Abstract:
The invention relates to an electrophotographic photoreceptor in which protrusions from the surface of the photoreceptor are partly or completely covered by a resin. The effect of such resin covering is to reduce toner deposition, thereby reducing or eliminating spotting defects in images formed using the electrophotographic photoreceptor. The invention further relates to an image-forming apparatus incorporating such an electrophotographic photoreceptor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2007-116095, filed Apr. 25, 2007, entitled “ELECTROPHOTOGRAPHIC PHOTORECEPTOR, METHOD FOR MANUFACTURING THE SAME, AND IMAGE-FORMING APPARATUS” The contents of this application are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic photoreceptor, a method for manufacturing the electrophotographic photoreceptor, and an image-forming apparatus that includes the electrophotographic photoreceptor. 
     2. Description of the Related Art 
     The production of electrophotographic photoreceptors that include an amorphous silicon (hereinafter referred to as “a-Si”) photosensitive layer is increasing year by year because of their high abrasion resistance, high heat resistance, high photosensitivity, and nonpolluting characteristics. 
     One of such electrophotographic photoreceptors includes an a-Si photosensitive layer that is formed on a cylindrical aluminum alloy substrate by a thin-film forming method (for example, a glow discharge decomposition method). This a-Si photosensitive layer includes an a-Si photoconductive layer and a surface layer formed thereon. The a-Si photosensitive layer may include a carrier injection preventing layer between the cylindrical substrate and the photoconductive layer. 
     However, in electrophotographic photoreceptors having such a structure, the photosensitive layer may have a protrusion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of an image-forming apparatus according to an embodiment of the present invention; 
         FIG. 2A  is a schematic cross-sectional view of an electrophotographic photoreceptor according to an embodiment of the present invention, and  FIG. 2B  is an enlarged view of a principal part thereof; 
         FIG. 3  is a schematic cross-sectional view of a plasma chemical vapor deposition (CVD) apparatus for forming a photosensitive layer in the electrophotographic photoreceptor illustrated in  FIG. 2A ; 
         FIG. 4  is a schematic cross-sectional view of a principal part of an electrophotographic photoreceptor to illustrate a protrusion disposed on a photosensitive layer; 
         FIGS. 5A to 5C  are schematic cross-sectional views of a principal part of an electrophotographic photoreceptor to illustrate a resin portion that partly ( 5 A,  5 B) or entirely ( 5 C) covers a protrusion disposed on a photosensitive layer; 
         FIG. 6  is a schematic view of an apparatus for forming a resin portion; 
         FIG. 7  is a schematic cross-sectional view illustrating how a protrusion disposed on a photosensitive layer scrapes resin off a resin film; 
         FIGS. 8A to 8C  are schematic cross-sectional views of a principal part of an electrophotographic photoreceptor to illustrate a resin portion that partly ( 8 A,  8 B) or entirely ( 8 C) covers a protrusion disposed on a photosensitive layer; 
         FIG. 9  is a schematic cross-sectional view of a principal part of a lapping sheet for grinding a resin portion; and 
         FIG. 10  is a schematic view of another apparatus for forming a resin portion upon a electrophotographic photoreceptor. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to an aspect of the present invention, an electrophotographic photoreceptor includes a substrate, a photosensitive layer and a resin portion. The photosensitive layer is disposed on the substrate and has a protrusion thereon. The resin portion partly or entirely covers the protrusion. 
     According to another aspect of the present invention, a method for manufacturing an electrophotographic photoreceptor includes the steps of: forming a photosensitive layer on the outer surface of a cylindrical substrate; and partly or entirely covering a protrusion disposed on the photosensitive layer with a resin. 
     According to further aspect of the present invention, an image-forming apparatus includes the electrophotographic photoreceptor. 
     Some embodiments of the invention will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings. 
     As illustrated in  FIG. 1 , an image-forming apparatus  1  utilizes a Carlson process as an image-forming method. The image-forming apparatus  1  includes an electrophotographic photoreceptor  2 , a charger  41 , an exposure unit  42 , a developing unit  43 , a transfer unit  44 , a fixing unit  45 , a cleaning unit  46 , and a static eliminator  47 . 
     The charger  41  is of a non-contact type. The charger  41  includes a housing  41 B having an opening  41 A facing the electrophotographic photoreceptor  2 , the grid electrode  41 C disposed at the opening  41 A, and a charging wire  41 D disposed inside the housing  41 B. 
