Image forming apparatus having rubbing member in contact with image bearing member

An image forming apparatus includes an image bearing member, a charging member, an exposing device, a developing device, a transfer member, and a rubbing member. The image bearing member includes a plurality of recess portions on a surface thereof. The rubbing member is configured to come into contact with the image bearing member to form a rubbing nip portion between the rubbing member and the image bearing member. The recess portions each have an opening portion whose maximum length in the rotational direction is 20 μm to 120 μm. When a linear speed of the image bearing member in the rubbing nip portion is S1 and a linear speed of the rubbing member in the same direction as the linear speed of the image bearing member in the rubbing nip portion is S2, a relationship of a linear speed ratio of S2/S1<1.0 is satisfied.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an image forming apparatus such as a copier, a laser beam printer, or a process cartridge employing an electrophotographic system or an electrostatic recording system.

Background Art

Conventionally, an image forming apparatus of an electrophotographic system is widely used as a copier, a printer, a plotter, a facsimile machine, or a multifunctional printer having functions of a plurality of these. As an image forming apparatus of this kind, an image forming apparatus that develops an electrostatic image formed on a photosensitive member by using two-component developer including nonmagnetic toner and magnetic carrier is widely used. As the photosensitive member, an organic electrophotographic photosensitive member, in which an organic photosensitive layer for which an organic material is used as a photoconductive substance such as a charge-generating substance or a charge-transporting substance is provided on a support body is widely used from the viewpoint of low cost and high productivity. Examples of the organic electrophotographic photosensitive member include a photosensitive drum and an image bearing member. As this organic electrophotographic photosensitive member, a photosensitive member including a laminated photosensitive layer formed by laminating a charge generating layer containing a charge-generating substance of a photosensitive dye or a photosensitive pigment and a charge transport layer containing a charge-transporting substance of a photosensitive polymer or a photosensitive low-molecular-weight compound is mainly used. Such a photosensitive member including a laminated photosensitive layer is advantageous in terms of sensitivity and variety of material design.

Since electric external force or mechanical external force is directly applied to the surface of the photosensitive member during charging, exposure, developing, transfer, and cleaning, the photosensitive member is required to have durability against these external forces. Specifically, the photosensitive member is required to have durability against generation of scratches or wear on the surface by these external forces, that is, scratch resistance and wear resistance. As a photosensitive member whose scratch resistance and wear resistance of the surface thereof are improved, for example, a photosensitive member including, as a surface layer, a cured layer formed by using a curable resin as a binder resin is known. In addition, a photosensitive member including, as a surface layer, a charge-transporting cured layer formed by curing polymerization of a monomer having a carbon-carbon double bond and a charge-transporting property is also known. Further, a photosensitive member including, as a surface layer, a charge-transporting cured layer formed by causing curing polymerization of a hole-transporting compound having a chain-polymerizable functional group in the molecule by energy of an electron beam is also known. As described above, as a technique of improving the scratch resistance and wear resistance of a peripheral surface of the photosensitive member, a technique of using a cured layer as a surface layer of a photosensitive member and thus increasing the mechanical strength of the surface layer has been established in recent years.

However, when image formation is performed by using a photosensitive member having a high hardness, blur of an electrostatic latent image called image deletion is likely to occur particularly in a high-humidity environment. The cause of this image deletion is considered as follows. Electric discharge products such as ozone and NOx are generated mainly in a charging portion, and attach to the surface of the photosensitive member. The surface of the photosensitive member has a low surface friction coefficient, is hard, and thus is not easy to wear, and therefore the electric discharge products attached to the surface are difficult to remove. It is considered that such electric discharge products that have attached to the surface and are difficult to remove absorb moisture in the high-humidity environment, thus degrade charge retaining capability of the surface of the photosensitive member, and cause the blur of electrostatic latent image. Therefore, particularly in the case where the hardness of the photosensitive member is high, the electric discharge products attached thereto become more difficult to remove, and the image deletion becomes more likely to occur.

A typical measure to suppress the occurrence of image deletion is drying the surface of the photosensitive member by installing a heater inside the photosensitive member or in the vicinity of the photosensitive member and raising the surface temperature of the photosensitive member. However, in the case where image formation is performed at a time when the effect of this means cannot be sufficiently obtained, for example, immediately after turning the power on, image deletion sometimes occurs. Particularly, in recent years, some apparatuses do not incorporate a heater from the viewpoint of saving energy or the like.

Therefore, an image forming apparatus, in which toner containing an abrasive such as titanium oxide is used, an abrasive portion such as an abrasive roller is disposed between a cleaning unit and a transfer member, and the surface of the photosensitive drum is polished by rubbing, has been developed to prevent image deletion. This is disclosed in, for example, Japanese Patent Laid-Open No. 2005-134776. In this image forming apparatus, the electric discharge products such as ozone and NOx present on a photosensitive drum can be removed by polishing a smooth surface of the photosensitive drum, and thus the image deletion can be prevented. In this image forming apparatus, an abrasive roller is preferably rotated in a direction following the rotational direction of the photosensitive drum at a linear speed ratio of about 1.1 to 1.2 with respect to the photosensitive drum. As a result of this, the electric discharge products can be efficiently removed while suppressing occurrence of insufficiency of polishing force and occurrence of jitter. In contrast, when the abrasive roller is rotated in the direction following the rotational direction of the photosensitive drum at a linear speed ratio smaller than 1.1 with respect to the photosensitive drum or in a direction opposite to the rotational direction of the photosensitive drum, there is a possibility that the surface layer of the roller is abraded due to increase in the torque.

