JP2011232433A - Charging component, image formation unit, and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a foreign matters such as developer or external additive from being adhered onto a charged component.SOLUTION: A charged component is rotatable in a predetermined direction. Grooves along a rotating direction and those in a direction perpendicular to the rotating direction are formed on the surface of the charged component. The relationship between a maximum height Ry1 of surface roughness of the charged component in the rotating direction and a maximum height Ry2 in the direction perpendicular to the rotating direction is expressed as Ry1<Ry2. In addition, the relationship between an average irregularity interval Sm1 of the surface roughness of the charged component in the rotating direction and an average irregularity interval Sm2 in the direction perpendicular to the rotating direction is expressed as Sm1>Sm2. With this configuration, adhesion of foreign matters onto the charged component can be prevented.

Description

  The present invention relates to a charging member used in an electrophotographic process, and an image forming unit and an image forming apparatus using the charging member.

  In an image forming apparatus using an electrophotographic process, such as a printer, a copying machine, a facsimile machine, and a multifunction machine, a photosensitive roller is brought into contact with the surface of a photosensitive drum as a latent image carrier by contacting a charging roller as a charging member. The drum surface is uniformly charged. In addition, a cleaning blade for scraping off toner (transfer residual toner) adhering to the surface of the photosensitive drum is disposed upstream of the above-described charging roller in the rotational direction of the photosensitive drum (for example, a patent) Reference 1).

Japanese Patent Laying-Open No. 2008-122915 (FIG. 2, paragraph 0018)

  However, when the image forming apparatus is used for a long period of time, the cleaning blade wears out, and foreign matters such as toner and its external additives easily pass through the gap between the photosensitive drum and the cleaning blade. Foreign matter that has passed through the gap between the photosensitive drum and the cleaning blade may adhere to the surface of the charging roller. If the amount of foreign matter that has adhered to the surface of the charging roller is large, the surface of the photosensitive drum should be made uniform. It becomes impossible to charge and causes uneven density of the image.

  The present invention has been made to solve the above-described problems, and an object thereof is to prevent foreign matter from adhering to the charging member.

  The charging member according to the present invention is a charging member that can rotate in a predetermined direction, and a groove along the rotation direction and a groove along a direction perpendicular to the rotation direction are formed on the surface of the charging member. . The maximum height Ry1 in the rotation direction of the surface roughness of the charging member and the maximum height Ry2 in the direction perpendicular to the rotation direction are in a relationship of Ry1 <Ry2, and the unevenness average in the rotation direction of the surface roughness of the charging member. The interval Sm1 and the uneven average interval Sm2 in the direction perpendicular to the rotation direction have a relationship of Sm1> Sm2.

  According to the present invention, adhesion of foreign matter to the charging member can be suppressed.

1 is a side view showing a basic configuration of an image forming apparatus according to a first embodiment of the present invention. It is a perspective view which shows the structure of the charging roller in the 1st Embodiment of this invention. It is the front view (A) and side view (B) for demonstrating the measurement direction of the resistance value of the charging roller in the 1st Embodiment of this invention. It is a figure which shows typically the surface shape of the axial direction of the charging roller in the 1st Embodiment of this invention, and the circumferential direction. It is a schematic diagram which expands and shows the surface of the charging roller in the 1st Embodiment of this invention. It is a graph which shows the relationship between log (R1) and Ry2 in the 1st Embodiment of this invention.

  FIG. 1 is a diagram showing an overall configuration of an image forming apparatus 10 according to an embodiment of the present invention. The image forming apparatus 10 includes a detachable image forming unit 20 (also referred to as a process cartridge) that forms an image on a recording medium P using toner (developer) in a housing 11, and this image forming unit 20. Is provided with a substantially S-shaped medium conveyance path for conveying the recording medium P. The image forming apparatus 10 may be configured to form a color image by arranging a plurality of image forming units 20, but here, for convenience of explanation, a single color (for example, black) is used by using a single image forming unit 20. ) Image is formed.

  A cassette 21 that accommodates a recording medium P such as printing paper in a stacked state is attached to the lower part of the image forming apparatus 10. On the upper side (upper right side in FIG. 1) of the cassette 21, a paper feed roller 22 that feeds the recording media P stored in the cassette 21 one by one, and a conveyance that guides the recording media P toward the image forming unit 20. A guide 23 and a transport roller pair 24 (feed mechanism) for transporting the recording medium P to the image forming unit 20 are provided.

  A transfer member 17 (transfer roller) is disposed so as to face a photosensitive drum 1 (described later) of the image forming unit 20 with a medium conveyance path interposed therebetween. The transfer member 17 transfers the toner image (developer image) formed in the image forming unit 20 to the recording medium P, and a transfer voltage is applied by a high voltage power source (not shown).

  A fixing unit 25 is disposed on the downstream side (left side in the drawing) of the image forming unit 20 along the medium conveyance path. The fixing unit 25 includes a heat roller 25a including a heat source and a backup roller 25b that sandwiches the recording medium P between the heat roller 25a, and heats and pressurizes the recording medium P on which the toner image is transferred, The toner image is fixed on the recording medium P.

  On the downstream side of the fixing unit 25 along the medium conveyance path, a conveyance guide 26 that guides the recording medium P toward the discharge port, and a discharge roller pair 27 that discharges the recording medium P to the outside of the image forming apparatus 10. It is arranged. Note that a stack unit 28 on which the discharged recording medium P is stacked is provided on the upper portion of the image forming apparatus 10.

  The image forming unit 20 includes a photosensitive drum 1 as an image carrier, a charging roller 2 (charging member) for uniformly charging the surface of the photosensitive drum 1, and an electrostatic formed on the surface of the photosensitive drum 1. A developing device 12 that develops the latent image with toner and a cleaning member 19 that removes toner remaining on the photosensitive drum 1 are provided. The charging roller 2 will be described later.

