JP2009080369A - Charging roll - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a charging roll advantageously improved in contact property (grounding property) with an image carrier by ensuring, when assembling the charging roll to contact with the outer circumferential surface of the image carrier within an electrophotographic machine, a sufficient nip width between the charging roll and the image carrier even at the axial center thereof. <P>SOLUTION: The charging roller comprises a conductive base layer 14 provided around a shaft body 12, and a resistance adjustment layer 16 provided on the outer circumferential surface of the conductive base layer 14. The conductive base layer 14 and the resistance adjustment layer 16 are formed, respectively, to have the largest thickness at the axial center and be gradually thinned toward both the ends, and to have the smallest thickness at the axial center and be gradually thickened toward both the ends, while having a center-swollen shape in which the sum of thicknesses of the conductive layer 14 and the resistance adjustment layer 16 is continuously increased from both the axial ends to the center. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging roll used for charging an image carrier made of an electrophotographic photosensitive member or an electrostatic recording dielectric in an image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic system. It is.

  2. Description of the Related Art Conventionally, in an image forming apparatus (hereinafter referred to as an electrophotographic apparatus) such as a copying machine, a printer, or a facsimile using an electrophotographic method, an image carrier such as a photosensitive member (photosensitive drum) is provided on the outer peripheral surface of a charging roll. A so-called roll charging method is employed in which the surface of the image carrier is charged by bringing the image carrier and the charging roll into contact with each other and rotating them.

  In addition, as a charging roll used in the roll charging system related to the contact charging system, those having various structures have been proposed and used. For example, a conductive base layer composed of a solid rubber layer or a rubber foam layer, which has a low hardness by containing a large amount of a softening agent around a conductive shaft (core metal), has a predetermined thickness. And a softener transition prevention layer for preventing bleeding (bleeding) of the softener contained in the conductive base layer to the roll surface, if necessary, on the outside of the conductive base layer. In general, a resistance adjustment layer for adjusting the resistance value of the entire roll is provided, and a protective layer for preventing contamination of the roll surface is provided as the outermost layer. Has been.

  By the way, in the electrophotographic apparatus, the charging roll is usually applied with a predetermined load applied to the core metal portions at both ends in the axial direction so that the outer peripheral surface thereof contacts the surface of the image carrier (photosensitive drum). For example, when a charging roll having a cylindrical shape is used, the nip width between the cylindrical charging roll and the image carrier is continuously increased from both ends in the axial direction toward the center. It is known to become smaller. On the other hand, in order to uniformly charge the surface of the image carrier by contact with the charging roll, the nip width between the charging roll and the image carrier generally extends in a direction parallel to the axial direction of the charging roll. More specifically, the nip width at the contact surface between the axial end of the roll and the image carrier, and the nip at the contact surface between the axial center of the roll and the image carrier. It is preferable that the difference from the width is small.

  For this reason, conventionally, there have been proposed charging rolls having various structures as a charging roll capable of ensuring a sufficient nip width even in the axial central portion when brought into contact with the image carrier (photosensitive drum). Although being used, as one of such charging rolls, the diameter of the central portion in the axial direction of the roll is slightly larger (about several tens of μm) than the diameter of both end portions in the axial direction to form a medium-high shape. Things have been used. The present applicant has also proposed an elastic roll exhibiting such a medium-high shape, which can be advantageously used as a charging roll (see Patent Document 1).

  However, when the present inventors repeated further detailed examination from the viewpoint of using it as a charging roll for the medium and high elasticity roll proposed in Patent Document 1, there is still room for improvement. found.