In addition, an image forming apparatus in which a plurality of independent recess portions are defined on the surface of a photosensitive drum in order to suppress the occurrence of abnormal electric discharge between the photosensitive drum and a charging portion to maintain uniformity of an image has been developed. This is disclosed in Japanese Patent Laid-Open No. 2015-152640.

However, in the case where the plurality of independent recess portions are defined on the surface of the photosensitive drum to suppress the occurrence of abnormal electric discharge between the photosensitive drum and the charging portion in the image forming apparatus of Japanese Patent Laid-Open No. 2005-134776 described above, there is a possibility that the following problem occurs. That is, in the case where an abrasive roller is rotated in the direction following the rotational direction of the photosensitive drum having recess portions at a linear speed faster than the photosensitive drum, capability of removing electric discharge products is sometimes degraded. Therefore, in the case where the abrasive roller is rotated further faster in order to secure the polishing performance, image defects caused by scattering of toner and abrasion of the surface layer of the roller caused by increase in torque sometimes simultaneously occur.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an image forming apparatus includes an image bearing member comprising a plurality of recess portions on a surface thereof and configured to rotate, a charging member configured to charge the image bearing member, an exposing device configured to expose the charged image bearing member to form an electrostatic image, a developing device configured to develop the electrostatic image formed on the image bearing member by toner, a transfer member configured to form a transfer portion between the transfer member and the image bearing member and transfer a toner image formed on the image bearing member onto a transfer material at the transfer portion, and a rubbing member disposed downstream of the transfer member and upstream of the charging member in a rotational direction of the image bearing member, formed from a rotary member including a surface layer formed from an elastic body, and configured to come into contact with the image bearing member to form a rubbing nip portion between the rubbing member and the image bearing member. The recess portions each have an opening portion whose maximum length in the rotational direction is 20 μm to 120 μm. In a case where a linear speed of the image bearing member in the rubbing nip portion is S1and a linear speed of the rubbing member in the same direction as the linear speed of the image bearing member in the rubbing nip portion is S2, a relationship of a linear speed ratio of S2/S1<1.0 is satisfied.

DESCRIPTION OF THE EMBODIMENTS

An exemplary embodiment of the present invention will be described below in detail with reference toFIGS. 1 to 3. In the present exemplary embodiment, a full-color printer of a tandem type is described as an example of an image forming apparatus1. However, the present invention is not limited to the image forming apparatus1of a tandem type, and may be an image forming apparatus of another system. In addition, the present invention is not limited to a full-color printer, and may be a monochromatic printer. Further, the present invention can be implemented in various applications such as a printer, various printing machines, a copier, a facsimile machine, and a multifunctional printer.

As illustrated inFIG. 1, the image forming apparatus1includes an apparatus body10, an unillustrated sheet feeding portion, an image forming portion40, an unillustrated sheet discharge portion, and a controller11. The image forming apparatus1is capable of forming a four-color image on a recording material in accordance with an image signal from an unillustrated image reading apparatus, a host device such as a personal computer, or an external device such as a digital camera or a smartphone. To be noted, a sheet S serving as a recording material is configured to carry a toner image formed thereon, and specific examples thereof include a plain paper sheet, a synthetic resin sheet that is a substitute for a plain paper sheet, a cardboard, and a sheet for an overhead projector.

The image forming portion40is capable of forming an image on a sheet S fed from the sheet feeding portion on the basis of image information. The image forming portion40includes image forming units50y,50m,50c, and50k, unillustrated toner bottles, exposing units42y,42m,42c, and42kserving as exposing devices, an intermediate transfer unit44, a secondary transfer portion45, and a fixing portion46. To be noted, the image forming apparatus1of the present exemplary embodiment is capable of full-color printing, and the image forming units50y,50m,50c, and50khave similar configurations and are separately provided for respective four colors of yellow, magenta, cyan, and black. Therefore, inFIG. 1, each component of the four colors is denoted by a combination of the same reference sign and a color identifier added at the end thereof. In this case, y corresponds to yellow, m corresponds to magenta, c corresponds to cyan, and k corresponds to black. However, inFIG. 2and other figures and in the description, each component is sometimes denoted only by the reference sign without the color identifier.

An image forming unit50includes a photosensitive drum51serving as an image bearing member on which a toner image is to be formed, a charging roller52serving as a charging member, a developing unit20serving as a developing device, an abrasive roller54, i.e. a rubbing roller, serving as a rubbing member, a cleaning blade55, and an electricity removing device56. The image forming unit50is formed as an integral unit with a process cartridge, and is configured to be attachable to and detachable from the apparatus body10.

The photosensitive drum51is rotatable, and carries an electrostatic image to be used for image formation. The photosensitive drum51is a negatively-chargeable organic photosensitive member (OPC) having a length of 340 mm and an outer diameter of 30 mm, and is rotationally driven in an arrow direction at a process speed, which is a peripheral speed, of, for example, 300 mm/sec. As illustrated inFIG. 2, the photosensitive drum51includes an aluminum cylinder as a base body30, and a surface layer formed on the surface thereof by laminating a charge generation layer31formed from an organic material and a charge transport layer32having a thickness of about 20 μm in this order from the bottom to the top. The surface layer of the photosensitive drum51is a cured layer formed by using a curable resin as a binder resin.

The surface layer of the photosensitive drum51includes a plurality of independent specific recess portions32aand a flat portion32b, and the details thereof will be described later. That is, the photosensitive drum51rotates with the specific recess portions32aon the surface thereof. To be noted, although a cured layer formed from a curable resin is used for surface curing treatment of the photosensitive drum51in the present exemplary embodiment, the configuration is not limited to this. For example, a charge-transporting cured layer formed by causing curing polymerization of a monomer having a carbon-carbon double bond and a charge-transporting monomer having a carbon-carbon double bond by energy of heat or light may be used. Alternatively, a charge-transporting cured layer formed by causing curing polymerization of a hole-transporting compound having a chain-polymerizable functional group in the molecule by energy of an electron beam may be used.