  The photosensitive drum 1 has, on the surface of a conductive support made of aluminum, stainless steel, or the like, a charge generation layer mainly composed of a charge generation material and a binder resin, and a charge transport material and a binder resin as main components. It is configured by laminating a charge transport layer.

  Various organic pigments and dyes can be used as the charge generation material, but in particular, metal-free phthalocyanine, copper indium chloride, gallium chloride, tin, oxytitanium, zinc, vanadium and other metals or their oxide / chloride coordination. Azo pigments such as phthalocyanines, monoazo, bisazo, trisazo, polyazos and the like can be used. Fine particles of these substances are, for example, polyester resin, polyvinyl acetate, polyacrylic acid ester, polymethacrylic acid ester, polyester, polycarbonate, polyvinyl acetoacetal, polyvinyl propional, polyvinyl butyral, phenoxy resin, epoxy resin, urethane resin, cellulose ester. It can be used by binding with various binder resins such as cellulose ether.

  Examples of charge transport materials include heterocyclic compounds such as carbazole, indole, imidazole, oxazole, pyrazole, oxadiazole, pyrazoline, thiadiazole, aniline dielectrics, hydrazone compounds, aromatic amine dielectrics, stilbene dielectrics, or these An electron donating substance such as a polymer having a group consisting of the above compound in the main chain or side chain can be used. Binder resins include polycarbonate, polymethyl methacrylate, polystyrene, vinyl polymers such as polyvinyl chloride, polyester, polyester carbonate, polysulfone, polyimide, phenoxy, epoxy, silicone resins, and copolymers thereof, and partial cross-linking depression. These can be used alone or in combination, and polycarbonate is particularly suitable. Moreover, you may contain various additives, such as antioxidant and a sensitizer, as needed.

  The developing device 12 has a toner holding space in which toner T to be supplied to the developing roller 13 is held, and the toner T is supplied from a removable toner cartridge 16. The developing device 12 includes a developing roller 13 (developer carrying member) that carries toner T on its surface, a supply roller 14 (developer supply member) that supplies toner T to the developing roller 13, and a surface of the developing roller 13. And a blade 15 (regulating member) for regulating the thickness of the toner layer.

  The developing roller 13 is formed, for example, by forming a conductive elastic layer on the surface of a metal shaft and coating the surface of the elastic layer as necessary, and rotates in contact with the surface of the photosensitive drum 1. To do. The supply member 14 has a foaming elastic layer formed on the surface of a metal shaft, for example, and rotates in contact with the surface of the developing roller 13. The blade 15 is formed by bending, for example, a metal plate-like member, and is pressed against the surface of the developing roller 13 with a constant pressing force at the bent portion.

  The cleaning member 19 is a rubber blade, for example, and its tip is in contact with the surface of the photosensitive drum 1.

  An exposure device 18 that exposes the surface of the photosensitive drum 1 to form an electrostatic latent image is disposed above the photosensitive drum 1 of the image forming unit 20. The exposure device 18 is configured by, for example, an LED (Light Emitting Diode) head, and includes a light emitting element and a lens array. The exposure device 18 is attached to an upper cover (shown integrally with the housing 11 in FIG. 1) attached to the upper side of the housing 11 of the image forming apparatus 1 so as to be openable and closable.

Next, the basic operation of the image forming apparatus 10 will be described.
First, when print data is input to the image forming apparatus 10, a motor as a drive source is driven by a print control unit (not shown), and the paper feed roller 22 and the conveyance roller pair 24 start to rotate. The recording medium P is guided along the transport guide 23 one by one by the rotation of the paper feed roller 22 and the transport roller pair 24 and reaches the image forming unit 20.

  Power from a drive source (not shown) is transmitted to the photoconductive drum 1 of the image forming unit 20, whereby the photoconductive drum 1 rotates at a constant speed in the direction indicated by the arrow in FIG. Further, the charging roller 2 provided in contact with the surface of the photosensitive drum 1 rotates at a constant speed in the direction indicated by the arrow in FIG. 1 following the rotation of the photosensitive drum 1.

  A constant DC voltage is applied to the charging roller 2 by a power supply device (not shown), and the charging roller 2 uniformly charges the surface (outer peripheral surface) of the photosensitive drum 1 while being driven to rotate. Further, the exposure device 18 disposed opposite to the photosensitive drum 1 irradiates the surface of the photosensitive drum 1 with light according to the received image signal of the print data, and electrostatically attenuates the potential of the light irradiation portion. A latent image is formed.

  The toner T supplied from the supply roller 14 adheres to the surface of the developing roller 13 in the developing device 12, and a thin toner layer is formed. This thin toner layer is regulated to a certain thickness by the blade 15. The developing roller 13 causes the toner T on the surface to adhere to the electrostatic latent image formed on the surface of the photosensitive drum 1, and reversely develops the electrostatic latent image to form a toner image.

  A high voltage is applied to the transfer member 17 by a power supply device (not shown). The transfer member 17 transfers the toner image formed on the surface of the photosensitive drum 1 to the recording medium P in accordance with the timing when the recording medium P reaches between the photosensitive drum 1 and the transfer member 17. Further, the toner remaining on the surface of the photosensitive drum 1 without being transferred to the recording medium P is removed by the cleaning member 19.

  The recording medium P on which the toner image is transferred is conveyed to the fixing device 25. The heat roller 25a and the pressure roller 25b constituting the fixing device 25 sandwich the recording medium P and apply heat and pressure. The toner image transferred onto the recording medium P is melted by the heat and pressure applied from the fixing device 25 and fixed on the recording medium P.

