt drive roller <NUM>. The toner images of the respective colors are sequentially superimposed to form a color image on the intermediate transfer belt <NUM>.

A transfer material <NUM> is fed into the apparatus by a sheet feeding roller. The transfer material <NUM> is conveyed to between the intermediate transfer belt <NUM> and a secondary transfer roller <NUM>. A voltage is applied to the secondary transfer roller <NUM> from a secondary transfer bias power source, and the color image on the intermediate transfer belt <NUM> is transferred to the transfer material <NUM>. The transfer material <NUM> to which the color image has been transferred is subjected to a fixing process by a fixing device <NUM>. The transfer material <NUM> that has been subjected to the fixing process is discharged to the outside of the apparatus.

On the other hand, the toner which has remained on the photosensitive drum <NUM> without being transferred is scraped off by the cleaning blade <NUM>, and is stored in a waste toner storage container <NUM>. In addition, the toner which has remained on the intermediate transfer belt <NUM> without being transferred is also scraped off by a cleaning apparatus <NUM>.

According to one aspect of the present disclosure, an electrophotographic roller can be obtained that can suppress the adhesion of the blots in the end region of the electrophotographic roller, even in a high-speed electrophotographic apparatus. In addition, according to another aspect of the present disclosure, a process cartridge can be obtained that contributes to stable formation of a high-quality electrophotographic image. According to further another aspect of the present disclosure, an electrophotographic image forming apparatus can be obtained that can stably form a high-quality electrophotographic image.

The present disclosure will be described below in detail with reference to Examples, but the present disclosure is not limited to Examples.

An A-kneaded rubber composition was obtained by mixing the materials shown in the following Table <NUM>. As a mixing machine, a <NUM>-liter pressure kneader (product name: TD6-15MDX, manufactured by Toshin Co. ) was used. Mixing conditions were set at a filling rate of <NUM> vol%, a blade rotation speed of <NUM> rpm, and a period of <NUM> minutes.

Next, an unvulcanized rubber composition <NUM> was obtained by mixing materials shown in the following Table <NUM>. As a mixing machine, an open roll (product name: <NUM>×<NUM> test roll, manufactured by Kansai Roll Co. ) was used which had a roll diameter of <NUM> inches (<NUM>). As for the mixing conditions, the number of rotations of the front roll was set at <NUM> rpm, the number of rotations of the rear roll was set at <NUM> rpm, and the roll gap was set at <NUM>; left and right were turned back <NUM> times in total; and then the roll gap was set at <NUM>, and tight milling was performed <NUM> times.

As the mandrel, a stainless-steel rod was prepared which had a diameter of <NUM> and a length of <NUM>. In order to form the unvulcanized rubber composition <NUM> around the mandrel, an extrusion forming apparatus (manufactured by Mitsuba Mfg. ) was used which had the crosshead shown in <FIG>. As an extrusion machine of the extrusion molding apparatus, an extrusion machine provided with a degassing port was used in which an inner diameter of the cylinder was <NUM> and a ratio of the length to the diameter of the cylinder was <NUM>.

An extrusion molding temperature was set at <NUM> for each of the cylinder, a screw and a crosshead die. The number of rotations of the screw was set at <NUM> rotations per minute. These conditions were adopted, and the mandrel was coated with the unvulcanized rubber composition <NUM>.

For information, when the mandrel is coated, the feed speed of the feed roll was controlled and the wall thickness was adjusted so that the shape of the charging roller became a crown shape after the electrophotographic roller precursor was vulcanized and the end portion of the elastic layer was removed, and in addition, the shape of the electrophotographic roller after the end portion of the elastic layer was removed showed that the values of L1, L2, Z1 and Z2 illustrated in <FIG> were the respective values shown in Table <NUM>; and the electrophotographic roller precursor was formed.

Specifically, an inflow speed of the material for forming the elastic layer into the crosshead was kept constant at <NUM>×<NUM>-<NUM> mm<NUM>/second; and on the other hand, when the layer of the material for forming the elastic layer was formed, and the central portion in the longitudinal direction of the mandrel was formed, the feed speed of the mandrel to the crosshead was set at <NUM>/second, and when both of the end portions were formed, the feed speed of the mandrel to the crosshead was set at <NUM>/sec.

In addition, the thickness (diameter) of the layer of the material for forming the elastic layer around the mandrel immediately after having been extruded from the crosshead was measured with the use of a laser measuring device, and feedback control of the feed speed of the mandrel was performed, based on the relationship between the feed speed and the diameter of the mandrel acquired in advance in the preforming.

