Patent Publication Number: US-7907879-B2

Title: Development roller, development device, image forming apparatus, and method of manufacturing development roller

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a development roller having a roughness on the circumference thereof for transporting toner to a latent image bearing unit, a development device containing the development roller, an image forming apparatus containing the development device, and a method of manufacturing the development roller. 
     2. Related Art 
     Development devices developing a toner image from a latent image with one-component non-magnetic toner triboelectrically charge the toner on a development roller. A development roller known in the related art (such as the one disclosed in Japanese Unexamined Patent Application Publication No. JP-A-2007-121948) has a surface roughness on the circumference thereof, the roughness having a substantially flat top surface. With the surface roughness, the development roller triboelectrically charges the toner thereon. As illustrated in  FIG. 10A , a development roller a includes a base unit b and a surface layer c plated on the base unit b as a coverage. 
     The development roller a generally remains in contact with a toner feed roller and a toner regulator (both not shown). Silica having a high hardness is used serving as an external additive that coats toner mother particles of the toner. A roughness portion, composed of a plurality of recesses d and projections e, is formed on the circumference of the base unit b. A roughness portion, composed of a plurality of recesses f and projections g, is formed on the circumference of the surface layer c. 
     The surface layer c is worn by the toner feed roller and the toner regulator in an image forming operation. A demand for high-quality image and reduction in toner consumption is mounting today. The particle diameter of the toner currently becomes smaller. If the image forming operation has been performed with the small particle size toner for a long period of time, the surface of the top portion h of the projection g is relatively heavily worn in a generally flat configuration while the surface of the recess f is generally unworn as illustrated in  FIG. 10B . If the degree of wear is different from the recess f to the projection g, the depth of the roughness portion is reduced in the long service life of image forming of the development roller. The amount of toner transported by the development roller is thus reduced. It becomes difficult to maintain the image density level of each image and to continue the development process for a long period of time. 
     SUMMARY 
     An advantage of some aspects of the invention is that a development roller remains operative in an image forming operation thereof for a long period of time with a reduction of a depth of a roughness portion of the development roller controlled as much as possible. An advantage of the invention is also that a development device and an image forming apparatus, each containing the development roller, also remain operative in the image forming operation thereof for a long period of time. 
     In accordance with one embodiment of the invention, surface hardness of a projection is higher than surface hardness of a recess in the roughness portion of the development roller. In the long service life of image forming, the wearing of a surface layer at the projection, likely to be subject to wear, is controlled. A difference between the degree of wear of the surface layer at the recess subject to mild wearing and the degree of wear of the surface layer at the project is smaller than a difference caused in the related art. A change in the depth of the roughness portion of the development roller is controlled in the long service life of the development roller. The amount of toner transported by the development roller remains almost unchanged. The image density level of images developed is maintained substantially at a constant level. Excellent development process is thus performed for a long period of time. 
     Surface hardness of the recess of the development roller is set to be small so that the surface at the recess is positively abraded. This arrangement prevents filming from taking place. Filming is caused by degraded toner building up in the recess that typically suffers from a poor toner refreshing characteristics by the toner feed roller. Furthermore, since the recess is spaced from a toner regulator blade, a toner charging property tends to be lowered. A decrease in the toner charging property is controlled by keeping the recess amorphous. This arrangement controls toner coverage or toner splashing, leading to excellent development characteristics. 
     In a toner transport method in which toner is not transported to the surface of the projection with a toner regulator unit, a function of the recess for maintaining the toner charging property at the surface of the recess is separated from a function of the projection for maintaining wear proofness on the surface of the projection. The two functions are thus separately performed. 
     The toner charging property of the projection is lowered by crystallizing the top portion of the projection. A low toner charging property prevents chargeup from taking place between the toner regulator blade and the projection of the development roller, thereby improving development results. In a toner transport method, toner having a toner particle size smaller than a depth of the roughness portion of the development roller is transported to the recess of the development roller with a front edge of the toner regulator blade placed into contact with the development roller, and the toner is not transported to the projection. In such a toner transport method, the supply of the toner to the projection is more effectively controlled. Filming of the toner on a flat portion of the projection and chargeup of the toner are thus prevented. 