  The recording medium P on which the toner image is fixed is guided along the conveyance guide 26 to reach the discharge roller pair 27, and is provided outside the image forming apparatus 10 (here, on the upper cover) by the discharge roller pair 27. It is discharged to the stacker unit 28.

  Next, the configuration of the charging roller 2 will be described. FIG. 2 is a perspective view showing the external shape of the charging roller 2. The charging roller 2 has a conductive core 2a and a conductive elastic layer 2b (resistive layer) formed on the outer peripheral surface of the core 2a excluding both ends in the axial direction.

  As the metal core 2a of the charging roller 2, for example, a shaft made of metal such as free cutting steel (SUM) or stainless steel plated with electroless nickel is used. The elastic layer 2b of the charging roller 2 is pressed against the photosensitive drum 1 (that is, by forming a nip with the photosensitive drum 1) to obtain an appropriate discharge. For example, a resin such as rubber or thermoplastic elastomer It is formed with. The elastic layer 2b is not limited to a single layer, and may have a multilayer structure of two or more layers as necessary to adjust resistance, prevent contamination of the photosensitive drum 1, and the like.

  The constituent material of the elastic layer 2b of the charging roller 2 includes epichlorohydrin rubber (CO, ECO, GECO), ethylene propylene rubber (EPM, EPDM), nitrile rubber (NBR), chloroprene rubber (CR), urethane rubber, silicone rubber, etc. The rubber composition which has as a main component what mixed 1 type (s) or 2 or more types can be used. In particular, a material mainly composed of epichlorohydrin rubber (ECO) is desirable.

Generally, when the resistance of the elastic layer 2b of the charging roller 2 is too large, image defects due to uneven charging or charging failure occur on the surface of the photosensitive drum 1, and conversely, when the resistance is too small, the photosensitive drum 1 Leakage occurs due to scratches on the surface of the image and image defects occur. Therefore, an appropriate resistance range is obtained for the elastic layer 2b using an ion conductive material, an ion conductive agent, carbon black, a metallic oxide, or the like. If the elastic layer 2b has uneven resistance (resistance variation due to location), it leads to uneven charging of the photosensitive drum 1, and therefore, an ion conductive resistance layer is preferable to an electronic conductive resistance layer. The resistance value of the elastic layer 2b of the charging roller 2 is preferably in the range of 10 ⁶ to 10 ⁹ Ω.

  Here, a method of measuring the resistance value of the elastic layer 2b of the charging roller 2 will be described with reference to FIG. As shown in FIG. 3, a resistance measuring device 30 (“4339B high resistance meter” manufactured by Agilent Technologies) is used to measure the resistance of the elastic layer 2 b of the charging roller 2. One terminal 31 of the resistance measuring device 30 is attached to the core metal 2a of the charging roller 2, and the other terminal is provided with a SUS bearing 32 having a width of 2.0 mm and a diameter of 6.0 mm, and the surface of the elastic layer 2b. Is pressed with a force F of 10 gf, and the resistance is measured while rotating the charging roller 2. Since the resistance value of the charging roller 2 generally varies depending on temperature, humidity, or voltage, a DC voltage of −500 V is applied to the cored bar 2a in an environment of a temperature of 20 degrees and a humidity of 50% RH. The charging roller 2 is rotated once (360 °), and the resistance measured at 100 locations is averaged. The resistance value obtained in this way is R1.

  In order to uniformly charge the photosensitive drum 1, it is necessary to form an appropriate nip between the elastic layer 2 b of the charging roller 2 and the photosensitive drum 1. The hardness of the elastic layer 2b is measured using a micro rubber hardness meter “MD-1capa” type A (manufactured by Kobunshi Keiki Co., Ltd.) using a peak hold function. The hardness of the elastic layer 2b as measured above is preferably in the range of 35 degrees to 75 degrees, and particularly preferably in the range of 45 degrees to 65 degrees. Further, as the tensile property of the elastic layer 2b, the elongation at break (JIS-K6251) is preferably 50% or more, and particularly preferably 100% or more.

  On the surface (outer peripheral surface) of the elastic layer 2b, predetermined polishing marks are formed by cutting or polishing, thereby giving a predetermined surface roughness. 4A is a diagram schematically showing a surface shape along the axial direction (the direction of arrow A in FIG. 2) of the elastic layer 2b of the charging roller 2. FIG. FIG. 4B is a diagram schematically illustrating the circumferential surface shape of the elastic layer 2 b of the charging roller 2. FIG. 5A is a schematic diagram showing an enlarged surface shape of the elastic layer 2b of the charging roller 2, and FIG. 5B is an enlarged view of the convex portion on the surface of the elastic layer 2b of the charging roller 2. It is a schematic diagram shown. In FIGS. 4 and 5, the irregularities on the surface of the elastic layer 2b are shown enlarged.

  As shown in FIGS. 4A and 4B, a groove 201 along the circumferential direction (rotation direction) of the charging roller 2 and a groove 202 along the axial direction are formed on the surface of the elastic layer 2b of the charging roller 2. Is formed.

  For example, as shown in FIG. 5A, the surface of the charging roller 2 has a shape in which minute pyramids that are long in the circumferential direction are arranged. Here, the groove 201 along the circumferential direction is formed deeper than the groove 202 along the axial direction. Further, the arrangement interval of the grooves 201 along the circumferential direction is formed narrower than the arrangement interval of the grooves 202 along the axial direction.

  Here, the following index is used to represent the surface roughness of the charging roller 2. That is, the maximum height Ry in the circumferential direction (B direction) is Ry1, and the average interval Sm of the unevenness in the circumferential direction is Sm1. Further, the maximum height Ry in the axial direction (A direction) is Ry2, and the average interval Sm of the unevenness in the axial direction is Sm2.

  As described above, since the groove 201 along the circumferential direction is deeper than the groove 202 along the axial direction and is formed with a narrow array interval, as shown in FIG. 5B, Ry1 <Ry2 and The relationship Sm1> Sm2 is established.