     The charger may be a contact charger in place of the non-contact charger  41 . The contact charger may have a charging roller. For example, the charging roller includes an electroconductive rubber roller and a metal shaft disposed at the center of the electroconductive rubber roller. A direct-current (DC) voltage or a voltage of DC and alternating current (AC) is applied to the metal shaft to charge the electrophotographic photoreceptor  2  directly. 
     The exposure unit  42  can emit light having a particular wavelength (for example, in the range of 650 to 780 nm) and forms an electrostatic latent image on the electrophotographic photoreceptor  2 . The exposure unit  42  can irradiate the electrophotographic photoreceptor  2  with light corresponding to a picture signal to reduce the electric potential at the irradiated portion, thus forming an electrostatic latent image as a voltage contrast. The exposure unit  42  may include a light-emitting diode (LED) head including a plurality of LED devices (wavelength: about 680 nm). Alternatively, in place of the LED head, the exposure unit  42  may include an optical system that includes a laser beam and a polygonal mirror, or an optical system that includes a lens and a mirror each transmitting light reflected from an object to be printed. 
     The developing unit  43  develops an electrostatic latent image of the electrophotographic photoreceptor  2  to form a toner image. The developing unit  43  includes a developing roller  43 A for retaining a developing agent (toner), and a wheel (not shown) for maintaining a substantially constant gap between the developing unit  43  and the electrophotographic photoreceptor  2 . The developing agent composes a toner image that is formed on the electrophotographic photoreceptor  2 . The developing agent may be a one-component system containing a toner or a two-component system containing a toner and a carrier. 
     The developing roller  43 A conveys a developing agent to the surface (particularly to an area to be developed) of the electrophotographic photoreceptor  2 . The developing roller  43 A is charged at a predetermined electric potential with a predetermined polarity upon the application of a direct-current voltage or an alternating voltage. 
     In the developing unit  43 , the developing agent conveyed by the developing roller  43 A adheres to an area to be developed on the electrophotographic photoreceptor  2  by the electrostatic attraction force between the developing agent and an electrostatic latent image, thus visualizing the latent image. When a toner image is formed by normal development, the charge polarity of the toner image is opposite to the charge polarity of the surface of the electrophotographic photoreceptor  2 . When a toner image is formed by reversal development, the charge polarity of the toner image is the same as the charge polarity of the surface of the electrophotographic photoreceptor  2 . 
     The transfer unit  44  transfers a toner image formed on the electrophotographic photoreceptor  2  to a recording medium P supplied to a transfer area between the electrophotographic photoreceptor  2  and the transfer unit  44 . The transfer unit  44  includes a transfer charger  44 A and a detach charger  44 B. In the transfer unit  44 , the back (non-recording surface) of the recording medium P is charged oppositely to the toner image by the transfer charger  44 A. The electrostatic attraction force between the charged electricity and the toner image allows the toner image to be transferred to the recording medium P. In the transfer unit  44 , synchronously with the transfer of the toner image, the back of the recording medium P is charged by an alternating current by the detach charger  44 B. Consequently, the recording medium P is immediately separated from the surface of the electrophotographic photoreceptor  2 . 
     The transfer unit  44  may be a transfer roller, which is disposed facing to the electrophotographic photoreceptor  2  with a minute gap (typically 0.5 mm or less) therebetween. The transfer roller is designed to apply a transfer voltage to the recording medium P, for example, with a direct-current power source to attract a toner image formed on the electrophotographic photoreceptor  2  to the recording medium P. The use of the transfer roller can eliminate a detach apparatus, such as the detach charger  44 B. 
     The fixing unit  45  includes a pair of fixing rollers  45 A and  45 B, and fixes a transferred toner image on the recording medium P. The fixing rollers  45 A and  45 B may be a metal roller coated with Teflon (registered trademark). The fixing unit  45  fixes a toner image, for example, by heating the recording medium P and applying pressure on the recording medium P when the recording medium P passes between the pair of fixing rollers  45 A and  45 B. 