     The roughness portion of the surface layer is constructed of the same material and the degree of crystallization is differentiated between the projection and the recess (for example, the projection is set to be higher in the degree of crystallization than the recess). With this arrangement, the surface hardness and electrical resistance of the projection and recess can be controlled. The surface layer at the recess and the projection is not fully crystallized. The surface composition of the development roller is thus easily set up. Filming (toner fusion) takes place if the wear of the projection is too small as a result of high hardness thereof. By controlling the degree of crystallization, the generation of filming is controlled. 
     By allowing the projection of the surface layer to be heated in a localized fashion, the base unit is almost free from crystallization. The base unit is thus free from release of stress, and bowing and bending responsive to variations in the degree of crystallization. 
     An area of the projection where crystallization advances is limited to within an average particle diameter of toner in use from the top surface of the projection. The toner particles transported to the recess that is subject to a decrease in charging property are thus allowed to be in contact with the amorphous recess. This arrangement prevents the toner from being lowered the in toner charging property. More specifically, the toner is effectively charged by setting the toner charging property of the recess to be higher than the toner charging property of the projection. 
     The surface layer is on the base unit through electroless plating before the formation of the roughness portion on the base unit. Even if a material relatively hard to machine is used for the base unit, the configuration stability of the roughness portion is improved by the plated surface layer. The roughness portion has an increased surface smoothness, allowing the toner particles to be moved smoothly. Filming of the toner at the recess is thus controlled. The toner transportability and the toner charging property are excellently maintained. 
     The development device containing the development roller of one embodiment of the invention can perform the development process on electrostatic latent images on a latent image bearing unit for a long period of time. The image forming apparatus containing the development device can thus provide stable and excellent-quality images for a long period of time. 
     In accordance with another aspect of the invention, surface hardness of the base unit is set to be higher than surface hardness of the surface layer if the surface layer includes one layer only. Surface hardness of a layer immediately inside the outermost layer is set to be higher than surface hardness of the outmost layer if the surface layer includes a plurality of layers. If the surface layer at the flat portion of the projection of the base unit or the outermost surface layer at the flat portion of the projection of the base unit is worn by the toner regulator blade, the toner feed roller, or the toner external additive, the flat portion of the base unit or the surface layer immediately beneath the outermost layer is exposed. The wear rate of the projection of the development roller is then reduced. In this way, the durability of the development roller is increased. 
     If the surface layer or the outermost layer is worn out, the depth of the roughness portion of the development roller slightly changes. The wearing of the exposed flat portion or the surface layer immediately below the outmost layer is controlled. As a result, a change in the depth of the roughness portion of the development roller is controlled for a long period of time. The depth of the roughness portion is thus maintained for a long period of time. The amount of toner transported to the development roller remains almost unchanged. The density level of the images is maintained at a substantially constant level for a long period of time. An excellent development process is thus provided for a long period of time. 
     The toner charging property of the exposed flat portion or the exposed surface layer immediately below the outmost layer, at the projection is lowered. Toner particles pinched between the development roller and the toner regulator blade result in stronger frictional force than that at the recess. A decrease in the toner charging property is controlled accordingly. Toner coverage and toner splashing are controlled, and excellent development characteristics are thus provided. 
     In a toner transport method in which toner is not transported to the surface of the projection with a toner regulator blade, a function of the recess for maintaining the toner charging property at the surface of the recess is separated from a function of the projection for maintaining wear proofness on the surface of the projection (maintaining the depth of the roughness portion). The two functions are separately performed. 
     The thickness of one of the surface layer and the outermost layer is set to be within an average particle diameter (D50 particle diameter) of the toner in use. The toner transported to the recess subject to a decrease in the charging property is placed into contact with the amorphous recess. A decrease in the toner charging property is controlled. 
     One of the surface layer and the outermost layer of a plurality of layers is removed through a grinding process of a grinding machine or a polishing process of a polishing machine. Even if a development roller having an exposed flat portion of the base projection or an exposed surface layer immediately beneath the outermost layer is used from the start, the same operation and advantages as those described above may be provided. 