As illustrated inFIG. 3, a rubber roller that comes into contact with the surface of the photosensitive drum51and rotates in accordance therewith is used as the charging roller52, and the charging roller52uniformly charges the surface of the photosensitive drum51. In the present exemplary embodiment, the charging roller52has a length of 330 mm in the axial direction and a diameter of 14 mm, and is formed by providing a conductive rubber layer on the outside of a core metal of stainless steel. The charging roller52is rotatably held by bearing members at both end portions of the core metal thereof, and is urged toward the photosensitive drum51by a pressing spring to be in pressure contact with the surface of the photosensitive drum51at a predetermined pressing force. As a result of this, the charging roller52rotates in accordance with the rotation of the photosensitive drum51. In this case, the peripheral speed of the charging roller52is 300 mm/sec. The charging roller52charges the surface of the photosensitive drum51at a charging nip portion between the charging roller52and the photosensitive drum51by using an electric discharge phenomenon occurring in a minute gap therebetween.

The core metal of the charging roller52is connected to a charging bias power source60, and a charging bias voltage of a predetermined condition is applied thereto from the charging bias power source60. In the present exemplary embodiment, the charging bias power source60is constituted by, for example, a direct current (DC) power source and an alternate current (AC) power source. For example, in the case where a DC bias to be applied is set to −500 V and an AC bias is set to a peak-to-peak bias that is at least double the discharge inception voltage in the environment, an image forming part of the rotating photosensitive drum51is uniformly charged to about −500V immediately after passing through the charging nip portion. To be noted, the DC bias applied during image formation is not limited to this voltage, and is appropriately set to a potential suitable for good image formation in accordance with the environment and operation history of the photosensitive drum51and the charging roller52.

The exposing unit42is a laser scanner including a semiconductor laser, and emits laser light to form an electrostatic image by exposing the charged photosensitive drum51in accordance with color-divided image information output from the controller11. That is, the exposing unit42outputs laser light modulated in correspondence with an image signal transmitted to the controller11from a host processing apparatus such as an image reading apparatus, and thus performs, at an exposing position, laser scanning exposure on the surface of the rotating photosensitive drum51that has been uniformly charged. As a result of this laser scanning exposure, the potential of a part irradiated with the laser light on the surface of the photosensitive drum51decreases, and an electrostatic latent image corresponding to the image information is sequentially formed on the surface of the rotating photosensitive drum51.

The developing unit, serving as the developing device,20includes a developer container that accommodates developer, and a developing sleeve24. In the present exemplary embodiment, the length of the developing sleeve24in the axial direction is 325 mm. The developing sleeve24performs development by carrying magnetic brushes formed from two-component developer including toner and carrier and bringing the magnetic brushes into contact with the photosensitive drum51at a developing nip portion. The developing sleeve24is connected to a developing bias power source61that applies a predetermined developing bias, and the electrostatic image formed on the photosensitive drum51is developed with toner as a result of the developing bias applied thereto. In the present exemplary embodiment, the developing bias is an oscillating voltage in which a direct current voltage and an alternate current voltage are superimposed. For example, the developing bias is an oscillating voltage in which an alternate current voltage of a rectangular wave having a frequency of 8.0 kHz and a peak-to-peak voltage of 1.8 kV is superimposed. The direct current voltage is appropriately set such that an appropriate fog-removing potential is achieved with respect to the potential of the photosensitive drum51in the developing nip portion.

As illustrated inFIG. 1, the toner image developed on the photosensitive drum51is transferred onto an intermediate transfer belt44bof the intermediate transfer unit44through primary transfer. The intermediate transfer belt44bserves as a transfer material. The intermediate transfer unit44includes a plurality of rollers including a driving roller44a, a driven roller44d, and primary transfer rollers47y,47m,47c, and47k, and the intermediate transfer belt44bthat is looped over these rollers and carries a toner image. The primary transfer rollers47y,47m,47c, and47kserving as transfer members are respectively disposed opposite to the photosensitive drums51y,51m,51c, and51k, and abut the intermediate transfer belt44b. The primary transfer rollers47are connected to a primary transfer bias power source62illustrated inFIG. 3that applies a primary transfer bias.

The intermediate transfer belt44bcomes into contact with the photosensitive drums51and forms primary transfer portions between the intermediate transfer belt44band the photosensitive drums51, and thus toner images formed on the photosensitive drums51are transferred at the primary transfer portions through primary transfer as a result of the primary transfer bias being applied. By applying the primary transfer bias of a positive polarity to the intermediate transfer belt44bvia the primary transfer rollers47, respective toner images having a negative polarity on the photosensitive drums51are sequentially transferred onto the intermediate transfer belt44bso as to be superimposed on one another. That is, the primary transfer rollers47form primary transfer portions between the primary transfer rollers47and the photosensitive drums51, and toner images formed on the photosensitive drums51are transferred onto the intermediate transfer belt44bthrough primary transfer at the primary transfer portions.