     The cleaning unit  46  includes a cleaning blade  46 A, a spring  46 B, and a case  46 C, and removes a developing agent that remains on the electrophotographic photoreceptor  2 . The cleaning blade  46 A scrapes a residual toner off the surface of a surface layer  29  of the electrophotographic photoreceptor  2 . The cleaning blade  46 A is supported by the case  46 C via an urging means, such as the spring  46 B, such that the front-end of the cleaning blade  46 A is pressed against the outer surface (surface layer  29  in  FIG. 2B ) of the electrophotographic photoreceptor  2 . The cleaning blade  46 A may be formed of a rubber material mainly composed of a polyurethane resin. The front-end of the cleaning blade  46 A in contact with the surface layer  29  typically has a thickness in the range of 1.0 to 1.2 mm. The linear pressure of the cleaning blade  46 A against the surface layer  29  may be 0.14 gf/cm (typically in the range of 0.05 to 0.3 gf/cm). The cleaning blade  46 A may have a hardness of 74 (suitably in the range of 67 to 84) according to JIS K 6253 (ISO 7619). 
     The static eliminator  47  removes surface charges (a remaining electrostatic latent image) of the electrophotographic photoreceptor  2 . The static eliminator  47  irradiates the outer surface (surface layer  29  in  FIG. 2B ) of the electrophotographic photoreceptor  2  with light from a light source, such as an LED, thus removing surface charges of the electrophotographic photoreceptor  2 . 
     An electrostatic latent image and a toner image are formed on the electrophotographic photoreceptor in response to a picture signal. The electrophotographic photoreceptor can rotate in the direction of arrow A in  FIG. 1 . As illustrated in  FIG. 2A , the electrophotographic photoreceptor  2  includes a photosensitive layer  21  formed on a cylindrical substrate  20 . 
     The cylindrical substrate  20  is a base body of the electrophotographic photoreceptor  2 . At least the surface of the cylindrical substrate  20  is electrically conductive. More specifically, the cylindrical substrate  20  may be formed entirely of an electroconductive material, or may be an insulating cylindrical body having an electroconductive film thereon. Examples of the electroconductive material that forms the cylindrical substrate  20  include metals, such as Al, stainless steel (SUS), Zn, Cu, Fe, Ti, Ni, Cr, Ta, Sn, Au, and Ag, and alloys thereof. Examples of an insulating material that forms the cylindrical substrate  20  include resins, glasses, and ceramics. Examples of a material that forms the electroconductive film of the cylindrical substrate  20  include the same metals as the electroconductive material that forms the cylindrical substrate  20  and transparent electroconductive materials, such as indium tin oxide (ITO) and SnO 2 . Preferably, the cylindrical substrate  20  is formed entirely of an Al alloy material. An Al alloy material can reduce the weight and the cost of the electrophotographic photoreceptor  2 . In addition, when a charge injection preventing layer  27  and a photoconductive layer  28  of the photosensitive layer  21  described below are formed of an amorphous silicon (a-Si) material, the adhesiveness between the electrophotographic photoreceptor  2  and the charge injection preventing layer  27  or the photoconductive layer  28  increases. This also increases the reliability. 
     The photosensitive layer  21  may be composed of the charge injection preventing layer  27 , the photoconductive layer  28 , and the surface layer  29 . 
     The charge injection preventing layer  27  prevents electrons and/or holes of the cylindrical substrate  20  from being injected into the photoconductive layer  28 . A material of the charge injection preventing layer  27  depends on the material of the photoconductive layer  28 , and may be an inorganic material, such as an a-Si material. The charge injection preventing layer  27  may be omitted. Furthermore, the charge injection preventing layer  27  may be replaced with a layer absorbing long-wavelength light. The layer absorbing long-wavelength light can prevent incident light having a long wavelength of at least 0.8 μm from being reflected from the cylindrical substrate  20  and forming interference fringes on a recorded image during exposure. 
     In the photoconductive layer  28 , exposure to a laser beam from the exposure unit  42  excites electrons and generates carriers, such as free electrons or holes. The thickness of the photoconductive layer  28  depends on the photoconductive material and desired electrophotographic characteristics, and may be in the range of 5 to 100 μm (suitably in the range of 15 to 80 μm). 
     The photoconductive layer  28  is formed of an a-Si material. Examples of the a-Si material include a-Si, amorphous silicon carbide (a-SiC), amorphous silicon nitride (a-SiN), amorphous silicon oxide (a-SiO), amorphous silicon germanium (a-SiGe), amorphous silicon carbonitride (a-SiCN), amorphous silicon oxynitride (a-SiNO), amorphous silicon oxycarbide (a-SiCO), and amorphous silicon oxycarbonitride (a-SiCNO). In particular, a photoconductive layer  28  formed of a-Si or an a-Si alloy material composed of an a-Si and an element, such as C, N, or O, consistently has excellent electrophotographic characteristics, such as high photosensitivity, high responsivity, good repetition stability, good heat resistance, and high durability. In addition, this photoconductive layer  28  has high compatibility with a surface layer  29  formed of hydrogenated a-SiC (hereinafter referred to as a-SiC:H). The photoconductive layer  28  may contain particles of the a-Si material described above dispersed in a resin, or may be an organic photo conductor (OPC) layer. 