     The development device containing the development roller can develop toner images on the latent image bearing unit in accordance with the electrostatic latent images for a long period of time. The image forming apparatus containing the development device can provide stable and excellent-quality images for a long period of time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  illustrates an image forming apparatus in accordance with one embodiment of the invention. 
         FIG. 2  is a sectional view diagrammatically illustrating a development device illustrated in  FIG. 1 . 
         FIG. 3A  diagrammatically illustrates a development roller, a toner feed roller, and a toner regulator unit,  FIG. 3B  is a partial sectional view illustrating part of the development roller and taken along line IIIB-IIIB in  FIG. 3A , and  FIG. 3C  is a partial sectional view illustrating only a base unit of the development roller. 
         FIG. 4  is a partial sectional expanded view of the development roller illustrated in  FIG. 3B . 
         FIG. 5A  illustrates a size of a roughness of the development roller, and  FIG. 5B  illustrates a wear process of the development roller when a toner particle diameter is larger than a depth of the roughness of the development roller. 
         FIG. 6A  illustrates the behavior of toner particles when the toner particle diameter is smaller than the depth of the roughness of the development roller, and  FIG. 6B  illustrates the wear state of the development roller of  FIG. 6A . 
         FIGS. 7A-7C  illustrate a method of manufacturing the development roller illustrated in  FIGS. 3A-3C  and  4 . 
         FIGS. 8A-8C  illustrate another method of manufacturing the development roller illustrated in  FIGS. 3A-3C  and  4 . 
         FIG. 9A  illustrates toner rubbing test results and  FIGS. 9B and 9C  illustrate surface potential test results. 
         FIG. 10A  is a partial sectional view of a roughness portion of a known development roller, and  FIG. 10B  illustrates the wear of the roughness portion illustrated in  FIG. 10A . 
         FIG. 11A  diagrammatically illustrates a development roller, a toner feed roller, and a toner regulator unit,  FIG. 11B  is a partial sectional view illustrating part of the development roller and taken along line IIIB-IIIB in  FIG. 11A ,  FIG. 11C  is a partial sectional view illustrating part of the development roller with a surface layer thereof partially worn, and  FIG. 11D  is a partial sectional view of only the base unit of the development roller. 
         FIGS. 12A and 12B  are partial sectional views of the development roller illustrated in  FIG. 11B . 
         FIG. 13A  illustrates a size of a roughness of the development roller, and  FIG. 13B  illustrates a wear process of the development roller when a toner particle diameter is larger than a depth of the roughness of the development roller. 
         FIGS. 14A-14C  illustrate a method of manufacturing the development roller illustrated in  FIGS. 11A-11D  and  12 A and  12 B. 
         FIGS. 15A-15B  illustrate another method of manufacturing the development roller illustrated in  FIGS. 11A-11D  and  12 A and  12 B. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The embodiments of the invention are described below with reference to the drawings. 
       FIG. 1  diagrammatically illustrates an image forming apparatus  1  in accordance with one embodiment of the invention. 
     With reference to  FIG. 1 , a photoconductor unit  3  as an image bearing unit is supported in an apparatus body  2  in a manner such that the photoconductor unit  3  is clockwise rotated in a direction of rotation a. A charging device  4  is arranged in the vicinity of the circumference of the photoconductor unit  3 . Also arranged in the direction of rotation a of from the charging device  4  to the photoconductor unit  3  around the photoconductor unit  3  are a rotary development unit  5  as a development device, a primary transfer device  6 , and a cleaning device  7 . The rotary development unit  5  includes a development device  5 Y for yellow color, a development device  5 M for magenta color, a rotary development unit  5 C for cyan color, and a development device  5 K for black. These development devices  5 Y,  5 M,  5 C and  5 K are detachably supported in a rotary  5   a  that is rotatable about a center axis in a direction of rotation β (counterclockwise rotation in  FIG. 1 ). An exposure device  8  is arranged below the charging device  4  and the cleaning device  7 . 