The secondary transfer portion45includes a secondary transfer inner roller45aand a secondary transfer outer roller45b. The secondary transfer outer roller45bis connected to a secondary transfer bias power source63illustrated inFIG. 3that applies a secondary transfer bias. By applying a secondary transfer bias of a positive polarity to the secondary transfer outer roller45b, a full-color toner image formed on the intermediate transfer belt44bis transferred onto the sheet S. The secondary transfer outer roller45babuts the intermediate transfer belt44band forms a secondary transfer portion45between the secondary transfer outer roller45band the intermediate transfer belt44b, and the toner image transferred onto the intermediate transfer belt44bthrough primary transfer is transferred onto the sheet S at the secondary transfer portion45through secondary transfer by applying the secondary transfer bias.

The fixing portion46includes a fixing roller46aand a pressurizing roller46b. As a result of the sheet S being nipped and conveyed between the fixing roller46aand the pressurizing roller46b, the toner image transferred onto the sheet S is heated, pressurized, and thus fixed to the sheet S. The sheet discharge portion feeds the sheet S conveyed through a discharge path after the fixing, and, for example, discharges the sheet S through a discharge port and stacks the sheet S on a discharge tray.

Meanwhile, as illustrated inFIG. 3, the abrasive roller54is disposed downstream of the primary transfer roller47and upstream of the charging roller52in the rotational direction of the photosensitive drum51. Therefore, the surface of the photosensitive drum51after primary transfer is cleaned by the abrasive roller54. Details of the abrasive roller54will be described later.

Transfer residual toner remaining on the surface of the photosensitive drum51in a small amount after the cleaning by the abrasive roller54is removed from the surface of the photosensitive drum51by the cleaning blade55. The cleaning blade55in the present exemplary embodiment employs a counter blade system that is formed from urethane rubber and has a flat-plate shape having a length of 330 mm in the axial direction and a free blade length of 8 mm. The cleaning blade55is pressed against the photosensitive drum51at a linear pressure of 30 gf/cm. After the toner removal by the cleaning blade55, electricity is removed from the surface of the photosensitive drum51by the electricity removing device56, and the surface of the photosensitive drum51is charged again by the charging roller52.

The controller11is constituted by a computer, and includes, for example, a central processing unit (CPU)12, a read-only memory (ROM)13, a random access memory (RAM)14, and an input/output circuit15serving as an interface (I/F). The ROM13stores a program for controlling each component, the RAM14temporarily stores data, and the input/output circuit15inputs and outputs a signal from and to the outside. The CPU12is a microprocessor that performs overall control of the image forming apparatus1, and is a main component of a system controller. The CPU12is connected to the sheet feeding portion and the image forming portion40via the input/output circuit15, and thus communicates a signal with each component and controls the operation thereof. The ROM13stores an image formation control sequence or the like for forming an image on the sheet S.

The controller11is connected to the charging bias power source60, the developing bias power source61, the primary transfer bias power source62, the secondary transfer bias power source63, and driving motors for various rollers. Here, it is assumed that the linear speed of the photosensitive drum51in the abrasive nip portion N, i.e. a rubbing nip portion, is S1and the linear speed of the abrasive roller54in the same direction as the linear speed of the photosensitive drum51is S2. In this case, the controller11controls the rotational speed of the photosensitive drum51and the abrasive roller54such that a linear speed ratio S2/S1satisfies a relationship of S2/S1<1.0. In addition, the controller11controls the linear speed ratio S2/S1so as to satisfy a relationship of −1.0≤S2/S1.

Next, an image forming operation in the image forming apparatus1thus configured will be described.

When the image forming operation is started, the photosensitive drum51rotates and the surface thereof is charged by the charging roller52. Then, laser light is emitted from the exposing unit42, i.e. the exposing device, to the photosensitive drum51on the basis of image information, and thus an electrostatic latent image is formed on the surface of the photosensitive drum51. This electrostatic latent image is developed and visualized as a toner image by toner attaching thereto by the developing unit20, and the toner image is transferred onto the intermediate transfer belt44b.

Meanwhile, the sheet S is supplied in parallel with such a formation operation of toner image, and the sheet S is conveyed to the secondary transfer portion45through a conveyance path at a timing matching conveyance of the toner image on the intermediate transfer belt44b. Further, the toner image is transferred from the intermediate transfer belt44bonto the sheet S, and the sheet S is conveyed to the fixing portion46. Then, the unfixed toner image is heated and pressurized in the fixing portion46to be fixed to the surface of the sheet S, and the sheet S is discharged from the apparatus body10.

Next, the surface shape of the photosensitive drum51in the image forming apparatus1of the present exemplary embodiment will be described. As illustrated inFIG. 2, the surface of the photosensitive drum51includes a specific recess portion32aand a flat portion32b. In the present exemplary embodiment, the specific recess portion32ahas a circular shape as viewed in a depth direction. However, the shape of the specific recess portion32ais not limited to a circular shape, and may be a polygonal shape such as a triangular shape.

Here, definition of the specific recess portion32aand the flat portion32bin a square region of 500 μm×500 μm in the surface of the photosensitive drum51will be described below. The specific recess portion32aand the flat portion32bon the surface of the photosensitive drum51can be observed by using a microscope such as a laser microscope, an optical microscope, an electron microscope, or an atomic force microscope. First, the surface of the photosensitive drum51is observed in a magnified view by a microscope or the like. In the case where the surface of the photosensitive drum51in the rotational direction is a curved surface, a sectional profile of the curved surface is extracted, and the sectional profile is fitted by a curved line. The sectional profile is corrected such that the curved line becomes a straight line, and a surface obtained by extending the obtained straight line in the longitudinal direction of the photosensitive drum51is set as a standard surface.