  That is, when the medium-high elastic roll disclosed in Patent Document 1 is used as a charging roll, the coating layer in the medium-high elastic roll is formed as a resistance adjustment layer. When the coating layer (resistance adjustment layer) is provided, its operability is improved. From the viewpoint of improving (coating property of the coating liquid), various fillers may be blended in the coating liquid used for the coating operation. In addition, as such a coating liquid, there may be used a liquid mainly composed of an epichlorohydrin rubber material capable of giving a relatively high hardness to the dried coating layer (resistance adjusting layer). In the medium-high elastic roll (charging roll) disclosed in Patent Document 1 obtained by using such a coating liquid, the coating layer (resistance adjusting layer) is thickest at the center in the axial direction of the roll. Since the thickness of the roll surface is gradually reduced toward the part, the hardness of the roll surface is the highest in the axial center part of the roll, as is the thickness of the coating layer (resistance adjustment layer), and toward the both ends. Gradually lower. As described above, since the hardness of the conductive base layer in the charging roll is generally kept low, the hardness of the charging roll depends on the hardness of the resistance adjustment layer provided outside the conductive base layer. . Even if such a roll with high hardness in the axial center portion exhibits a medium-high shape, there is a possibility that a sufficient nip width cannot be secured in the vicinity of the axial center portion. The medium-high elastic roll disclosed in Document 1 still has room for improvement. In addition, it replaces with the coating layer as a resistance adjustment layer currently disclosed by patent document 1, according to shaping | molding methods other than the coating method, the resistance adjustment layer which becomes thickest in an axial direction center part, and becomes gradually thin as it goes to both ends As a result of studies by the present inventors on the charging roll provided with the above, it has been confirmed that such a charging roll has the same problem.

Japanese Patent No. 2753925

  Here, the present invention has been made in the background of such circumstances, and the problem to be solved is assembled in the electrophotographic apparatus so as to be in contact with the outer peripheral surface of the image carrier. In this case, the nip width between the image carrier and the image carrier is ensured to be sufficiently large even in the central portion in the axial direction, and the contact property (grounding property) with the image carrier is advantageously improved. It is in providing the charged roll.

  And in order to solve such a problem, the present inventors have made extensive studies based on the medium-high elastic roll (charging roll) described in Patent Document 1, and have completed the present invention. It is. That is, the present invention includes a conductive base layer provided around the shaft body and a resistance adjustment layer provided on the outer peripheral surface of the conductive base layer, and the thickness of the conductive base layer and the resistance adjustment layer. In the charging roll exhibiting a medium-high shape, in which the sum of the thickness of the conductive base layer continuously increases from both axial ends toward the central portion, the conductive base layer is thickest in the axial central portion and gradually increases toward both ends. A charging roll characterized in that the resistance adjusting layer is formed so as to be thinned, while the resistance adjusting layer is formed so as to be thinnest in the central portion in the axial direction and gradually thicker toward both ends. It is what.

  In one preferred embodiment of such a charging roll according to the present invention, the resistance adjusting layer is formed by a coating operation using a predetermined coating solution.

  In another preferred embodiment of the charging roll of the present invention, a softener migration preventing layer is formed between the conductive base layer and the resistance adjusting layer.

  Furthermore, in another desirable aspect of the charging roll according to the present invention, a protective layer is formed on the surface of the resistance adjusting layer.

  And in each aspect of the charging roll according to the present invention described above, advantageously, the difference between the diameter at the central portion in the axial direction and the average of the respective diameters at both end portions in the axial direction is 10 to 90 μm. is there.

  As described above, the charging roll according to the present invention exhibits a medium-high shape in which the sum of the thicknesses of the conductive base layer and the resistance adjusting layer continuously increases from both axial end portions toward the central portion, The conductive base layer is thickest in the central portion in the axial direction and is formed so as to be gradually thinner toward both ends, while the resistance adjusting layer is thinnest in the central portion in the axial direction and gradually thicker toward both ends. It is formed as follows. That is, in the charging roll of the present invention, since the thickness of the resistance adjustment layer that has a great influence on the hardness of the roll is the thinnest in the central portion in the axial direction, the hardness in the central portion in the axial direction is effectively reduced. It is kept low, and coupled with the fact that the entire roll exhibits a medium-high shape, the nip width when brought into contact with the image carrier is ensured with a sufficient size even in the central portion in the axial direction, Therefore, the contact property (grounding property) with the image carrier is excellent.