Here, a method for processing the surface of the elastic layer 2b of the charging roller 2 will be described.
First, the elastic layer 2b of the charging roller 2 is polished (here, dry polishing) so that the groove 202 is formed in the axial direction. That is, for example, in a state where the charging roller 2 is rotated, the surface of the elastic layer 2b is polished by bringing a rotating grindstone into contact therewith, thereby forming the groove 201 in the axial direction on the surface of the elastic layer 2b. Since the outer diameter of the charging roller 2 is determined by this polishing, it is also called outer diameter polishing.

  Next, the surface of the elastic layer 2b is polished (wet polishing here) so that a groove 201 is formed along the circumferential direction (rotational direction). More specifically, the surface of the elastic layer 2b is brought into contact with a water-resistant polishing paper (tape) while supplying the polishing liquid while the charging roller 2 is rotated. At this time, by forming the groove 201 along the circumferential direction so as to cut the mountain extending in the axial direction (the mountain between the grooves 202), the unevenness due to the axial groove 202 formed by the outer diameter polishing is formed. Try not to lose as much as possible.

  Here, a surface roughness measuring device “SE-3500” and a detector “PU-DJ2S” were used for measuring the surface roughness (both manufactured by Kosaka Laboratory Ltd.). The measurement conditions were based on JISB0601 (1994), the cutoff value (λc) was 2.5 mm, the reference length (L) was 2.5 mm, the measurement length was 12.5 mm, and the feed rate was 0.5 mm. .

  The surface roughness of the elastic layer 2b of the charging roller 2 includes the type and polishing speed of a rotating grindstone used for dry polishing, the type and polishing speed of a tape (water-resistant polishing paper) used for wet polishing, and the pressure applied to the tape. It can be adjusted by changing the polishing conditions.

  That is, in order to increase the maximum height Ry1 in the circumferential direction, the abrasive grains of the dry grinding rotary grindstone are coarsened or the rotational speed of the rotary grindstone is increased. In order to increase the maximum height Ry2 in the axial direction, the particle size of wet-resistant water-resistant paper is coarsened or the polishing pressure is increased. Further, in order to increase the average interval Sm1 between the irregularities in the circumferential direction, the rotational speed of the rotary grindstone for dry polishing is increased, or the rotational speed of the charging roller 2 is increased. In order to increase the average interval Sm2 between the irregularities in the circumferential direction, the particle size of the wet-resistant water-resistant paper is increased.

  Each index (Ry1, Ry2, Sm1, Sm2) representing the surface roughness of the charging roller 2 is determined by each of the above steps, but is affected by the hardness and tensile strength of the elastic layer 2b. To obtain the desired surface roughness.

  By imparting a certain surface roughness to the elastic layer 2b of the charging roller 2, a minute gap is formed between the surface of the elastic layer 2b of the charging roller 2 and the surface of the photosensitive drum 1, according to Paschen's law. A region that contributes to the discharge based thereon can be secured. The surface roughness (maximum height Ry1, Ry2) of the elastic layer 2b of the charging roller 2 depends on the voltage applied to the charging roller 2 and the surface state of the photosensitive drum 1, but is generally in the range of 4 to 40 μm. .

  By subjecting the surface of the elastic layer 2b of the charging roller 2 to surface treatment or coating, it is possible to prevent contamination of the photosensitive drum 1 and to adjust resistance. For example, the surface can be impregnated and coated with an isocyanate compound or polyol. Examples of the isocyanate compound and polyol here include toluene diisocyanate (TDI), methylene didiisocyanate (MDI), xylylene diisocyanate (XDI), naphthalene diisocyanate (NDI), hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI). , Polyester polyols, polycarbonate polyols, silicone diols, acrylic fluoropolymers, acrylic silicone polymers, and multiples and modified bodies thereof. The isocyanate compound or polyol preferably has a molecular weight Mw of 600 to 12000, particularly preferably 700 to 3000. Further, the surface of the charging roller 2 can be covered with not only an isocyanate compound and a polyol but also polyester, urethane, acrylic urethane, epoxy, nylon, fluorine-containing material, silicone, and the like. However, it is necessary to be careful not to lose irregularities such as polishing marks on the surface of the elastic layer 2b of the charging roller 2 even after the surface treatment or coating.

  Next, experimental results of an example in the present embodiment and a comparative example for the example will be described.

  Here, the cored bar 2a of the charging roller 2 is a metal shaft obtained by electroless plating a SUM material having an outer diameter of 6 mm and an axial length of 252 mm. The thickness of the elastic layer 2b of the charging roller 2 was 3 mm, and the outer diameter was 12 mm. The axial length of the elastic layer 2b was 224 mm. As the elastic layer 2b, epichlorohydrin rubber (ECO) having a hardness of 54 degrees and an elongation at break of 150% was used. Further, an ionic conductive agent was added to the elastic layer 2b for resistance adjustment.

  The surface of the elastic layer 2b was dry-polished to have an outer diameter of 12 mm, and an axial groove 202 was formed on the surface of the elastic layer 2b. Next, wet polishing using a tape was performed to form circumferential grooves 201 on the surface of the elastic layer 2b. Further, the charging roller 2 is immersed in a surface treatment liquid in which 20 parts by weight of hexamethylene diisocyanate compound (HDI) is mixed with 100 parts by weight of ethyl acetate, and then heated in an oven, whereby an elastic layer of the charging roller 2 is obtained. Surface treatment was applied to 2b.