     The image forming apparatus  1  further includes an intermediate transfer belt  9  having an endless structure as an intermediate transfer medium. The intermediate transfer belt  9  is entrained about a belt driving roller  10  and a driven roller  11 . A driving force of a motor (not shown) is conveyed to the belt driving roller  10 . The belt driving roller  10  causes the intermediate transfer belt  9  to rotate in a rotational direction  7  (counterclockwise rotation in  FIG. 1 ) while the intermediate transfer belt  9  is pressed by the primary transfer device  6  against the photoconductor unit  3 . 
     A secondary transfer device  12  is arranged next to the belt driving roller  10  of the intermediate transfer belt  9 . A transfer material cassette  13  is arranged below the exposure device  8 . The transfer material cassette  13  holds a sheet-like transfer material such as a transfer paper sheet (corresponding to a transfer medium in accordance with one embodiment of the invention). A pickup roller  15  and a gate roller  16  are arranged close to the secondary transfer device  12  in a transfer material transport path  14  extending from the transfer material cassette  13  to the secondary transfer device  12 . 
     A fixing device  17  is arranged above the secondary transfer device  12 . The fixing device  17  includes a heater roller  18  and a pressure roller  19  pressed against the heater roller  18 . A transfer material discharge tray  20  is arranged on the top portion of the apparatus body  2 . A pair of transfer material discharge rollers  21  are arranged between the fixing device  17  and the transfer material discharge tray  20 . 
     In the image forming apparatus  1  thus constructed, a yellow electrostatic latent image, for example, is formed on the photoconductor unit  3  uniformly charged by the charging device  4  in response to laser light L from the exposure device  8 . The yellow electrostatic latent image is developed on the photoconductor unit  3  by yellow toner of the yellow development device  5 Y at a development position (not shown) determined when the rotary  5   a  rotates. A yellow toner image is thus developed on the photoconductor unit  3 . The yellow toner image is then transferred to the intermediate transfer belt  9  by the primary transfer device  6 . Toner remaining on the photoconductor unit  3  subsequent to the transfer operation is scraped off by a cleaning blade or the like of the cleaning device  7  and then recycled. 
     Similarly, a magenta image is formed by the exposure device  8  on the photoconductor unit  3  that is uniformly charged by the charging device  4 . The magenta electrostatic latent image is developed by magenta toner of the magenta development device  5 M at the development position. The magenta image on the photoconductor unit  3  is transferred to the intermediate transfer belt  9  by the primary transfer device  6  in a manner such that the magenta image is superimposed on the yellow image. Toner remaining on the photoconductor unit  3  subsequent the transfer operation is recycled by the cleaning device  7 . A similar operation is repeated for cyan and black toners. The toner images are successively formed on the photoconductor unit  3 , and then superimposed on the preceding toner images on the intermediate transfer belt  9 . A full-color toner image is then formed on the intermediate transfer belt  9 . Similarly, toner remaining on the photoconductor unit  3  subsequent to each transfer operation is recycled by the cleaning device  7 . 
     The full-color toner image transferred onto the intermediate transfer belt  9  is then transferred by the secondary transfer device  12  to the transfer material transported from the transfer material cassette  13  via the transfer material transport path  14 . The transfer material is then transported to the secondary transfer device  12  at a timing with the full-color toner image of the intermediate transfer belt  9  by the gate roller  16 . 
     The toner image pre-fixed to the transfer material is heated and pressure-fixed by the heater roller  18  and the pressure roller  19  in the fixing device  17 . The transfer material having the image thereon is transported via the transfer material transport path  14 , discharged to the transfer material discharge tray  20  via the transfer material discharge roller pair  21  and then held there. 
     A characteristic structure of the image forming apparatus  1  is described below. 
     The development devices  5 Y,  5 M,  5 C, and  5 K in the image forming apparatus  1  are identical in structure. In the discussion that follows, the rotary development unit  5  is representatively discussed without individually referring to the development devices  5 Y,  5 M,  5 C, and  5 K. In this case, reference number  51  is used to discriminate the development device from the rotary development unit  5 . 
       FIG. 2  is a sectional view of the development device  5 ′ taken in a direction perpendicular to the longitudinal direction of the development device  5 ′ in accordance with one embodiment of the invention. 