  In addition, the nip width at the central portion in the axial direction is ensured to be sufficiently large, that is, the difference between the nip widths at both end portions in the axial direction is reduced, so that the image carrier (photosensitive drum) is charged in the axial direction. The variation in the amount is reduced, and it is advantageous to make the finally obtained image uniform and the amount of scraping on the surface of the image carrier, and the variation in the nip pressure with respect to the image carrier is small in the axial direction. Therefore, it is possible to effectively prevent the occurrence of image unevenness due to dirt adhesion unevenness.

  Hereinafter, in order to clarify the present invention more specifically, one of the representative embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows an embodiment of a charging roll according to the present invention in the form of a longitudinal section. In the charging roll 10 shown therein, reference numeral 12 denotes a metal long bar-like conductive shaft (core metal), and a conductive base layer 14 is formed on the outer peripheral surface of the shaft 12. ing. In addition, hereinafter, the structure in which the conductive base layer (14) is formed on the outer peripheral surface of the shaft body (12) is appropriately referred to as a base roll. In addition, a resistance adjustment layer 16 for adjusting the resistance value of the entire roll is provided outside the conductive base layer 14.

  In the charging roll 10 according to the present invention, as can be seen from FIG. 1, the conductive base layer 14 and the resistance adjustment layer 16 are formed so as to satisfy the following relationship. It has characteristics. That is, the sum of the thicknesses of the conductive base layer 14 and the resistance adjusting layer 16 continuously increases from both axial end portions toward the central portion, and the conductive base layer 14 has an axial direction. It is formed so as to be thickest at the center and gradually thin toward the both ends. On the other hand, the resistance adjustment layer 16 is thinnest at the center in the axial direction and gradually decreases toward both ends. It is formed to be thick. As described above, since the hardness of the conductive base layer 14 is generally low in order to advantageously secure the grounding property (contact property) with the image carrier (photosensitive drum), The hardness of the roll 10 greatly depends on the hardness of the resistance adjustment layer 16 provided outside the conductive base layer 14. The hardness of the resistance adjusting layer 16 depends on its thickness. In the charging roll 10 according to the present invention, the resistance adjusting layer 16 is the thinnest at the central portion in the axial direction, and approaches the both ends. Compared to conventional charging rolls that have been made to be thicker by increasing the thickness of the resistance adjustment layer in the axial center, the hardness in the axial center is kept low. It has been made. As described above, in combination with the fact that the hardness in the central portion in the axial direction is effectively reduced and the entire roll has a medium-high shape, the charging roll 10 according to the present invention has an electrophotographic apparatus. Inside, when assembled so as to be in contact with the outer peripheral surface of the image carrier, the nip width between the image carrier and the image carrier can be secured at a sufficient size even in the axially central portion. Therefore, the contact property (grounding property) with the image carrier is excellent.

  In the charging roll according to the present invention, by adopting the configuration as described above, it is possible to enjoy the effect advantageously. However, when actually manufacturing the charging roll, for example, as follows. The thickness of the resistance adjustment layer is set. Needless to say, the specific numerical values and the like described below are appropriately changed according to the material or the like constituting the resistance adjustment layer.

  As the thickness of the resistance adjustment layer increases, the nip width when brought into contact with the image carrier (photosensitive drum) becomes smaller. Therefore, the thickness of the resistance adjustment layer that can ensure the minimum nip width is determined according to the present invention. The thickness of the resistance adjusting layer of the charging roll according to the above is the thickest thickness, that is, the thickness (maximum thickness) at both axial ends. The maximum thickness is generally about 150 μm.

  On the other hand, since the resistance adjustment layer is also required to have electrical durability, the thickness of the resistance adjustment layer that satisfies the electrical durability is the thinnest in the resistance adjustment layer of the charging roll according to the present invention. The thickness, that is, the thickness at the center in the axial direction (minimum thickness). The minimum thickness is usually about 100 μm.