The surface roughness (Ry1, Sm2, Ry2, Sm2) of the charging roller 2 of Example 1 created as described above was as follows.
(1) Maximum height Ry1 in the circumferential direction: 15.74 μm
(2) Convex / concave average interval Sm1: 175 μm
(3) Maximum height Ry2 in the axial direction: 17.20 μm
(4) Axial unevenness average interval Sm2: 89 μm
In addition, the resistance value R1 of the charging roller 2 of Example 1 was 1.48 × 10 ⁸ Ω.

  The charging rollers of Examples 2 and 3 were prepared by changing the polishing conditions so that the surface roughness was smaller than that of the charging roller of Example 1.

  The charging roller of Example 4 was prepared by changing the polishing conditions so that the surface roughness of the charging roller of Example 1 was increased.

  The charging rollers of Comparative Examples 2 and 5 were prepared by changing the polishing conditions so that the surface roughness of the charging roller of Example 1 was larger (than that of Example 4).

  The material of the elastic layer 2b of the charging roller of Examples 2 to 4 and Comparative Examples 2 and 5 is the same as that of Example 1, but the resistance R1 (in FIG. 3) is changed by changing the surface roughness. The value measured by the method is also changing.

  The charging roller of Example 5 was prepared by changing the amount of ionic conductive agent added to the charging roller of Example 1 so that the resistance of the elastic layer 2b was reduced. In Example 5, the polishing conditions are the same as in Example 1, but the surface roughness is also changed by changing the material of the elastic layer 2b.

  The charging roller of Example 6 was prepared by changing the polishing conditions so as to reduce the surface roughness of the charging roller of Example 5.

  The charging rollers of Examples 7 and 8 were prepared by changing the polishing conditions so that the surface roughness of the charging roller of Example 5 was increased.

  The charging rollers of Comparative Examples 3 and 6 were prepared by changing the polishing conditions so that the surface roughness of the charging roller of Example 5 was larger (than that of Examples 7 and 8).

  The charging roller of Example 9 was prepared by changing the amount of ion conductive agent added to the charging roller of Example 2 so that the resistance of the elastic layer 2b was increased.

  The charging roller of Example 10 was prepared by changing the polishing conditions so as to reduce the surface roughness of the charging roller of Example 9.

  The charging rollers of Comparative Examples 4 and 7 were prepared by changing the polishing conditions so that the surface roughness of the charging roller of Example 9 was increased.

  In these Examples 1 to 10 and Comparative Examples 2 to 7, the circumferential maximum height Ry1 is smaller than the axial maximum height Ry2, and the circumferential uneven average interval Sm1 is larger than the axial uneven average interval Sm2. Large (Ry1> Ry2, Sm1 <Sm2).

  On the other hand, the charging roller of Comparative Example 1 was prepared without performing tape polishing for forming a circumferential groove. In the first comparative example, the circumferential maximum height Ry1 is larger than the axial maximum height Ry2, and the circumferential uneven average interval Sm1 is smaller than the axial uneven average interval Sm2 (Ry1> Ry2, Sm1 < Sm2).

  Table 1 shows the measurement results of the surface roughness and the resistance value of the charging rollers of Examples 1 to 10 and Comparative Examples 1 to 7. In Table 1, the unit of the surface roughness Ry1, Sm1, Ry2, Sm2 is μm, and the unit of the resistance value R1 is Ω.

Table 1 also shows values obtained by calculating the surface roughness (maximum height in the axial direction) Ry2 and the resistance value R1 by the following equation (1).
Ry2 + 20 × log (R1) (1)
Equation (1) is obtained based on the graph of FIG. 6 and will be described later.

  The charging rollers of Examples 1 to 10 and Comparative Examples 1 to 7 were mounted on the image forming apparatus 10 and a printing test was performed under the following conditions.

  The outer diameter of the photosensitive drum 1 was 30 mm, and the axial length was sufficiently longer than the elastic layer 2 b of the charging roller 2. The rotational speed of the photosensitive drum 1 was 150 rpm.

  As described above, the charging roller 2 is driven to rotate in contact with the photosensitive drum 1, but by applying a force of 350 gf to both ends of the core metal 2 a of the charging roller 2, the elastic layer 2 b of the charging roller 2 is exposed to light. Pressed against the surface of the body drum 1.

  As the developing method of the developing device 12, non-magnetic one-component contact development was used, and as the toner (developer), a negatively charged pulverized toner containing a polyester resin and having an average particle diameter of about 6.5 μm was used.

  In the printing test, images were continuously printed on 5000 sheets of 40000 recording media at room temperature. As a recording medium, A4 size “P paper” (manufactured by Fuji Xerox Co., Ltd.) was used. The conveyance direction of the recording medium was “vertical feed”, and the coverage (ratio of the image portion in the unit area) was 5%. The DC voltage (charging voltage) applied to the charging roller 2 was −1000V.

  Every time 5000 sheets were continuously printed (that is, 0 sheets, 5000 sheets, 10,000 sheets, 15000 sheets,..., 40000 sheets), printing for image evaluation was performed. Printing for image evaluation was performed in a low-temperature and low-humidity environment (LL environment) at a temperature of 8 ° C. and a relative humidity of 20% (LL environment). And halftone images (30% coverage) one by one (6 sheets in total). Also, even in a high temperature and high humidity environment (HH environment) with a temperature of 28 ° C. and a relative humidity of 85%, the charging voltage is changed in three ways: −850V, −1000V and −1200V, and a blank paper image and a halftone image are printed one by one ( 6 sheets in total). As a recording medium, A4 version “Excellent White” (manufactured by Oki Data Corporation) was used.

  Each member of the image forming apparatus 10 used for the printing test is stored in advance for 40 hours or more in the range of the test temperature (8 ° C. or 28 ° C.) ± 3 ° C. and the humidity (20% or 85%) ± 10%. Therefore, it was used by fully adapting to the LL environment or the HH environment. The surface potential of the photosensitive drum 1 was also measured in the same environment.