     When the photoconductive layer  28  is formed entirely of an inorganic substance, the photoconductive layer  28  may be formed by a known method, such as glow discharge decomposition, sputtering, vapor deposition, electron cyclotron resonance (ECR), photo-CVD, catalytic CVD, or reactive evaporation. 
     The photoconductive layer  28  may be formed with a plasma CVD apparatus  5  illustrated in  FIG. 3 . The plasma CVD apparatus  5  includes a substrate support  51  at the center of a cylindrical vacuum vessel  50 . An a-Si film is formed by glow discharge plasma on a cylindrical substrate  20  supported by the substrate support  51 . The vacuum vessel  50  is coupled to a high-frequency power source  52 . A high-frequency power is applied between the vacuum vessel  50  and the substrate support  51  (cylindrical substrate  20 ) which is grounded. The substrate support  51  can be rotated by a rotation mechanism  53 , and is heated by a heater  54  disposed in the substrate support  51 . The plasma CVD apparatus  5  further includes a plurality of gas-inlet pipes  55  surrounding the substrate support  51  (cylindrical substrate  20 ). Each of the gas-inlet pipes  55  includes a plurality of gas inlets  56  disposed in the axial direction. The gas inlets  56  face the cylindrical substrate  20  so that a reaction gas blows out from the gas inlets  56  toward the cylindrical substrate  20 . 
     In the formation of an a-Si film on the cylindrical substrate  20  with the plasma CVD apparatus  5 , a reaction gas having a predetermined composition is blown on the cylindrical substrate  20  at a predetermined flow rate from the gas-inlet pipes  55  via the gas inlets  56 , while the cylindrical substrate  20 , together with the substrate support  51 , is rotated by the rotation mechanism  53 . The high-frequency power source  52  applies a high-frequency power between the vacuum vessel  50  and the substrate support  51  (cylindrical substrate  20 ) to decompose the reaction gas by glow discharge, thereby forming an a-Si film on the cylindrical substrate  20 , which is maintained at a desired temperature. 
     As illustrated in  FIG. 2B , the surface layer  29 , which is formed on the photoconductive layer  28 , protects the photoconductive layer  28  from friction and abrasion. The surface layer  29  is formed of an inorganic material, such as an a-Si material. The thickness of the surface layer  29  is in the range of 0.2 to 1.5 μm (suitably in the range of 0.5 to 1.0 μm). The surface layer  29  having a thickness of at least 0.2 μm can reduce image flaws and inconsistencies in image density due to impression durability. The surface layer  29  having a thickness of 1.5 μm or less improves initial properties (for example, image defects due to residual potential). 
     A surface layer  29  formed of a-SiC:H can be formed with the plasma CVD apparatus  5  illustrated in  FIG. 3 , in the same way as the photoconductive layer  28  formed of an a-Si material. When an a-Si photosensitive layer  21  is formed with the plasma CVD apparatus  5 , a protrusion  22  may be formed at the surface of the photosensitive layer  21 , as illustrated in  FIG. 4 . This protrusion  22  may be formed by abnormal growth of a foreign particle  23  deposited on the surface of the cylindrical substrate  20  or during the formation of the photosensitive layer  21 . The protrusion  22  may cause an image defect, as described above. 
     In the electrophotographic photoreceptor  2 , as illustrated in  FIGS. 5A to 5C , resin portion  6   a ,  6   b , or  6   c  partly or entirely covers a protrusion  22 . 
     As illustrated in  FIG. 5A , the resin portion  6   a  is formed on the front side of the protrusion  22  in the rotation direction of the electrophotographic photoreceptor  2  so that the resin portion  6   a  reduces a difference in level between the protrusion  22  and a normal surface  24 . 
     As illustrated in  FIG. 5B , the resin portion  6   b  is formed on the front and rear sides of the protrusion  22  in the rotation direction of the electrophotographic photoreceptor  2  so that the resin portion  6   b  reduces a difference in level between the protrusion  22  and the normal surface  24 . 
     As illustrated in  FIG. 5C , a resin portion  6   c  entirely covers the protrusion  22 . More specifically, the resin portion  6   c  covers the top  22 A of the protrusion  22 , as well as the front and rear sides of the protrusion  22  in the rotation direction of the electrophotographic photoreceptor  2 . 
     As illustrated in  FIG. 6 , the resin portions  6   a ,  6   b , and  6   c  illustrated in  FIGS. 5A to 5C  can be formed by making a resin sheet  60  contact with the photosensitive layer  21  (see  FIG. 2A ) disposed on the cylindrical substrate  20 , which is rotatably supported, for example, by an umbrella-shaped center pin (not shown) for use in a lathe. The resin sheet  60  supplied from a resin roller  61  is pressed against the photosensitive layer  21  by a rear roller  62 . The rear roller  62  generally rotates in the different direction as the cylindrical substrate  20 . The resin sheet  60  is wound around a recovery roller  63 . 