When the charging roll of the present invention is brought into contact with the image carrier (photosensitive drum), in order to make the nip width at the central portion in the axial direction sufficiently large, the following formula (1) is satisfied. The thickness of the resistance adjustment layer is set in the roll design stage.
Ae-Ac> 30 (1)
However, Ae: design thickness [μm] at the axial end of the resistance adjustment layer,
Ac: Design thickness [μm] at the axially central portion of the resistance adjustment layer.

  On the other hand, although the charging roll finally obtained varies depending on its use conditions, etc., the difference between the diameter in the central portion in the axial direction and the average of the respective diameters at both ends in the axial direction (hereinafter referred to as product outer diameter difference). ) Is generally designed to be about 10 to 90 μm. Considering this and the above formula (1) comprehensively, in the base roll in which the conductive base layer is provided around the shaft body, the diameter in the axial center portion and the axial end portions The difference from the average of each diameter is preferably about 70 to 150 μm.

  By the way, in manufacturing the charging roll 10 according to the present invention as shown in FIG. 1, the forming material used for forming the conductive base layer 14 and the resistance adjusting layer 16 is not particularly limited in the present invention. In addition, various known materials that have been conventionally used in charging rolls can be appropriately used depending on the purpose.

Specific examples of the material for forming the conductive base layer 14 include styrene-butadiene rubber (SBR), polynorbornene rubber, ethylene-propylene-diene terpolymer rubber (EPDM), nitrile rubber (NBR), and hydrogenated nitrile. Rubber materials such as rubber (H-NBR), butadiene rubber (BR), isoprene rubber (IR), natural rubber (NR), silicone rubber and the like can be exemplified, and these can be used alone or in combination of two or more. Mixed and used. In order to provide conductivity, conventionally known conductive agents such as conductive carbon black, graphite, potassium titanate, iron oxide, c-TiO ₂ , c-ZnO, c-SnO ₂ , and various ionic conductive agents are used. Further, if necessary, a foaming agent, a vulcanizing agent, a vulcanization accelerator, a plasticizer, and the like are appropriately blended according to each purpose to prepare the material for forming the conductive base layer 14. It will be.

  Examples of the material for forming the resistance adjusting layer 16 include those obtained by blending conductive carbon black and various ionic conductive agents with epichlorohydrin rubber, EPDM, SBR, NBR, H-NBR, polyurethane elastomer, and the like. In forming the resistance adjusting layer, when using a dipping method or a roll coating method, such a component is mixed in a predetermined solvent to be prepared as a liquid material.

  As described above, in the charging roll according to the present invention, since the thickness of the resistance adjustment layer is the thinnest at the central portion in the axial direction, the hardness is relatively high, such as an epichlorohydrin rubber material. When a rubber material capable of providing a resistance adjusting layer is used as the rubber material of the resistance adjusting layer forming material, a particularly remarkable effect is recognized. When such a rubber material having a relatively high hardness is used as the resistance adjusting layer forming material, the appropriate hardness difference between the rubber material and the rubber material constituting the conductive base layer forming material is about 10 to 30 degrees. It is. Incidentally, the hardness of the rubber material means that each rubber material is vulcanized under a vulcanization condition of 60 ° C. × 30 minutes, and the obtained test piece is type A durometer according to JIS-K-6253. Mean measured hardness.

  As a technique for manufacturing the charging roll 10 according to the present invention using such a forming material, conventionally known various techniques can be employed. For example, when the conductive base layer 14 is integrally formed around the shaft body 12, a cylindrical conductive elastic layer is provided around the shaft body by press molding, and then the conductive elastic layer is ground. Thus, using a method of making the conductive base layer 14 exhibiting a target base roll outer diameter difference (crown shape), or using a cylindrical mold having a predetermined crown shape on the inner peripheral surface in advance, Any method of forming the target conductive base layer 14 around the shaft body by molding can be employed.