The obtained electrophotographic roller precursor was heated in a hot air oven at <NUM> for <NUM> hour, and then both of the end portions of the layer of the material for forming the elastic layer in the electrophotographic roller precursor were removed by <NUM>; and an electrophotographic roller <NUM> was obtained. The length of the elastic layers in the longitudinal direction of the electrophotographic roller <NUM> was <NUM>; and L1 was <NUM> and L2 was <NUM>.

The surface of the obtained electrophotographic roller was irradiated with ultraviolet rays, and an electrophotographic roller <NUM>' was obtained which had UV-treated region on the surface of the elastic layer. A low-pressure mercury-vapor lamp (trade name: GLQ500US/<NUM>, manufactured by Toshiba Lighting & Technology Corporation Co. ) was used for the irradiation of ultraviolet rays, and the electrophotographic roller was uniformly irradiated while having been rotated at a constant speed while the mandrel <NUM> was regarded as a rotational axis. The amount of the ultraviolet rays was set so as to be <NUM> mJ/cm<NUM> in terms of the sensitivity of the sensor for <NUM>.

The outer diameter of the electrophotographic roller <NUM>' was measured with the use of a non-contact laser measuring instrument, according to a configuration illustrated in <FIG>. In the present Example, the shape of the roller was measured with the use of a non-contact laser measuring instrument (LS-<NUM>; manufactured by Keyence Corporation), by the operation of moving the roller at a <NUM> pitch in the longitudinal direction and at a pitch of <NUM> degree in the rotational direction.

The diameter in the present disclosure is defined as an average value of diameters of one circumference in each cross section. The diameters at the central position O, X1 and X2 of the charging roller <NUM>' in <FIG> were defined as Do, DX1 and DX2, respectively, and Z1 and Z2 were defined as Z1 = (Do - DX1)/<NUM> and Z2 = (DX1 - DX2)/<NUM>; and each value was calculated. As a result of calculation, Do was <NUM>. <NUM>, and Z1 and Z2 became as shown in Table <NUM>.

An indentation test was performed on the produced electrophotographic roller <NUM>' according to a method according to ISO <NUM>, with the use of a nanoindentation apparatus (apparatus name: PICODENTOR, model number: HM500, manufactured by Fischer Instruments K. ), and the ηIT was analyzed.

As for a measurement condition, an approach speed of an indenter was <NUM>/s; as for a load condition, the maximum load was <NUM> mN, and an indentation time was <NUM>; and an unloading condition was the reverse of the load condition.

The measurement points were <NUM> points in total of <NUM> points at intervals of <NUM> degrees in the circumferential direction, for three points in total of the central position in the longitudinal direction of the electrophotographic roller and two positions close to the center by <NUM> from both end portions of the elastic layer, and measurement was performed there.

The test was performed under the above conditions to obtain a load-displacement curve of the electrophotographic roller <NUM>', and the ηIT was calculated for each curve. In the present disclosure, the median value of ηITs at <NUM> points is denoted by the ηIT.

As a result, in the present Example, the ηIT was <NUM> [%] as shown in Table <NUM>.

The electrophotographic roller <NUM>' is incorporated into the process cartridge as a charging roller, the vibration of the charging roller is measured at the time when an electrophotographic apparatus is operated, and a magnitude of the amplitude is evaluated as a magnitude of the difference in the peripheral speed. As a measuring instrument, a laser Doppler vibrometer (trade name: LV-<NUM>, manufactured by Ono Sokki Co. ) was used. The measurement position was set at a position close to the center by <NUM> from the end portion of the elastic portion of the electrophotographic roller, on a position opposite to the contact position with the photosensitive member (position on the opposite side by <NUM> degree). As an electrophotographic apparatus for measurement of the amplitude, Lasejet M608dn (manufactured by HP Inc. ) was prepared, and was used after having been modified so that the number of output sheets per unit time became <NUM> sheets/min (A4 vertical output), which was larger than the original number of output sheets, in order to perform evaluation in a high-speed process. An electrophotographic process cartridge for the above printer was used as the electrophotographic process cartridge. Vibrations were measured at the time when the printer was operated, and the frequencies were analyzed; as a result, the amplitude was large at a frequency of <NUM>; and the magnitude of the amplitude at the frequency of <NUM> was adopted as the magnitude of the vibration of the charging roller <NUM>. The measurement results are shown in Table <NUM>.