  The blank image (coverage 0%) and halftone image (coverage 30%) formed in this way were observed, and the presence or absence of “image contamination” due to the potential drop of the photosensitive drum 1 was determined. Specifically, the blank paper image and the halftone image were visually observed, and if a dark portion (spots or the like) was seen in either one, it was determined that “image was dirty”.

  Further, the halftone image (coverage 30%) was visually observed to determine the presence or absence of a fine horizontal stripe pattern (that is, a stripe pattern in the axial direction of the charging roller 2) with a high density (or a light density).

  Further, when the rotational speed of the photosensitive drum 1 was 150 rpm and 75 rpm, the surface potential at the exposed portion of the photosensitive drum 1 was measured with a DC voltage of −1000 V applied to the charging roller 2.

  Table 2 shows the results of image evaluation. Table 2 shows the presence / absence of image stains and horizontal streak patterns as the image evaluation results (in the case of occurrence, the number of printed sheets at the start of occurrence).

  Of Examples 1 to 10 and Comparative Examples 1 to 7, the occurrence of a horizontal streak pattern was observed only in Comparative Example 1. In Comparative Example 1, a horizontal streak pattern was generated at the charging voltage of −850 V and −1000 V in the HH environment from the start of continuous printing (0 sheets of continuous printing). In addition, image smearing occurred in the LL environment after 10,000 sheets of continuous printing and a charging voltage of −850 V, and the image smearing deteriorated as the number of printed sheets increased.

  On the other hand, in Examples 1 to 10 and Comparative Examples 2 to 4, neither image stain nor horizontal stripe pattern was seen. That is, in comparison with Comparative Example 1, good results were obtained.

  In Comparative Examples 5 to 7, image smear occurred at the charging voltage of −850 V in the LL environment from the start of continuous printing, but no image smudge was observed at the charging voltages of −1000 V and −1200 V in the same environment. No horizontal stripe pattern was observed at any of the charging voltages of -850V, -1000V and -1200V. That is, compared with the comparative example 1, the favorable result (especially improvement of a horizontal stripe pattern) was obtained. This is presumably because the circumferential groove 201 on the surface of the charging roller 2 contributed to the suppression of the occurrence of the horizontal stripe pattern.

  Accordingly, by forming axial grooves and circumferential grooves on the surface of the charging roller 2 so as to satisfy Ry1 <Ry2, Sm1> Sm2 (Examples 1 to 10, Comparative Examples 2 to 7). It can be seen that the effect of suppressing the adhesion of foreign matters to the surface of the charging roller 2 is obtained.

  Note that the image smear occurred at the charging voltage of −850 V in the LL environment of Comparative Examples 5 to 7 because the surface potential of the photosensitive drum 1 decreased due to the surface roughness of the charging roller 2 or the high resistance. This is considered to be due to image smearing.

  In Examples 1 to 10 and Comparative Examples 2 to 4 described above, the value of the above formula (1), that is, the value of Ry2 + 20 × log (R1) was 190.6 or less. In Comparative Examples 5 to 7, the value of Ry2 + 20 × log (R1) was 195.3 to 195.6.

  Table 3 shows the measurement results of the surface potential (also referred to as drum potential) of the photosensitive drum 1 described above. The surface potential (unit: V) of the photosensitive drum 1 is measured at the time of starting continuous printing and after printing 40000 sheets.

  Furthermore, from the results shown in Table 3, the difference between the drum potential when the rotational speed of the photosensitive drum 1 was 75 rpm and 150 rpm in the LL environment (referred to as the LL speed difference) was obtained. The drum potential in the HH environment and the drum potential in the LL environment (referred to as HH-LL difference) were determined for each of the cases where the rotational speed of the photosensitive drum 1 was 75 rpm and 150 rpm. The LL speed difference and the HH-LL difference were obtained at the start of continuous printing and after printing 40000 sheets. Table 4 shows the results.

  The determination results shown in Table 4 were determined as follows. That is, the LL speed difference (drum potential difference at the drum rotation speed of 75 rpm / 150 rpm in the LL environment) is less than 20 V both at the start of continuous printing and after the printing of 40000 sheets, and the LL-HH difference ( When the drum potential difference between the HH environment and the LL environment was less than 100 V, the determination result was “◯”.

  On the other hand, if the LL speed difference exceeds 20 V or the LL-HH difference exceeds 100 V, either at the start of continuous printing or after printing 40000 sheets, the determination result is “Δ”. did.

  In the case of “Δ”, when the LL−HH difference (the difference in drum potential between the HH environment and the LL environment) exceeds 120 V, the determination result is “x”.

  The reason why such a determination is made is that it is difficult to use the drum potential when the drum potential changes greatly depending on the use environment.

  Here, the difference in the drum potential (LL speed difference) at the drum rotation speed of 75 rpm / 150 rpm in the LL environment between the HH environment and the LL environment is examined. This is due to the following reason. That is, generally, a discharge based on Paschen's law occurs between the photosensitive drum 1 and the charging roller 2, and the surface of the photosensitive drum 1 is charged by using this discharge. The environment has a higher discharge start voltage and is more susceptible to the resistance, surface shape, and surface condition (dirt, etc.) of the charging roller 2. Therefore, here, a difference in drum potential (LL speed difference) at a drum rotation speed of 75 rpm / 150 rpm in the LL environment is examined.

  In Comparative Example 1, both the LL-HH difference and the LL speed difference were small at the start of continuous printing, but after continuous printing of 40000 sheets, the LL-HH difference exceeded 100 V (when the drum rotation speed was 150 rpm). The speed difference also exceeded 20V.