  Further, when forming the resistance adjustment layer 16, a known dipping method is applied to a previously prepared base roll having a crown shape, or the resistance adjustment layer forming material is extruded and coated on the base roll. Any of the methods of making the target resistance adjusting layer by grinding the surface after the sulfur operation, etc. can be adopted. In the present invention, in particular, a roll using the apparatus shown in Patent Document 1 The resistance adjusting layer 16 is preferably provided by a coating method. In this patent document 1, the relative movement speed of the application roll with respect to the base roll (the roll body in patent document 1) is determined from one end side in the axial direction of the base roll using the roll coating apparatus disclosed therein. While continuously decreasing toward the center, while continuously increasing from the center toward the other end, the thickness of the coating layer (resistance adjustment layer) formed on the outer peripheral surface of the base roll is It is said that it is possible to manufacture a medium-high elastic roll that is thickest in the central portion in the axial direction and gradually becomes thinner toward both ends. By applying this method, the relative movement speed of the coating roll with respect to the base roll is continuously increased from one end side in the axial direction of the base roll toward the central portion, while from the central portion to the other end side. By continuously decreasing the resistance, the resistance adjustment layer 16 that is thinnest in the central portion in the axial direction and gradually becomes thicker toward both ends can be advantageously formed.

  The exemplary embodiments of the present invention have been described in detail above. However, the embodiments are merely examples, and the present invention is limited in any way by specific descriptions according to such embodiments. It should be understood that it is not interpreted.

  In the charging roll according to the present invention, the bleeding (bleeding) of the softening agent contained in the conductive base layer to the roll surface is prevented between the conductive base layer and the resistance adjusting layer as in the conventional charging roll. It is, of course, possible to provide a softening agent migration preventing layer for this purpose, or to provide a protective layer for the purpose of preventing contamination of the roll surface as the outermost layer on the outside of the conductive base layer.

  As the material for forming each layer, any conventionally known material can be used. For example, as a material for forming the softener migration preventing layer, a conductive material such as polyamide resin such as N-methoxymethylated nylon or polyester can be used. Examples of the protective layer forming material include polyamide resins such as N-methoxymethylated nylon, fluorine-modified acrylate resins, fluorine resins, acrylic resins, and the like. The thing formed by mix | blending various additives with resin materials, such as a urethane resin and a silicone resin, can be mentioned.

  Some examples of the present invention will be shown below to clarify the present invention more specifically. However, the present invention is not limited by the description of such examples. Needless to say. In addition to the following examples, the present invention includes various changes and modifications based on the knowledge of those skilled in the art without departing from the spirit of the present invention, in addition to the specific description described above. It should be understood that improvements and the like can be added.

  While preparing a core [diameter: 6 mm, made of stainless steel (SUS304)] as a shaft body, as a forming material of the conductive base layer, SBR (JIS-A hardness: 20 degrees): 100 parts by weight and carbon black: 10 weights A rubber composition prepared by blending parts was prepared.

  A cylindrical mold having a predetermined crown shape on the inner peripheral surface in advance, and using the five types of molds having different crown shapes, the above-mentioned conductive base layer in a state where the core metal is arranged at the center of the axis By injecting the forming material into a mold and vulcanizing, five types of base rolls having a crown shape and different outer diameter differences (base roll outer diameter differences) were produced.

In addition, the outer diameter difference of each base roll ("(A) Base roll outer diameter difference" in Table 1) and the outer diameter difference of each finally obtained charging roll ["(B) product in Table 1" “Outside diameter difference” refers to the center of the base roll (charging roll) in the axial direction and both ends in the axial direction (end part α and end part β, 140 mm away from the axial center, respectively). It is measured by a meter (manufactured by Mitutoyo Corporation, trade name: LSM6000), and is calculated from the following formula using the obtained result.
(A) Base roll outer diameter difference
= [Outer diameter of the base roll in the axial center]
-[Average of outer diameter of end α of base roll and outer diameter of end β]
(B) Product outer diameter difference
= [Outer diameter at the axial center of the charging roll]
-[Average of outer diameter of end portion α and outer diameter of end portion β of charging roll]