     The nip width between the photosensitive layer  21  and the resin sheet  60  is controlled by the hardness of the resin sheet  60  and the pressing force of the rear roller  62  against the photosensitive layer  21  via the resin sheet  60 . The resin sheet  60  may be a monolayer of a resin material of the resin portion  6   a ,  6   b , or  6   c , or a multilayer composed of the resin materials on a base sheet. Examples of the resin material include “fluorocarbon resins” (which include at least one fluorine atom and may include other halogen atoms), polystyrene resins, and polyethylene resins. Fluorocarbon resins are preferred in terms of the prevention of toner deposition. Examples of the fluorocarbon resins include polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, perfluoroalkoxy fluorocarbons, tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-tetrafluoroethylene copolymers, and ethylene-chlorotrifluoroethylene copolymers. The pressing force of the rear roller  62  against the photosensitive layer  21  via the resin sheet  60  may be in the range of 0.01 to 0.2 kgf/cm 2  (9.806×10 to 1.961×10 Pa) per unit axial length. The nip width may be in the range of 0.01 to 2 mm. 
     As illustrated in  FIG. 7 , when the resin sheet  60  moves relative to the cylindrical substrate  20  while the resin sheet  60  is in contact with the photosensitive layer  21 , the protrusion  22  scrapes a resin  64  off the resin sheet  60 . The resin  64  adheres to the photosensitive layer  21  to reduce a difference in level between the protrusion  22  and the normal surface  24 . The nip width between the photosensitive layer  21  and the resin sheet  60 , the hardness of the resin sheet  60  (the composition of the resin material), or the moving speed of the resin sheet  60  is appropriately controlled to apply the resin portion  6   a ,  6   b , or  6   c  to the side and/or the top of the protrusion  22  while the resin adhering to the normal surface  24  is minimized, as illustrated in  FIGS. 5A to 5C . 
     In the electrophotographic photoreceptor  2 , at least part of the side of the protrusion  22  disposed on the photosensitive layer  21  is covered with the resin portion  6   a ,  6   b , or  6   c . This can reduce the toner deposition at a stepped portion  25  between the protrusion  22  and the normal surface  24 , and allows toner around the protrusion  22  to be removed easily with the cleaning blade  46 A. Furthermore, the reduction in toner deposition around the protrusion  22  can reduce toner adherence to the photosensitive layer  21 . 
     Thus, in the electrophotographic photoreceptor  2  and the image-forming apparatus  1  including the electrophotographic photoreceptor  2 , image defects caused by the toner deposition around the protrusion  22 , as well as insufficient cleaning and black-striped image defects, can be reduced. 
     Other examples of the resin portion are described below with reference to  FIGS. 8A to 8C  and  FIG. 9 . 
     As illustrated in  FIGS. 8A to 8C , a resin portion  7   a ,  7   b , or  7   c  partly or entirely covers the side of the protrusion  22 , or entirely covers the protrusion  22  having a flat top  70 . 
     As illustrated in  FIG. 8A , the resin portion  7   a  corresponds to a truncated form of the protrusion  22  illustrated in  FIG. 5A . As illustrated in  FIG. 8B , the resin portion  7   b  corresponds to a truncated form of the protrusion  22  illustrated in  FIG. 5B . As illustrated in  FIG. 8C , the resin portion  7   c  corresponds to a truncated form of the protrusion  22  illustrated in  FIG. 5C . 
     The resin portions  7   a ,  7   b , and  7   c  can be formed in the same way as the resin portions  6   a ,  6   b , and  6   c  illustrated in  FIGS. 5A to 5C , except that the protrusion  22  is truncated. 
     The protrusion  22  may be truncated using a lapping sheet in place of the resin sheet  60  of the apparatus illustrated in  FIG. 6 . The direction of movement of the lapping sheet may follow the rotation direction of the cylindrical substrate  20 , or may be the opposite direction thereof. 
     As illustrated in  FIG. 9 , a lapping sheet  71  includes a base sheet  72  and a polymer resin binder  74  containing abrasive particles  73 . Preferably, the base sheet  72  is a polyester that does not expand and contract significantly and has a uniform thickness. Examples of the abrasive particles  73  include silicon carbide, iron oxide, chromium oxide, aluminum oxide, and diamond particles. The size of the abrasive particles  73  may be in the range of 0.3 to 20 μm. 
     The protrusion  22  is ground with the lapping sheet  71  to form a truncated protrusion  22 . The height of the truncated protrusion  22  is appropriately determined in a manner that depends on the size of the protrusion  22 . For example, when the protrusion  22  has a diameter of 0.3 mm or less and a height of 60 μm or less, the height of the truncated protrusion  22  may be in the range of about 0.1 to 1 μm. 
     After the protrusion  22  is ground, the lapping sheet  71  of the apparatus is replaced with the resin sheet  60 . In the same way as the resin portions  6   a ,  6   b , and  6   c  illustrated in  FIGS. 5A to 5C , the resin portion  7   a ,  7   b , or  7   c  is formed to reduce a difference in level between the protrusion  22  (flat top  70 ) and the normal surface  24 . 