Then, a region within ±0.2 μm from the obtained standard surface in terms of height is regarded as the flat portion32bin the square region of 500 μm×500 μm. A portion positioned below the flat portion32bis regarded as a recess portion, and the maximum distance from the flat portion32bto the bottom surface of the recess portion is regarded as the depth of the recess portion. In addition, a section taken along the flat portion32b, that is, a plane having a height level of the flat portion32bis regarded as an opening portion of the recess portion, and the length of the longest line segment among line segments included in the opening portion is regarded as an opening portion maximum diameter D1of the recess portion. Among recess portions included in the square region of 500 μm×500 μm, recess portions whose depths obtained as described above are within a range of 0.5 μm to 6.0 μm and whose opening portion maximum diameters are within a range of 20 μm to 120 μm will be referred to as specific recess portions32ain the square region of 500 μm×500 μm. That is, the opening portion of each of the specific recess portions32ahas a maximum length of 20 μm to 120 μm in the rotational direction.

In a region including the specific recess portions32a, the specific recess portions32aare defined in a predetermined area ratio with respect to the flat portion32bthat occupies most part of the surface of the photosensitive drum51. Due to how the specific recess portions32aare defined, projections32chaving a rim shape, which are neither recess portions nor flat portions, are formed around the specific recess portions32a. The specific recess portions32aof the present exemplary embodiment include two kinds of recess portions including a plurality of first recess portions having a depth of 5 μm serving as a first depth and a plurality of second recess portions having a depth of 2 μm serving as a second depth, and these are alternately arranged.

The specific recess portions32aare provided in the surface of the photosensitive drum51so as to occupy the following area. The square region of 500 μm×500 μm whose one side is parallel to the rotational direction of the photosensitive drum51is disposed in an arbitrary position in the surface of the photosensitive drum51. In this case, the specific recess portions32aare provided such that the area of the specific recess portions32ain the square region of 500 μm×500 μm is 7500 μm2to 88000 μm2. That is, the specific recess portions32aare provided such that the area ratio of the total area of the opening portions of the plurality of specific recess portions32awith respect to the surface area of an image forming region of the photosensitive drum51is 3.00% to 3.52%. In addition, the flat portion32bis provided in the surface of the photosensitive drum51so as to occupy the following area. The square region of 500 μm×500 μm whose one side is parallel to the rotational direction of the photosensitive drum51is disposed in an arbitrary position in the surface of the photosensitive drum51. In this case, the flat portion32bis provided such that the area of the flat portion32bin the square region of 500 μm×500 μm is 81000 m2to 240000 m2.

Next, the abrasive roller54in the image forming apparatus1of the present exemplary embodiment will be described. As illustrated inFIG. 3, in the present exemplary embodiment, the abrasive roller54has a length of 330 mm in the axial direction, and is formed by providing, for example, an elastic foam layer54bserving as a surface layer formed from an elastic foam body as an elastic body on the outside of a core metal54aof stainless steel. The elastic foam layer54bis an elastic layer having a foam structure formed from a rubber material or the like. That is, the abrasive roller54is constituted by a rotary member including the elastic foam layer54b, abuts the photosensitive drum51to form the abrasive nip portion N, i.e. the rubbing nip portion, between the abrasive roller54and the photosensitive drum51, and polishes the photosensitive drum51at the abrasive nip portion N by relative rotation. Although the thickness of the elastic foam layer54bis not limited, for example, the overall thickness thereof is about 4 mm to 10 mm. Although physical properties of the elastic foam layer54bare not limited, for example, the average cell diameter thereof is about 100 μm to 1000 μm, the number of air bubble cells thereof is about 10 to 200 per inch, the air permeability thereof is about 0.5 to 10.0 L/min, and the density thereof is about 0.08 to 0.20 g/cm3. To be noted, cells are exposed on the surface of the elastic foam layer54b, and part of these projects as projection portions54ccapable of engaging with the specific recess portions32aof the photosensitive drum51as illustrated inFIG. 5A. In addition, the elastic body is not limited to an elastic foam body, and may be an elastic body of another material.

When obtaining the average cell diameter of the elastic foam layer54b, a region of about 20 mm2in the surface of the elastic foam layer54bis observed with an electron microscope or the like, and the maximum length of an opening portion in each cell present in the observed field of view is measured. The average cell diameter can be obtained as an average length obtained by arithmetically averaging the measured maximum length. The average cell diameter of the cells can be adjusted by adjusting the kind and content of a foaming agent contained in a silicone rubber foam composition that forms the elastic foam layer54b, the content of a reaction control agent contained in the silicone rubber foam composition, curing conditions of the silicone rubber foam composition, or the like.

As the rubber material for the elastic foam layer54b, for example, general purpose rubbers such as butadiene rubber, isoprene rubber, chloroprene rubber, and styrene-butadiene rubber, and rubbers such as acrylonitrile, silicone rubber, and polyurethane rubber can be used alone or in combination of two or more kinds. Polyol serving as a raw material for polyurethane rubber is not particularly limited, and polyol to be used can be appropriately selected from various polyols that are conventionally known as raw materials for polyurethane foam. For example, the polyol to be used can be selected from known polyols such as polyether polyol, polyester polyol, and polymer polyol, which are typically used for producing soft polyurethane foams, and these can be used alone or in combination of two or more kinds. To be noted, among the polyols described above, polyether polyol is preferably used for producing a highly-elastic soft polyurethane foam having excellent durability against humidity and heat.