  Next, the material for forming the softener migration preventing layer was N-methoxymethylated nylon (made by Teikoku Chemical Co., trade name: Toresin EF30T): 100 parts by weight, and a conductive agent (made by Ketjen Black International Co., Ltd., trade name: Ketjen Black EC): Prepared by mixing 20 parts by weight with 400 parts by weight of methyl ethyl ketone. Using this forming material, a softening agent migration preventing layer having a thickness of 5 μm was provided on the outer peripheral surface of each base roll obtained previously by dipping.

Furthermore, the material for forming the resistance adjusting layer is NBR (manufactured by ZEON Corporation, trade name: Nipol DN401): 100 parts by weight and quaternary ammonium salt (trade name: TBAHS, manufactured by Wako Pure Chemical Industries, Ltd.): 1 Parts by weight, 30 parts by weight of acetylene black (manufactured by Denki Black HS-100, manufactured by Denki Kagaku Kogyo Co., Ltd.), stearic acid (manufactured by Kao Corporation, product name: LUNAC S30): 0.5 parts by weight, zinc Hana (manufactured by Sakai Chemical Industry Co., Ltd.): 5 parts by weight, vulcanizing aid (manufactured by Sanshin Chemical Co., Ltd., trade name: Sunseller CZ-G): 1.07 parts by weight, vulcanizing aid (Ouchi Shinsei Chemical) Made by company, product name: Noxeller BZ-P): 0.49 parts by weight and sulfur (manufactured by Tsurumi Chemical Co., Ltd., product name: Sulfax PTC): 1 part by weight was mixed and prepared by stirring. . Using the obtained forming material, a resistance adjusting layer was formed using the method described in Patent Document 1. Specifically, for the two rolls (charged rolls No. 1 and 2 finally obtained), the relative movement speed of the coating roll with respect to the base roll (provided with the softener migration prevention layer) Roll coating was carried out by continuously increasing from one end side of the roll in the axial direction toward the center while continuously decreasing from the center toward the other end. On the other hand, the remaining three rolls (finally obtained charging rolls Nos. 3 to 5) were roll-coated according to the method described in Patent Document 1.

  Further, a methanol-toluene mixed solution (methanol: toluene = 7: 3): 500 parts by weight, N-methoxymethylated nylon (product name: Toresin EF30T): 100 parts by weight, and melamine resin (Sumitomo) A protective layer forming material was prepared by adding and mixing 20 parts by weight of Sumitex Resin M3) manufactured by Kagakusha. And using this protective layer forming material, five types of charging rolls were obtained by providing a protective layer having a thickness of 14 μm on the outer peripheral surface of the resistance adjusting layer in accordance with a conventionally known roll coating method. (Charging roll No. 1-5).

  Each charging roll was measured according to the above-described method, and the outer diameter difference [(B) product outer diameter difference] of each charging roll was calculated from the result. The calculated values are shown in Table 1 below. Moreover, the difference [(B)-(A)] produced by subtracting the outer diameter difference [(A)] of the used base roll from the (B) product outer diameter difference is also shown in Table 1 below. From this difference, the charging roll No. It was confirmed that 1 and 2 are charging rolls according to the present invention.

  About each obtained charging roll, using a spring-type hardness tester (micro rubber hardness meter / MD-1 type, manufactured by Kobunshi Keiki Co., Ltd.) in which a cantilever leaf spring type load system is adopted, The tip of the push needle of the spring type hardness tester is brought into contact with the surface of the central portion of each charging roll held in the axial direction, and the tester is pressurized vertically with a load of 33.85 g and immediately calibrated. Was read to measure the micro rubber hardness (MD-1 hardness). The measurement results are also shown in Table 1 below.