     The development device  5 ′ has a form of an elongated container. With reference to  FIG. 2 , the development device  5 ′ has the same structure as the development device disclosed in Japanese Unexamined Patent Application Publication No. JP-A-2007-121948. More specifically, the development device  5 ′ includes in an elongated housing  22  a toner container  23 , a toner feed roller  24 , a development roller  25 , and a toner regulator member  26 . The toner container  23 , the toner feed roller  24 , the development roller  25 , and the toner regulator member  26  extend in the longitudinal direction of the development device  5 ′ (i.e., in a direction perpendicular to the plane of the page of  FIG. 2 ). 
     The toner container  23  is partitioned into two toner compartments  23   a  and  23   b  by a partitioning wall  27 . The toner container  23  includes a common section  23   c  through which the first and second toner compartments  23   a  and  23   b  are open to each other in  FIG. 2 . The partitioning wall  27  limits the movement of toner  28  between the first and second toner compartments  23   a  and  23   b . When the development device  5 ′ is turned upside down from the position illustrated in  FIG. 2  with the rotary  5   a  of the rotary development unit  5  rotated, the toner  28  stored in each of the first and second toner compartments  23   a  and  23   b  moves to the common section  23   c . The rotary  5   a  further rotates, causing the development device  5 ′ to be positioned to the state illustrated in  FIG. 2 . The toner  28  then moves back to each of the first and second toner compartments  23   a  and  23   b . In this way, part of the toner  28  previously held in the first toner compartment  23   a  is moved to the second toner compartment  23   b  and part of the toner  28  previously held in the second toner compartment  23   b  is moved to the first toner compartment  23   a . The toner  28  is thus agitated within the toner container  23 . The toner  28  is one-component, non-magnetic toner with toner mother particles thereof coated with an external additive. In accordance with one embodiment of the invention, the external additive contains at least silica. 
     Referring to  FIG. 2 , the toner feed roller  24  is arranged in the lower portion of the first toner compartment  23   a  in a manner such that the toner feed roller  24  is clockwise rotatable. The development roller  25  is counterclockwise rotatably supported on the outside of the housing  22  as illustrated in  FIG. 2 . The development roller  25  is arranged close to the photoconductor unit  3  (in a non-contact fashion). The development roller  25  is pressed against the toner feed roller  24  at a predetermined pressure through an opening  22   a  of the housing  22 . The toner regulator member  26  is also arranged on the housing  22 . The toner regulator member  26  remains in contact with the development roller  25  downstream of a nip (contact point) between the development roller  25  and the toner feed roller  24 . The toner regulator member  26  regulates a thickness of the toner  28  fed to the development roller  25  from the toner feed roller  24 . The toner  28  regulated by the toner regulator member  26  is transported to the photoconductor unit  3  by the development roller  25 . The electrostatic latent image is thus developed into the toner image on the photoconductor unit  3  by the toner  28  transported by the development roller  25 . The toner image of each color thus results on the photoconductor unit  3 . 
       FIGS. 3A-3C  illustrate the circumference surface of the development roller  25  that has the same mesh roughness pattern as the one on the development roller discussed with reference to Japanese Unexamined Patent Application Publication No. JP-A-2007-121948. In the development roller  25 , grooves  29  are formed in a roughness pattern in predetermined positions in the axial direction thereof on the whole circumference surface. The grooves  29  include first grooves  29   a  of a predetermined number continuously spiraling at a predetermined angle with respect to the axial direction of the development roller  25  (the predetermined angle is 45° in  FIG. 3A , but not limited to 45°), and second grooves  29   b  of a predetermined number continuously spiraling at an angle opposite to the slant angle of the first grooves  29   a . The first and second grooves  29   a  and  29   b  are formed at the respective slant angles at a predetermined pitch p with regular interval of W along the axial direction of the development roller  25 . The first and second grooves  29   a  and  29   b  may be different from each other in slant angle and pitch. 