As the polyol, prepolymer that has been polymerized with polyisocyanate in advance may be used. The polyisocyanate is not particularly limited, and polyisocyanate to be used can be appropriately selected from various polyisocyanates that are conventionally known as raw materials for polyurethane foam. For example, the following compounds can be used alone or in combination of two or more kinds: 2,4- and 2,6-tolylene diisocyanate: TDI; tolidine diisocyanate: TODI; naphtylene diisocyanate: NDI; xylylene diisocyanate: XDI; 4,4′-diphenylmethane diisocyanate: MDI; carbodiimide-modified MDI; polymethylene polyphenyl polyisocyanate; and polymeric polyisocyanate. To be noted, as the polyisocyanate, isocyanate-terminated prepolymer obtained by reacting polyisocyanate with one or more kinds of known active hydrogen compounds can be also used.

In addition, the elastic foam layer54bof the abrasive roller54preferably has an ASKER FP hardness of 30 to 100. Here, an ASKER FP hardness is a hardness detected by a predetermined durometer, that is, an ASKER rubber durometer FP type manufactured by Kobunshi Keiki Co., Ltd.

Next, an operation of the image forming apparatus1of the present exemplary embodiment in the abrasive nip portion N between the abrasive roller54and the photosensitive drum51will be described. First, as an index of measuring speeds of the abrasive roller54and the photosensitive drum51at the abrasive nip portion N, observation is performed by using a high-speed video camera, and speed difference between the abrasive roller54and the photosensitive drum51is exponentialized by using video analysis. A high-speed camera MEMRECAN_GX-8F manufactured by nac Image Technology Inc. is used for the observation. The frame rate of the high-speed camera is 10 KFPS, and the resolution thereof is 640×480 pixels. A semi telephoto lens of 105 mm/f2.8 manufactured by Nikon Corporation is used for the lens. In the observation of behavior of the abrasive roller54, a cylindrical tube of transparent glass with a transparent conductive film of an indium tin oxide (ITO film) formed thereon is used as the base body30of the photosensitive drum51. In the present exemplary embodiment, the photosensitive drum51is formed by applying three layers of an undercoat layer, the charge generation layer31, and the charge transport layer32on the base body30in this order from the bottom to the top. When calculating the speeds of the abrasive roller54and the photosensitive drum51, video analysis software, that is, motion analysis software TEMA available from Photron Limited, is used.

Hereinafter, it is assumed that the linear speed of the photosensitive drum51is S1and the linear speed of the abrasive roller54is S2. First, a case where the abrasive roller54is rotated quickly in a direction following the photosensitive drum51as illustrated inFIGS. 4A and 4B, that is, a case where 1<S2/S1holds will be described. In this case, when the abrasive roller54enters the abrasive nip portion N, part of the projection portions54cof the cells of the abrasive roller54engage with the specific recess portions32awhen abutting the photosensitive drum51. The elastic foam layer54bis squashed in the peripheral direction as a result of this engagement and friction at the abrasive nip portion N, and thus the abrasive roller54rotates in accordance with the photosensitive drum51at a speed lower than the linear speed S2that is the aimed linear speed. Therefore, on the upstream side of the photosensitive drum51at the abrasive nip portion N in the rotational direction, the speed difference becomes smaller, and the abrasive roller54becomes less likely to rub the surface of the photosensitive drum51. To be noted, in the case where S1=S2holds, that is, where S2/S1=1 holds, there is no speed difference between the photosensitive drum51and the abrasive roller54, and thus the abrasive roller54becomes less likely to rub the surface of the photosensitive drum51.

The linear speed S1of the photosensitive drum51and the linear speed S2of the abrasive roller54when passing through the abrasive nip portion N have a relationship illustrated inFIG. 4C. As illustrated inFIG. 4C, although the linear speed S1of the photosensitive drum51is stable, the linear speed S2of the abrasive roller54unstably following the photosensitive drum51is unstable, thus the abrasive nip portion N becomes smaller, and it is expected that the capability of removing the electric discharge products is degraded.

Next, a case where the abrasive roller54is rotated slowly in the direction following the photosensitive drum51as illustrated inFIGS. 5A and 5B, that is, a case where 0<S2/S1<1 holds, will be described. In this case, when the abrasive roller54enters the abrasive nip portion N, part of the projection portions54cof the cells of the abrasive roller54engage with the specific recess portions32awhen abutting the photosensitive drum51. As a result of this engagement and friction at the abrasive nip portion N, the elastic foam layer54bis pulled downstream in the peripheral direction, and thus the abrasive roller54rotates in accordance with the photosensitive drum51at a speed higher than the linear speed S2that is the aimed linear speed. Therefore, the speed difference is maintained constant in the entirety of the abrasive nip portion N, and the abrasive roller54is likely to rub the surface of the photosensitive drum51. To be noted, the same behavior is exhibited in the case where the abrasive roller54is stopped, that is, where S2/S1=0 holds.

The linear speed S1of the photosensitive drum51and the linear speed S2of the abrasive roller54when passing through the abrasive nip portion N have a relationship illustrated inFIG. 5C. As illustrated inFIG. 5C, similarly to the linear speed S1of the photosensitive drum51that is stable, the linear speed S2of the abrasive roller54is also stable, the abrasive nip portion N becomes larger, and thus it is expected that the capability of removing the electric discharge products improves.

In addition, a case where the abrasive roller54is rotated in an opposite direction with respect to the photosensitive drum51as illustrated inFIGS. 6A and 6B, that is, a case where S2/S1<0 holds will be described. Here, since S1and S2are opposite in a positive/negative relationship, the linear speed S2of the abrasive roller54and the linear speed S1of the photosensitive drum51have a relationship of S1>S2. Also in this case, when the abrasive roller54enters the abrasive nip portion N, part of the projection portions54cof the cells of the abrasive roller54engage with the specific recess portions32awhen abutting the photosensitive drum51. As a result of this engagement and friction at the abrasive nip portion N, the elastic foam layer54bis pulled downstream in the peripheral direction, and thus the abrasive roller54rotates in accordance with the photosensitive drum51at a speed higher than the linear speed S2that is the aimed linear speed. Therefore, the speed difference is maintained constant in the entirety of the abrasive nip portion N, and the abrasive roller54is likely to rub the surface of the photosensitive drum51.