  In Comparative Example 1, since the circumferential groove is not formed in the charging roller 2, the contact area with the photosensitive drum 1 is increased, and the external additive adhering to the charging roller 2 from the photosensitive drum 1 is increased. This is thought to be due to the increase in quantity. That is, it is considered that due to the deposits on the surface of the charging roller 2, the charging roller 2 cannot uniformly charge the surface of the photosensitive drum 1, and the image is stained.

  In Comparative Examples 2 to 4, the LL-HH difference exceeded 100 and the LL speed difference exceeded 20 V from the start of continuous printing. This is presumably because the surface roughness of the charging roller 2 is relatively large (coarse) and the resistance of the charging roller 2 is further increased due to low humidity (LL environment).

  In these Comparative Examples 2 to 4, the values of the above Ry2 + 20 × log (R1) were both greater than 185.2 and 190.6 or less.

  In Comparative Examples 5 to 7, the LL-HH difference exceeded 120 from the start of continuous printing, and the LL speed difference exceeded 20V. This is presumably because the surface roughness of the charging roller 2 is larger (rough) and the resistance of the charging roller 2 is further increased due to low humidity (LL environment).

  In these Comparative Examples 5 to 7, the above Ry2 + 20 × log (R1) values were all greater than 190.6.

  Here, the influence of the maximum height Ry2 in the axial direction of the charging roller 2 and the resistance value R1 of the charging roller 2 (when 500 V is applied) on the stability of the surface potential of the photosensitive drum 1 will be examined.

  First, the reason why the maximum height Ry2 in the axial direction is selected from among Ry1, Ry2, Sm1, and Sm2 representing the surface roughness of the charging roller 2 will be described. The charging roller 2 of the present embodiment has a groove 201 along the circumferential direction and a groove 202 along the axial direction, and the maximum height Ry1 in the circumferential direction is that of the groove 202 along the axial direction of the charging roller 2. Corresponding to the size, the maximum height Ry2 in the axial direction corresponds to the size of the groove 201 along the circumferential direction of the charging roller 2. In this embodiment, since Ry1 <Ry2 has a relationship as described above, Ry2 has a greater influence on the contact state between the charging roller 2 and the photosensitive drum 1. Further, since the average interval Sm1 and Sm2 of the unevenness is an average value of the length corresponding to the unevenness formed on the surface, it affects the ease of adhesion of foreign matter, but the charging roller 2 and the photosensitive drum 1 The influence on the contact state is smaller than the maximum height Ry2. For these reasons, Ry2, Ry2, Rm2, Sm1, and Sm2, which represent the surface roughness of the charging roller 2, was selected to have the greatest influence on the contact state between the charging roller 2 and the photosensitive drum 1.

  FIG. 6 is a graph in which log (R1) is taken on the horizontal axis, Ry2 is taken on the vertical axis, and the determination results shown in Table 4 are indicated by ◯, Δ, and X. In FIG. 6, it can be seen that a determination result of ◯ or Δ is obtained in the region of Ry2 + 20 × log (R1) ≦ 190.6 (the one-dot chain line and the region below it). Furthermore, in the region of Ry2 + 20 × log (R1) ≦ 185.2 (broken line and the region below it), it can be seen that a determination result of “◯” is obtained.

  Therefore, in the range where Ry2 + 20 × log (R1) ≦ 190.6 is satisfied, the surface potential variation of the photosensitive drum 1 is small with respect to the change in the rotational speed of the photosensitive drum 1 (75 rpm / 150 rpm). In addition, it can be seen that the surface potential of the photosensitive drum 1 varies little with respect to the environmental change (LL / HH).

  Further, it can be seen that the variation in the surface potential of the photosensitive drum 1 is further less with respect to the change in the rotational speed of the photosensitive drum 1 and the environmental change in a range where Ry2 + 20 × log (R1) ≦ 185.2 is satisfied.

  Therefore, when Ry2 + 20 × log (R1) ≦ 190.6 is satisfied, the surface potential of the photosensitive drum 1 is stable with respect to changes in the rotational speed and environment (LL / HH) of the photosensitive drum 1. It can be seen that Furthermore, it can be seen that a more stable surface potential of the photosensitive drum 1 can be obtained when Ry2 + 20 × log (R1) ≦ 185.2 is satisfied.

  As described above, in the first embodiment of the present invention, the surface of the charging roller 2 is provided with the groove 201 along the rotation direction (circumferential direction) and the groove 202 along the direction perpendicular to the rotation direction, By configuring so as to satisfy Ry1 <Ry2 and Sm1> Sm2, adhesion of foreign matters to the surface of the charging roller 2 can be suppressed, and image defects such as image stains and horizontal stripes can be reduced.

  In particular, by satisfying Ry2 + 20 × log (R1) ≦ 190.6 (more preferably 185.2), the photosensitive drum 1 can be controlled against changes in the rotational speed and environment (LL / HH) of the photosensitive drum 1. The surface potential can be stabilized.

Here, the effect by this Embodiment is further demonstrated.
In conventional charging rollers, irregularities are formed by adding particles of a specific size to the coating layer on the surface, and the projections are brought into contact with the photosensitive drum, thereby contacting the charging roller and the photosensitive drum. There is one in which the area is reduced (Japanese Patent Laid-Open No. 9-258523).

  However, in the charging roller having such a configuration, when the same portion of the charging roller is pressed against the photosensitive drum for a long time, for example, at the time of shipment, particles are sunk inside the charging roller by the pressing force. And nip marks may occur, leading to image defects. Moreover, since the irregularities are formed by adding particles to the surface, the surface roughness in the axial direction and the circumferential direction are substantially the same, and the roughness cannot be controlled separately in the circumferential direction and the axial direction.

  On the other hand, according to the above-described embodiment, the surface roughness in the axial direction and the circumferential direction of the charging roller is controlled so that Ry1 <Ry2 and Sm1> Sm2 are satisfied. Adherence of foreign matter to the surface of the charging roller can be suppressed without generating nip marks on the surface of the charging roller (due to the sinking).