     With reference to  FIG. 3B , the development roller  25  includes a base unit  25   a , and a surface layer  25   b  formed on the circumference surface of the base unit  25   a . The base unit  25   a  is a metal sleeve made of an aluminum based metal such as 5056 aluminum alloy or 6063 aluminum alloy, or an iron based metal such as STKM steel. The surface layer  25   b  is a nickel-based or chromium-based layer plated on the base unit  25   a.    
     Referring to  FIG. 3C , first and second grooves  29   a ′ and  29   b , for forming the first and second grooves  29   a  and  29   b  are formed on the circumference surface of the base unit  25   a  of the development roller  25  through component rolling. The machining method of forming the first and second grooves  29   a ′ and  29   b ′ may be any known method. The discussion of the machining method is thus omitted here. The base unit  25   a  has island projections  30 ′ of a predetermined number surrounded by the first and second grooves  29   a ′ and  29   b ′. In the discussion of the specification, the base recess refers to a portion of the base unit  25   a  deeper than half the depth of each of the first and second base grooves  29   a ′ and  29   b ′ and the base projection  30 ′ refers to a projection of the base unit  25   a  externally protruded from half the depth of each of the first and second base grooves  29   a ′ and  29   b′.    
     Referring to  FIGS. 3C and 4 , the top portion of the base projection  30 ′ is a the base flat surface  30   a   1 . The base flat surface  30   a  of the base projection  30 ′ is square if the first and second base grooves  29   a ′ and  29   b ′ have a slant angle of 45.degree. and the same pitch p, and is diamond if the first and second slant base grooves  29   a ′ and  29   b ′ have a slant angle of other than 450 and the same pitch p. The base flat surface  30   a ′ of base projection  30 ′ is rectangular if the first and second base grooves  29   a ′ and  29   b ′ have a slant angle of 45.degree. and different pitches p, and is parallelogrammic if the first and second base grooves  29   a ′ and  29   b ′ have a slant angle of other than 45.degree, and different pitches p. Regardless of the type of quadrilateral of the flat surface  30   a ′, the base flat surface  30   a ′ of the base projection  30 ′ becomes a quadrangular pyramid frustum with four inclined walls. 
     Each of the first and second base grooves  29   a ′ and  29   b ′ has a curved recess surface in a sinusoidal wave configuration along an inclination direction. Each of the four side walls of the quadrangular pyramid frustum of the base projection  30 ′ is continued to the curved recess surface in a sinusoidal wave configuration. The four side walls of the quadrangular pyramid frustum of the base projection  30 ′ are respectively continued to the four side walls of the sinusoidal wave curved recesses at half the depth of the roughness portion. 
     The circumference surface of the base unit  25   a  having the first and second base grooves  29   a ′ and  29   b ′ and the base projections  30 ′ is electroless nickel plated. The surface layer  25   b  is thus formed on the surface of the base unit  25   a . A first and second grooves  29   a  and  29   b  and a projection  30  are formed on the surface layer  25   b  in a configuration similar to the first and second base grooves  29   a ′ and  29   b ′ and the base projection  30 ′. 
     A flat top portion  30   a  having a quadrilateral shape is formed on the projection  30 . With the surface layer  25   b  formed on the base unit  25   a , the top portion  30   a  continued to the first and second grooves  29   a  and  29   b  has a quadrangular pyramid frustum with four inclined side walls. The four side walls of the quadrangular pyramid frustum are respectively continued to the four side walls of the first and second grooves  29   a  and  29   b  having a sinusoidal wave configuration. 
     The development roller  25  has on the surface layer  25   b  at the top portion  30   a  of the projection  30  a high-hardness portion  30   a ″ having hardness higher than surface hardness of the other portions (see  FIG. 4 ). An area of the projection  30  within which the high-hardness portion  30   a ″ is formed (to a depth t from the top surface of the projection  30 ) is set to be within an average particle diameter of the toner in use. The area of the surface layer  25   b  including the first and second grooves  29   a  and  29   b  but excluding the high-hardness portion  30   a ″ provides a toner charging property higher than that of the high-hardness portion  30   a″.    