     Grinding of the protrusion  22  and the formation of the resin portion  7   a ,  7   b , or  7   c  may be performed in a single step by controlling the type of the lapping sheet  71  (the type and the particle size of abrasive particles, the type of a binder, etc.), the nip width between the photosensitive layer  21  and the lapping sheet  71 , and the moving speed of the lapping sheet  71  relative to the cylindrical substrate  20 . 
     Thus, in the electrophotographic photoreceptor  2  having the resin portion  7   a ,  7   b , or  7   c  and the image-forming apparatus  1  including the electrophotographic photoreceptor  2 , image defects caused by the toner deposition around the protrusion  22 , as well as insufficient cleaning and black-striped image defects, can be reduced. 
     Furthermore, even when the protrusion  22  comes into contact with the cleaning blade  46 A, a reduction in height of the protrusion  22  can reduce frictional heat and damage to the cleaning blade  46 A. This can further reduce adhesion of toner to the photosensitive layer  21 , and further reduce the occurrence of insufficient cleaning and black-striped image defects. 
     The resin portions  6   a ,  6   b ,  6   c ,  7   a ,  7   b , and  7   c  may be formed not only by the methods described above, but also using a liquid material containing a resin material. 
     For example, as illustrated in  FIG. 10 , a liquid material (resin coating material) is applied to the photosensitive layer  21  with a spray coater  80 , while the electrophotographic photoreceptor  2  is rotated. After the resin coating material is heat-treated with a heat treatment apparatus  81 , an excessive resin coating material is removed with a blade  82  and a finishing roller  83 . 
     The present invention will be further described with Examples. The present invention is not limited to the Examples. 
     Example 1 
     Toner deposition was observed after image forming in both cases where a protrusion disposed on an electrophotographic photoreceptor is or is not covered with a resin portion. 
     (Production of Electrophotographic Photoreceptor) 
     Electrophotographic photoreceptors A and B according to Comparative Examples and electrophotographic photoreceptors C1, C2, D1 and D2 according to Examples were produced. 
     The electrophotographic photoreceptor A was produced as follows. A cylindrical substrate was mirror-finished and washed. A photosensitive layer was formed on the cylindrical substrate with a plasma CVD apparatus  5  illustrated in  FIG. 3 . The cylindrical substrate was an aluminum cylindrical substrate having a diameter of 84 mm and a length of 370 mm. The photosensitive layer had a three-layered structure composed of a p-type charge injection preventing layer, an a-Si photoconductive layer, and an a-Si surface layer. The p-type charge injection preventing layer was formed using SiH 4 , B 2 H 6 , H 2 , and NO as reaction gases, and had a thickness of 4 μm. The a-Si photoconductive layer was formed using SiH 4 , H 2 , and B 2 H 6  as reaction gases, and had a thickness of 27 μm. The a-Si surface layer was formed using SiH 4  and CH 4  as reaction gases, and had a thickness of 0.7 μm. 
     The electrophotographic photoreceptor B was produced as follows. A photoreceptor produced in the same way as the electrophotographic photoreceptor A was attached to a umbrella-shaped center pin, and was installed in a rotator. The photoreceptor was rotated at 60 revolutions per minute (rpm) to grind a protrusion disposed on the photoreceptor with a lapping sheet (planarization), thus producing the electrophotographic photoreceptor B. 
     The electrophotographic photoreceptors C were produced as follows: a photoreceptor subjected to the planarization as in the electrophotographic photoreceptor B was brought into contact with a resin-coated sheet to cover the protrusion and its periphery with a resin portion (see  FIG. 8C ). The resin portion was formed of a fluorocarbon resin (C2) or a polystyrene resin (C1). 
     The electrophotographic photoreceptors D were produced as follows: a photoreceptor (not subjected to planarization) produced in the same way as the electrophotographic photoreceptor A was brought into contact with a resin-coated sheet to cover the protrusion and its periphery with a resin portion (see  FIG. 5C ). The resin portion was formed of a fluorocarbon resin (D2) or a polystyrene resin (D1). 
     (Evaluation of Toner Deposition) 
     The electrophotographic photoreceptor A, B, C, or D was installed in an image-forming apparatus (KM-8030, Kyocera Mita Corporation). After a plate wear test of 10,000 sheets, the electrophotographic photoreceptor was visually inspected. This test was performed in quintuplicate for each of the electrophotographic photoreceptors A, B, C, and D. The toner deposition was evaluated as the incidence of the toner deposition. The incidence of the toner deposition was defined by the ratio of the number of protrusions to which toner adhered to the total number of protrusions. 
     