The electrophotographic roller <NUM>' was incorporated into an electrophotographic apparatus as a charging roller, and was subjected to a paper passing endurance test under a low-temperature and low-humidity environment. As the electrophotographic apparatus, a laser printer (trade name: Laserjet M608dn; manufactured by HP Inc. ) was prepared. The laser printer was used after having been modified so that the number of output sheets per unit time became <NUM> sheets/min (A4 vertical output), which was larger than the original number of output sheets, in order to perform evaluation in a high-speed process. The resolution of an image of the above laser printer is <NUM> dpi, and the output of primary charging is a DC voltage of -1100V. An electrophotographic process cartridge for the above printer was used as the electrophotographic process cartridge. For information, the electrophotographic process cartridge includes a drum-shaped electrophotographic photosensitive member and a charging roller which is arranged in contact with the electrophotographic photosensitive member. In addition, a pressure of <NUM> is applied to each of both ends of the mandrel body of the charging roller, and thereby the charging roller is pressed against the electrophotographic photosensitive member.

Firstly, the electrophotographic roller <NUM>', the electrophotographic apparatus and the process cartridge were left to stand in an environment at a temperature of <NUM> and a relative atmospheric humidity of <NUM>% for <NUM> hours, for the purpose of being acclimated to the measurement environment.

Subsequently, the electrophotographic roller <NUM>' was incorporated into the process cartridge as a charging roller. The output image was evaluated with the use of the process cartridge.

Specifically, an E character image having a print density of <NUM>% was subjected to the endurance (image output) of continuous <NUM> sheets, under a low-temperature and low-humidity environment (temperature of <NUM>, and relative humidity of <NUM>%). After <NUM> sheets of images were output, a halftone image (image in which horizontal lines each having width of <NUM> dot in direction perpendicular to rotation direction of electrophotographic photosensitive member and interval of <NUM> dots were drawn) was output. The obtained image was visually observed, and an image defect was observed which originates in the adhesion of the blot on the surface of the charging roller.

After the image for the evaluation was output, the degree of the adhesion of the blots of the toner and external additives on the surface of the electrophotographic roller <NUM>' was observed, with a digital microscope (model number VH-<NUM>, manufactured by Keyence Corporation). As for the observation positions, two positions were observed which were sides by <NUM> inner from the end portions of the elastic layer, respectively, and a position at which the adhesion state of the blot was inferior was determined to be places to be evaluated.

The results of the observation of the image defects originating in the adhesion of the blot on the halftone image and the results of microscopic observation of the surface of the electrophotographic roller <NUM>' were evaluated according to the criteria shown in Table <NUM>. The results are shown in Table <NUM>.

An electrophotographic roller <NUM>' was obtained in the same manner as in Example <NUM>, except that the numerical values of Z1, Z2 and (Do-DX2)/<NUM> were controlled as shown in Table <NUM>.

The obtained electrophotographic roller <NUM>' was evaluated in the same manner as in Example <NUM>. The results are shown in Table <NUM>.

Electrophotographic rollers <NUM>' were obtained in the same manner as in Example <NUM>, except that the numerical values of Z1, Z2 and (Do-DX2)/<NUM> were controlled as shown in Table <NUM>.

For the measurement of the amplitudes of the obtained electrophotographic rollers <NUM>', the evaluation of the image, and the observation of the blot of the charging roller, Laserjet M608dn (manufactured by HP Inc. ) was prepared as an electrophotographic apparatus, was modified so that the number of output sheets per unit time became <NUM> sheets/min (A4 vertical output), which was larger than the original number of output sheets, in order to perform the evaluation in a high-speed process, and was used.

The surface of the obtained electrophotographic roller was irradiated with an electronic beam, and an electrophotographic roller was obtained which had EB-treated region on the surface of the elastic layer. For the electron-beam irradiation, an electron-beam irradiation apparatus having a maximum acceleration voltage of <NUM> kV (trade name: "low-energy electron-beam irradiation source EB-ENGINE" manufactured by Hamamatsu Photonics K. ) was used. For information, before the electron beam irradiation, the air in the irradiation chamber was purged with nitrogen gas, and the oxygen concentration in the irradiation chamber was adjusted. The treatment conditions were as follows: acceleration voltage: <NUM> kV, electronic current (irradiation current): <NUM> mA, treatment speed (scanning speed): <NUM>/min, and concentration of oxygen: <NUM> ppm. At this time, the apparatus constant at the acceleration voltage of <NUM> kV in the electron-beam irradiation apparatus was <NUM>, and the dose calculated by Expression (<NUM>) was <NUM> kGy.

An electrophotographic roller <NUM>" shown in Table <NUM> was obtained in the same manner as in Example <NUM>, except for the above. The obtained electrophotographic roller <NUM>" was evaluated in the same manner as in Example <NUM>.