  Conventionally, it has also been proposed to provide a cleaning roller for removing deposits on the surface of the charging roller (Japanese Patent Application Laid-Open No. 8-44158). However, in this case, since the installation space for the cleaning roller is required, the image forming unit must be enlarged, and peripheral members such as a bearing for the cleaning roller are required. Cost and assembly man-hour increase.

  On the other hand, according to the above-described embodiment, it is possible to suppress the adhesion of foreign matters to the surface of the charging roller without providing a cleaning roller or the like, increasing the number of parts, and the associated manufacturing cost and assembly man-hour. Can be reduced. Further, it is possible to eliminate a horizontal streak pattern that cannot be eliminated by the cleaning roller.

Second embodiment.
In the first embodiment described above, the resistance value R1 is measured by applying 500 V to the charging roller 2 in the measurement method shown in FIG. On the other hand, the resistance value of the elastic layer 2b of the charging roller 2 varies depending on the applied voltage, and even with the same material, the larger the surface roughness Ry, the more susceptible to the voltage. Therefore, in the second embodiment, attention is paid to the resistance value R2 when 300 V is applied to the charging roller 2 in addition to the resistance value R1 when 500 V is applied to the charging roller 2. The method of measuring the resistance values R1, R2 is as shown in FIG.

  In the second embodiment, the configuration of the charging roller 2 is the same as that of the first embodiment. Further, the configuration and operation of the image forming apparatus 10 are the same as those in the first embodiment.

  Table 5 shows the resistance value R1 when 500 V is applied to the charging roller 2 and the resistance value when 300 V is applied to each of the charging rollers 2 of Examples 1 to 10 and Comparative Examples 1 to 7 described in the first embodiment. R2 and the calculated value of log (R2) -log (R1) are shown. In addition, the image evaluation (image contamination, horizontal stripe pattern) and the drum potential determination result described in the first embodiment are also shown.

  From Table 5, in Examples 1 to 10, there is no image stain or horizontal stripe pattern (as described in Embodiment 1), and the determination result of the drum potential is good (◯). Furthermore, log (R2 ) -Log (R1) is in the range of 0.298 to 1.222.

  On the other hand, in the results of Comparative Examples 1 to 7, image smudges or horizontal streaks are observed, or the drum potential determination result is Δ or ×, and the value of log (R2) −log (R1) is 0. It can be seen that it is smaller than 192 or larger than 1.213.

  From this result, the surface of the charging roller 2 is provided with a groove 201 along the rotation direction (circumferential direction) and a groove 202 along the direction perpendicular to the rotation direction, satisfying Ry1 <Ry2 and Sm1> Sm2, By configuring so as to satisfy 0.298 ≦ log (R2) −log (R1) ≦ 1.2, it is possible to suppress image smearing and horizontal streak patterns, and the rotational speed and environment of the photosensitive drum 1 (LL / It can be seen that the surface potential of the photosensitive drum 1 can be stabilized against changes in HH).

  Each of the above-described embodiments can be applied to a rotatable charging member having a groove formed on the surface by polishing or the like, an image forming unit having the charging member, and an image forming apparatus. The charging member is not limited to a charging roller having a single layer structure, but may be one in which a groove is formed on the surface of a charging roller having a multilayer structure. In addition, the charging member is not limited to the charging roller as long as it is a rotatable charging member, and may be, for example, a belt-type rotating member.

  Needless to say, a cleaning member (for example, a cleaning roller) for removing foreign matter on the surface of the charging roller may be added to the configuration of each embodiment of the present invention.

1 Photosensitive drum (image carrier)
2 Charging roller (charging member)
2a Core 2b Elastic layer 10 Image forming device 12 Developing device 13 Developing roller 16 Toner cartridge 17 Transfer member 18 Exposure device 19 Cleaning member 20 Image forming unit 21 Cassette 22 Feed roller 25 Fixing unit 30 Resistance measuring device 31 Terminal 32 Bearing 201 Groove 202 along the circumferential direction (rotation direction) Groove along the axial direction (direction perpendicular to the rotation direction)

Claims (8)

A charging member rotatable in a predetermined direction,
On the surface of the charging member, a groove along the rotation direction and a groove along the direction perpendicular to the rotation direction are formed,
The maximum height Ry1 in the rotation direction of the surface roughness of the charging member and the maximum height Ry2 in the direction perpendicular to the rotation direction are in a relationship of Ry1 <Ry2.
The charging member is characterized in that the average roughness interval Sm1 in the rotation direction and the average roughness interval Sm2 in the direction perpendicular to the rotation direction of the surface roughness of the charging member have a relationship of Sm1> Sm2.   The charging member according to claim 1, wherein the groove along the rotation direction and the groove in a direction perpendicular to the rotation direction are formed by polishing. When the resistance value when applying 500V to the charging member is R1,
The maximum height Ry2 of the surface roughness and the resistance value R1 are:
Ry2 + 20log (R1) ≦ 190.6
The charging member according to claim 1, wherein: When the resistance value when applying 500V to the charging member is R1,
The maximum height Ry2 of the surface roughness and the resistance value R1 are:
Ry2 + 20log (R1) ≦ 185.2
The charging member according to claim 3, wherein: A resistance value R1 when 500V is applied to the charging member and a resistance value R2 when 300V is applied to the charging member,
0.298 ≦ log (R2) −log (R1) ≦ 1.2
The charging member according to any one of claims 1 to 4, wherein the charging member is satisfied.   The charging member according to any one of claims 1 to 5, wherein the charging member has a roller shape and has an elastic layer on a surface thereof.   An image forming unit comprising the charging member according to claim 1. An image forming apparatus comprising the image forming unit according to claim 7.

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