       
         
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Resin portion 
               
             
          
           
               
                   
                 None 
                 Polystyrene resin 
                 Fluorocarbon resin 
               
               
                   
                   
               
             
          
           
               
                   
                 A 
                 25% 
                 — 
                 — 
               
               
                   
                 B 
                  5% 
                 — 
                 — 
               
               
                   
                 C 
                 — 
                 0% (C1) 
                 0% (C2) 
               
               
                   
                 D 
                 — 
                 8% (D1) 
                 2% (D2) 
               
               
                   
                   
               
             
          
         
       
     
     Table 1 shows that, in a comparison of the electrophotographic photoreceptors A and D1 in which the protrusion was not truncated, the presence of the polystyrene resin portion reduced the incidence of the toner deposition from 25% to 8%, and the presence of the fluorocarbon resin portion (Example D2) reduced the incidence of the toner deposition to 2%. In a comparison of the electrophotographic photoreceptors B, C1 and C2 in which the protrusion was truncated, the presence of the polystyrene (C1) or fluorocarbon (C2) resin portion reduced the incidence of the toner deposition from 5% to 0%. 
     These results demonstrate that the formation of a resin portion can reduce the toner deposition. 
     Furthermore, the incidence of the toner deposition was lower in the electrophotographic photoreceptors C1 and C2, in which the truncated protrusion was covered with the resin portion, than in the electrophotographic photoreceptors D1 and D2, in which the full protrusion was covered with the resin portion. 
     This result demonstrates that the formation of a resin portion covering a truncated protrusion can further reduce the toner deposition. 
     Furthermore, the incidence of the toner deposition was lower in the electrophotographic photoreceptor D2 having the fluorocarbon resin portion than in the electrophotographic photoreceptor D1 having the polystyrene resin portion. 
     This result demonstrates that the fluorocarbon resin portion is preferred to the polystyrene resin portion. 
     Example 2 
     The surfaces of the photosensitive layers of the electrophotographic photoreceptors A and D1 in Example 1 were observed. Images after a plate wear test were evaluated. 
     Surface observation of the electrophotographic photoreceptor A before use showed that there were 20 protrusions having a diameter in the range of 0.1 to 0.2 mm and a height in the range of 15 to 50 μm. 
     The electrophotographic photoreceptor A was installed in an image-forming apparatus (KM-8030, Kyocera Mita Corporation). After a plate wear test of 10,000 sheets, surface observation of the photosensitive layer showed that there was discharge breakdown of one protrusion and that five of the 20 protrusions had toner deposition. Inspection of images after the plate wear test showed that there were two black spots in a white solid portion and six white spots in a gray image formed by performing an intermediate exposure (exposure at an intermediate point of the light decay from the dark voltage) to a black solid portion. The term “white solid portion”, as used herein, refers to a printed portion corresponding to an unexposed portion of an electrophotographic photoreceptor. 
     Surface observation of the electrophotographic photoreceptor D1 before use showed that there were 25 protrusions having a diameter in the range of 0.1 to 0.2 mm and a height in the range of 15 to 50 μm. 
     The electrophotographic photoreceptor D1 was installed in the image-forming apparatus (KM-8030, Kyocera Mita Corporation). After a plate wear test of 10,000 sheets, two of the 25 protrusions had toner deposition, and the discharge breakdown of the protrusions was not observed. Inspection of images after the plate wear test showed that there was no black spot in a white solid portion and two white spots in a gray image formed by performing an intermediate exposure to a black solid portion. 
     Example 3 
     Surface observation of a photosensitive layer of an electrophotographic photoreceptor E and the evaluation of images after a plate wear test were performed as in Example 2. In the electrophotographic photoreceptor E, grinding of a protrusion and the formation of a resin portion were performed in a single step. 
     A photoreceptor (not subjected to planarization) produced in the same way as the electrophotographic photoreceptor A was treated with a lapping sheet to produce the electrophotographic photoreceptor E. The lapping sheet was composed of a polyethylene terephthalate base sheet and a fluorocarbon resin binder containing abrasive particles. 