An electrophotographic roller <NUM>" having an EB-treated region on the surface of the elastic layer was obtained in the same manner as in Example <NUM>, except that among the conditions of the electron-beam irradiation in Example <NUM>, an electronic current was set at <NUM> mA and the electron-beam irradiation was performed. Z1, Z2 and the like are shown in Table <NUM>.

The obtained electrophotographic roller <NUM>" was evaluated in the same manner as in Example <NUM>.

An electrophotographic roller was obtained in the same manner as in Example <NUM>, except that the surface of the electrophotographic roller was not treated. Z1, Z2 and the like are shown in Table <NUM>.

With the use of the crosshead extrusion molding machine used in Example <NUM>, a coated roller was formed so that an outer diameter of <NUM> at the time when having been extruded from the molding machine became a straight shape. After that, the coated roller was heated in a hot air oven at <NUM> for <NUM> minutes, and an electrophotographic roller precursor was produced. Subsequently, both end portions of the vulcanized rubber layer were each removed by <NUM>, the surface of the vulcanized rubber layer of the electrophotographic roller precursor was ground by a plunge cut grinding type of grinding machine, and an electrophotographic roller <NUM>‴ was obtained of which the numerical values of Z1, Z2, (Do-DX2)/<NUM> were as shown in Table <NUM>. Except for that, the same procedure as those in Example <NUM> was employed.

The electrophotographic roller <NUM>' produced in Example <NUM> was incorporated into an electrophotographic apparatus as a developing roller, and was subjected to a paper passing endurance test. Fogging was evaluated with the use of Laserjet Pro M102w Printer (manufactured by HP Inc. ) as the electrophotographic apparatus. Firstly, the electrophotographic apparatus loaded with the electrophotographic roller <NUM>' was placed in an environment of a temperature of <NUM> and a relative atmospheric humidity of <NUM>%, and then was left to stand for <NUM> hours or longer.

Next, the reflectance R0(%) of the unprinted paper was measured.

Subsequently, an image of a black color having a printing rate of <NUM>% was continuously output on <NUM> sheets, and then the reflectance R1(%) was measured at the end portion of the image on the 1000th sheet.

For the measurement, a reflection densitometer (trade name: TC-6DS/A; manufactured by Tokyo Denshoku Co. ) was used.

Then, the fogging value calculated with the use of the following expression is shown in Table <NUM>.

The rubber among the materials of the unvulcanized rubber composition in Example <NUM> was changed from NBR (grade N230SV, produced by JSR Corporation) to NBR (grade N230SL, produced by JSR Corporation). Except for that, the same procedure as those of Example <NUM> was employed. Z1, Z2 and the like are shown in Table <NUM>.

The rubber among the materials of the unvulcanized rubber composition in Example <NUM> was changed from NBR (grade N230SV, produced by JSR Corporation) to NBR (grade N230S, produced by JSR Corporation). Except for that, the same procedure as those in Example <NUM> was employed. Z1, Z2 and the like are shown in Table <NUM>.

Electrophotographic rollers <NUM>' were obtained in the same manner as in Example <NUM>, except that the values of Z1, Z2 and (Do-DX2)/<NUM> were controlled as shown in Table <NUM>. The obtained electrophotographic rollers <NUM>' were evaluated in the same manner as in Example <NUM>.

Electrophotographic rollers <NUM>' were obtained in the same manner as in Example <NUM>, except that the values of Z1, Z2 and (Do-DX2)/<NUM> were controlled as shown in Table <NUM>. The obtained electrophotographic roller <NUM>' was evaluated in the same manner as in Examples <NUM> to <NUM>.

Claim 1:
An electrophotographic roller (<NUM>, <NUM>) comprising a mandrel (<NUM>) and an elastic layer (<NUM>) on the mandrel (<NUM>); and having a crown shape in which a diameter decreases from a central position O in a longitudinal direction (X) of the mandrel (<NUM>) toward both ends thereof, characterised in that when
a position of an end portion in the longitudinal direction (X) of the elastic layer (<NUM>) is defined as an end position X2,
a position between the central position O and the end position X2 is defined as a position X1,
a distance between the central position O and the position X1 is defined as L1, and
a distance between the position X1 and the end position X2 is defined as L2, <MAT> and when
outer diameters of the electrophotographic roller (<NUM>, <NUM>) at the central position O, the position X1 and the end position X2 are defined as Do, DX1 and DX2, respectively, and Z1 and Z2 are defined as follows: <MAT> and <MAT> Z1, Z2, L1 and L2 satisfy a relationship expressed by the following calculation expression (<NUM>): <MAT> wherein α is <NUM>, and
wherein the electrophotographic roller (<NUM>, <NUM>) satisfies <NUM> < [(Z2/L2)/(Z1/L1)] < <NUM>.