When measuring the nip width in the abrasive nip portion N, the nip width is exponentialized by measuring the width in which the abrasive roller54is in contact with the photosensitive drum51based on an image captured by a high-speed camera.FIG. 7Aillustrates a relationship between the linear speed S2/S1and the nip width obtained in this manner together with presence/absence of the specific recess portions32a. As illustrated inFIG. 7A, in the case where the abrasive roller54is rotated quickly in the direction following the photosensitive drum51, that is, where 1<S2/S1holds, cells of the abrasive roller54are caught in the specific recess portions32aof the photosensitive drum51when the photosensitive drum51includes the specific recess portions32a. In addition, since the cells of the abrasive roller54are squashed due to friction as a result of the linear speed S2of the abrasive roller54being higher than the linear speed S1of the photosensitive drum51, the nip width is narrower than in the case of a photosensitive drum not including the specific recess portions32a.

In addition, in the case where the abrasive roller54is rotated slowly or in an opposite direction with respect to the photosensitive drum51, that is, where S2/S1<1 holds, the cells of the abrasive roller54are caught in the specific recess portions32aof the photosensitive drum51when the photosensitive drum51includes the specific recess portions32a. In addition to this, since the cells of the abrasive roller54are pulled downstream due to friction as a result of the linear speed S1of the photosensitive drum51being higher than the linear speed S2of the abrasive roller54, the nip width is wider than in the case of a photosensitive drum not including the specific recess portions32a. Particularly, the nip width is wider in a range where −1<S2/S1<1 holds.

Further, a relationship between the linear speed ratio S2/S1and polishing performance is obtained on the basis of the speed difference between the abrasive roller54and the photosensitive drum51, the nip width, and further the presence/absence of the specific recess portions32aof the photosensitive drum51, and results thereof are shown inFIG. 7B. The polishing performance is defined as total rubbing distance. As illustrated inFIG. 7B, it is confirmed that the capability of removing the electric discharge products can be improved in the case where the abrasive roller54is rotated slowly or in an opposite direction with respect to the photosensitive drum51, that is, where S2/S1<1 holds.

In the present exemplary embodiment, since the photosensitive drum51including the specific recess portions32aon the surface thereof is used and the abrasive roller54is rotated in the opposite direction or slowly in the direction following the photosensitive drum51, the effect of suppressing the image deletion can be improved. In addition, in the case where the abrasive roller54is rotated in the opposite direction or slowly in the direction following the photosensitive drum51, the nip width increases, the speed difference between the abrasive roller54and the photosensitive drum51is maintained constant, and thus the polishing performance is improved when the photosensitive drum51includes the specific recess portions32aon the surface thereof. However, in the case where the abrasive roller54is rotated quickly in the direction following the photosensitive drum51, the nip width decreases, the speed difference between the photosensitive drum51and the abrasive roller54decreases, and thus the polishing performance is degraded. To effectively suppress the image deletion, it is preferable that the linear speed ratio S2/S1is smaller than 1.0, and it is more preferable that the linear speed ratio S2/S1is equal to or larger than −1.0 and smaller than 1.0.

As described above, according to the image forming apparatus1of the present exemplary embodiment, the linear speed ratio S2/S1of the linear speed S2of the abrasive roller54and the linear speed S1of the photosensitive drum51is set to a value smaller than 1.0. Therefore, the linear speed S2of the abrasive roller54is in the opposite direction to the liner speed S1of the photosensitive drum51or low in the same direction as the linear speed S1. Therefore, the capability of removing the electric discharge products by the abrasive roller54being degraded as in the case where the abrasive roller54is rotated in the direction following the rotational direction of the photosensitive drum51including the specific recess portions32aat a linear speed higher than that of the photosensitive drum51can be suppressed. Hence, the capability of removing the electric discharge products by the abrasive roller54being degraded while using the photosensitive drum51including the specific recess portions32aon the surface thereof can be suppressed. In addition, according to the image forming apparatus1of the present exemplary embodiment, since the linear speed ratio S2/S1is set to −1.0 or larger, the capability of removing the electric discharge products by the abrasive roller54can be improved more.

In addition, according to the image forming apparatus1of the present exemplary embodiment, since the opening portion maximum diameter of the specific recess portions32ais set to 20 μm to 120 μm, the capability of removing the electric discharge products by the abrasive roller54being degraded can be effectively suppressed. To be noted, by setting the opening portion maximum diameter of the specific recess portions32ato 20 μm to 100 μm, the capability of removing the electric discharge products by the abrasive roller54being degraded can be further effectively suppressed.

In addition, according to the image forming apparatus1of the present exemplary embodiment, since the ASKER FP hardness of the elastic foam layer54bis set to 30 to 100, the capability of removing the electric discharge products by the abrasive roller54being degraded can be more effectively suppressed. To be noted, by setting the ASKER FP hardness of the elastic foam layer54bto 40 to 90, the capability of removing the electric discharge products by the abrasive roller54being degraded can be further effectively suppressed.

EXAMPLES

Printing was performed on 1000 sheets by using the image forming apparatus1of the present exemplary embodiment described above in an environment of a room temperature of 30° C. and a humidity of 80%, and then the image forming apparatus1was left to stand for 12 hours in the same environment. Thereafter, image formation was performed, and occurrence conditions of image deletion were evaluated. Here, evaluation was performed while changing the ASKER FP hardness of the elastic foam layer54band the opening portion maximum diameter of the specific recess portions32afor each linear speed ratio S2/S1of the abrasive roller54. To be noted, the linear speed ratio S2/S1was set to be smaller than 1.0. The results thereof are shown in Tables 1 to 5. Each table corresponds to a different value of linear speed ratio.