     The lapping sheet was brought into contact with a photosensitive layer with an apparatus illustrated in  FIG. 6 . Grinding of a protrusion with the abrasive particles and the formation of a fluorocarbon resin portion were performed in a single step. The pressing force of the lapping sheet against the photosensitive layer was appropriately controlled to selectively cover a protrusion with a resin portion (see  FIG. 8C ). 
     Surface observation of the electrophotographic photoreceptor E before use showed that there was no protrusion having a diameter in the range of 0.1 to 0.2 mm and a height in the range of 15 to 50 μm. 
     The electrophotographic photoreceptor E was installed in an image-forming apparatus (KM-8030, Kyocera Mita Corporation). After a plate wear test of 10,000 sheets, surface observation of the photosensitive layer showed that there was no discharge breakdown and no substantial toner deposition. Inspection of images after the plate wear test showed that there was no black spot in a white solid portion and no white spot in a gray image formed by performing an intermediate exposure to a black solid portion. 
     Example 4 
     Surface observation of a photosensitive layer of an electrophotographic photoreceptor F and the evaluation of images after a plate wear test were performed as in Example 2. In the electrophotographic photoreceptor F, a resin portion was formed with an apparatus illustrated in  FIG. 10 . 
     The electrophotographic photoreceptor F was produced as follows. A photoreceptor produced in the same way as the electrophotographic photoreceptor A was attached to a umbrella-shaped center pin, and was installed in an rotator. A resin coating material was applied to a photosensitive layer with a spray coater, while the electrophotographic photoreceptor F was rotated. A rubber blade was then brought into contact with the rotated photoreceptor to scrape the resin coating material off a normal surface. A finishing roller was then brought into contact with the rotated photoreceptor to further scrape the resin coating material off the normal surface. Thus, a protrusion disposed on the electrophotographic photoreceptor F was selectively covered with the resin coating material (see  FIG. 5C ). 
     Surface observation of the electrophotographic photoreceptor F before use showed that there were 20 protrusions having a diameter in the range of 0.1 to 0.2 mm and a height in the range of 15 to 50 μm. 
     The electrophotographic photoreceptor F was installed in an image-forming apparatus (KM-8030, Kyocera Mita Corporation). After a plate wear test of 10,000 sheets, surface observation of the photosensitive layer showed that there was no discharge breakdown and that two of the 20 protrusions had toner deposition. Inspection of images after the plate wear test showed that there was one black spot in a white solid portion and two white spots in a gray image formed by performing an intermediate exposure to a black solid portion. 
     Table 2 summarizes the results of Examples 2, 3, and 4. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Comparative 
                   
                   
                   
               
               
                   
                 Example 
                 Example 2 
                 Example 3 
                 Example 4 
               
               
                   
                 Electrophotographic 
                 Electrophotographic 
                 Electrophotographic 
                 Electrophotographic 
               
               
                   
                 photoreceptor A 
                 photoreceptor D1 
                 photoreceptor E 
                 photoreceptor F 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Toner 
                 25% 
                 8% 
                 0% 
                 10%  
               
               
                 deposition 
               
               
                 Discharge 
                  0% 
                 0% 
                 0% 
                 0% 
               
               
                 breakdown 
               
               
                 Black spot 
                 10% 
                 4% 
                 0% 
                 5% 
               
               
                 Evaluation 
                 Poor 
                 Fair 
                 Good 
                 Fair 
               
               
                   
               
             
          
         
       
     
     Table 2 shows that toner deposition, discharge breakdown, and black spots were reduced in the electrophotographic photoreceptors D1, E, and F according to the present embodiments, as compared with the conventional electrophotographic photoreceptor A having no resin portion. Thus, the present invention can provide an electrophotographic photoreceptor exhibiting less toner deposition and fewer black spots than before without deterioration of discharge breakdown. Furthermore, according to the present invention, damage to a cleaning blade by a protrusion, insufficient cleaning, and black-striped image defects can be reduced.