The obtained charging rolls were assembled in a commercially available drum cartridge (Fuji Xerox Co., Ltd., trade name: CT350376) (assembly force: P = 1400 [gf]). ) And a pulse transmitter (NF circuit design: manufactured by Gr), a charging bias in which an AC voltage with a peak-to-peak voltage (Vp-p): 1800 V and a frequency: 2 kHz is superimposed on a DC voltage of −700 V, Applied for 5 minutes. Thereafter, from the discharge trace on the photosensitive drum to the nip width [nip width (center portion)] at the position corresponding to the central portion of the charging roll in the axial direction on the photosensitive drum, and the axial end of the charging roll. The corresponding nip widths [nip width (end 1), nip width (end 2)] were measured. The measurement results are also shown in Table 1 below. In addition, the difference caused by subtracting the value of the nip width (center portion) from the average value of the nip width (end portion 1) and the nip width (end portion 2) is also shown in Table 1 below, Evaluation was performed according to the evaluation criteria.
Difference in nip width
○: Less than 50 μm
Δ: 50 to 100 μm
×: Over 100 μm

Furthermore, each obtained charging roll was pressed against the photosensitive drum taken out from a commercially available drum cartridge (same as above) in the dark room with a force of 1400 gf, and in this state, the photosensitive drum was rotated (60 rpm). ), The charging roll was rotated. Using a high-voltage power supply (manufactured by TREK) and a pulse transmitter (NF circuit design: manufactured by Gr), the DC charging voltage of -700 V is applied to the rotating charging roll under a normal temperature / normal pressure environment. A surface potential at a position corresponding to the central portion in the axial direction of the charging roll on the rotating photosensitive drum by applying a charging bias superimposed with an AC voltage of peak-to-peak voltage (Vp-p): 1600 V and frequency: 2 kHz. A) and surface potentials (B1, B2) at positions corresponding to the axial ends of the charging roll were measured. The average value of the above B1 and B2 was obtained (referred to as Ba), and the drum surface potential difference was calculated by subtracting Ba from A. The results (drum surface potential difference) are shown in Table 1 below, and the results of evaluating the drum surface potential difference based on the following evaluation criteria are also shown in Table 1 below.
Drum surface potential difference
○: Potential difference is less than 3V
×: Potential difference is 3V or more

  As is clear from the results of Table 1, in the charging rolls (charging rolls No. 1 and 2) according to the present invention, the difference between the nip width at the axial center and the nip width at the axial end. Is relatively small, and variation in the amount of charge in the axial direction of the photosensitive drum is confirmed to be small. From this result, it is presumed that by using the charging roll of the present invention, it is possible to advantageously achieve uniformity of the finally obtained image and uniformity of the scraping amount of the image carrier surface.

It is a longitudinal cross-sectional explanatory drawing which shows an example of the charging roll which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Charging roll 12 Shaft body 14 Conductive base layer 16 Resistance adjustment layer

Claims (5)

A conductive base layer provided around the shaft body and a resistance adjustment layer provided on the outer peripheral surface of the conductive base layer, and the sum of the thicknesses of the conductive base layer and the resistance adjustment layer is the axial direction In the charging roll exhibiting a medium-high shape that continuously increases from both end portions toward the central portion,
The conductive base layer is formed so as to be thickest at the central portion in the axial direction and gradually thinner toward both ends, while the resistance adjusting layer is thinnest at the central portion in the axial direction and toward both ends. The charging roll is formed so as to gradually become thicker according to the above.   The charging roll according to claim 1, wherein the resistance adjusting layer is formed by a coating operation using a predetermined coating solution.   The charging roll according to claim 1 or 2, wherein a softening agent migration preventing layer is formed between the conductive base layer and the resistance adjusting layer.   The charging roll according to any one of claims 1 to 3, wherein a protective layer is formed on a surface of the resistance adjustment layer. The charging roll according to any one of claims 1 to 4, wherein a difference between a diameter at a central portion in the axial direction and an average of respective diameters at both end portions in the axial direction is 10 to 90 µm.

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