In the examples described above, as shown in Tables 1 to 5, image deletion occurred when the ASKER FP hardness was 20. This is considered to be because tear strength generally also decreases when hardness decreases, and the polishing force decreased as a result of occurrence of wear of the surface layer of the abrasive roller54. Further, when the ASKER FP hardness was 110, drum scratches were generated regardless of the linear speed ratio and the opening portion maximum diameter of the specific recess portions32a. This is considered to be because the hardness of the abrasive roller54was high and the surface layer of the photosensitive drum51was abraded.

When the opening portion maximum diameter of the specific recess portions32awas 10 μm and the ASKER FP hardness was 20 to 100, image deletion and abrasion of the surface layer of the sponge both occurred. This is considered to be because the diameter of the specific recess portions32awas small, thus the torque of the drum increased, and wear of the abrasive roller54was promoted more. In addition, when the opening portion maximum diameter of the specific recess portions32awas 130 μm, drum scratches were generated. This is considered to be because the specific recess portions32awere wide, thus contact pressure between the surface layer of the photosensitive drum51and the cleaning blade55increased, and therefore wear of the surface layer of the photosensitive drum51was promoted to generate scratches.

Therefore, in the examples described above, good results were obtained when the opening portion maximum diameter of the specific recess portions32awas in the range of 20 μm to 120 μm and the ASKER FP hardness of the elastic foam layer54bwas in the range of 30 to 100. That is, the electric discharge products on the surface of the photosensitive drum51were successfully removed, and a good image free from charging failure caused by image deletion was obtained. Particularly, more effective results were obtained when the opening portion maximum diameter of the specific recess portions32awas in the range of 20 μm to 100 μm and the ASKER FP hardness of the elastic foam layer54bwas in the range of 40 to 90.

Comparative Examples

In contrast with the examples described above, occurrence conditions of image deletion were evaluated while setting the linear speed ratio S2/S1to 1.0 or larger and using the same values for the other conditions. The results thereof are shown in Tables 6 and 7. Tables 6 and 7 correspond to different linear speed ratios.

In the comparative examples described above, as shown in Table 6, in the case where the linear speed ratio was 1.0, image deletion occurred regardless of the ASKER FP hardness and the opening portion maximum diameter of the specific recess portions32aindependently provided on the surface of the photosensitive drum51. This is considered to be because the abrasive roller54and the photosensitive drum51rotated at the same speed and the surface of the photosensitive drum51was not rubbed.

As shown in Table 7, in the case where the linear speed ratio exceeded 1.0, sponge abrasion occurred when the ASKER FP hardness was 20. This is considered to be because tear strength generally also decreases when hardness decreases, and thus the polishing force decreased as a result of occurrence of wear of the surface layer of the abrasive roller54. Similarly, in the case where the linear speed ratio exceeded 1.0, toner scattering occurred when the ASKER FP hardness was 30 to 100. This is considered to be because the linear speed of the abrasive roller54was high, thus toner on the photosensitive drum51was blown off to be scattered, and thus an image of a good quality was not obtained. Similarly, in the case where the linear speed ratio exceeded 1.0, drum scratches were generated when the ASKER FP hardness was 110. This is considered to be because the hardness of the abrasive roller54was high, and thus the surface layer of the photosensitive drum51was abraded.

As described above, it was confirmed that it is difficult to output an image of a good quality in the case where the linear speed ratio S2/S1of the abrasive roller54is 1.0 or larger.

To be noted, although a case where the specific recess portions32aof the image forming apparatus1of the exemplary embodiment described above are a plurality of independent recess portions has been described, the configuration is not limited to this. For example, the recess portions may have long groove shapes extending along the axial direction of the photosensitive drum51, and also in this case, by setting the maximum length of the opening portion in the rotational direction to, for example, 20 μm to 120 μm, an effect equivalent to the case of employing the specific recess portions32acan be obtained.

In addition, although a case where an image forming apparatus of an intermediate transfer system that forms an image on a recording material by secondary transfer from the intermediate transfer belt44bis used as the image forming apparatus1of the exemplary embodiment described above has been described, the configuration is not limited to this. For example, the present invention may be applied to an image forming apparatus of a system that directly transfers a toner image from a photosensitive drum onto the recording material.

According to the present invention, the maximum length in the rotational direction of an opening portion of the recess portion is set to 20 μm to 120 μm and the linear speed ratio S2/S1of the linear speed S2of the rubbing member and the linear speed S1of the image bearing member is set to a value smaller than 1.0. Therefore, the linear speed S2of the rubbing member is in the opposite direction to the liner speed S1of the image bearing member or low in the same direction as the linear speed S1. Therefore, the capability of removing the electric discharge products by the rubbing member being degraded as in the case where the rubbing member is rotated in the direction following the rotational direction of the image bearing member including the recess portions at a linear speed higher than that of the image bearing member can be suppressed. Hence, the capability of removing the electric discharge products by the rubbing member being degraded while using the image bearing member including the recess portions on the surface thereof can be suppressed.

INDUSTRIAL APPLICABILITY

The present invention can be applied to image forming apparatuses such as copiers and laser beam printers that employ an electrophotographic system or an electrostatic recording system, and is particularly preferably used for an image forming apparatus that includes a photosensitive drum including recess portions on the surface thereof.