     The top portion g of the development roller a is relatively heavily worn in a flat configuration while the surface layer c of the recess formation portion f of the first and second grooves is not worn in practice as illustrated in  FIG. 10B . The inventor of the invention has studied this phenomenon by conducting durability tests. The wear trace was measured using Keyence VK-9500 as a three-dimensional measuring laser microscope. The image forming apparatus used in the tests is printer model LP9000C manufactured by Seiko Epson. A development roller  25  to be discussed below was used instead of the original development roller in the printer model LP9000C. Printer model LP9000C was modified to employ the development roller  25 . Image forming conditions in the durability tests were the standard image forming conditions of the printer model LP9000C. 
     Before forming the roughness portion on the base unit  25   a , the base unit  25   a  of the development roller  25 , made of STKM steel, was centerless machined in surface finishing. The first and second base grooves  29   a ′ and  29   b ′ were formed on the base unit  25   a  through component rolling. A nickel-phosphorus (Ni—P) layer is electroless plated to a thickness of 3 μm as the surface layer  25   b  on the base unit  25   a . As illustrated in  FIG. 5A , the development roller  25  was machined as below. In the development roller  25 , the roughness depth (height from the bottom of the grooves  29   a  and  29   b  to the top surface of the projections  30 ) was 6 μm, the roughness pitch was 100 μm, the width of the projection  30  along a line extending at half the roughness depth was 60 μm, and the width of the recess along the half line was 40 μm. 
     The toner feed roller  24 , made of urethane foam, was installed to press against the development roller  25  by an amount of sink of 1.5 mm. The toner regulator member  26  was constructed of a blade made of urethane rubber, and installed to be pressed against the development roller  25  under a pressure of 40 g/cm. 
     Two types of toner were used. A first type of toner was produced by manufacturing polyester particles through a pulverizing process, and by internally dispersing proper amounts of a charge control agent (CCA), a wax, and a pigment with the polyester particles into toner mother particles. Then externally added to the toner mother particles were small silica particles having a size of 20 nm, median silica particles having a size of 40 nm, large silica particles having a size of 100 nm, and titania particles having a size of 30 nm. The process resulted in small size toner having an average diameter D50 of 4.5 μm, and smaller than the roughness depth of 6 μm. A second type of toner was produced by manufacturing styrene acrylate particles through a polymerization process, and by internally dispersing proper amounts of a wax, and a pigment with the styrene acrylate particles into toner mother particles. Then externally added to the toner mother particles were small silica particles having a size of 20 nm, median silica particles having a size of 40 nm, large silica particles having a size of 100 nm, and titania particles having a size of 30 nm. The process resulted in small size toner having an average diameter D50 of 4.5 μm. 
     Durability image forming tests were conducted on A4 size standard sheets using a text pattern having a monochrome image occupancy rate of 5% under the standard image forming condition of the printer model LP9000C. When the first type small size toner was used, the top four side edges of the top portion  30   a  of the surface layer  25   b  at the projection  30  having an initial profile denoted by a solid line in  FIG. 5B  tended to be worn into a flat profile denoted by a dot-and-dash chain line as the number of image forming cycles increased. When the second type small size toner was tested, the projections  30  tended to be worn into a profile similarly curved profile obtained when the first type toner was used. 
     The possible reason why such a curved wear profile occurred is described below. As the development roller  25  rotates in  FIG. 6A , the toner feed roller  24  and the toner regulator member  26  are respectively pressed against the development roller  25 . Toner particles present on the flat surfaces  30   a  of the projections  30  move into the first and second grooves  29   a  and  29   b . Since the average diameter (D50 particle diameter) of the toner particles is smaller than the roughness depth, almost all the toner particles of the toner  28  having moved into the first and second grooves  29   a  and  29   b  are arranged in a plurality of layers. As the development roller  25  further rotates, toner particles present in the first and second grooves  29   a  and  29   b  move onto the flat surfaces  30   a  of the projections  30 . Since the top layer of toner particles is then about at the same level as the flat surface  30   a  of the projection  30 , mainly the toner particles at the top layer out of the toner particles in the first and second grooves  29   a  and  29   b  horizontally move, and most of the remaining toner particles at the lower layers remain stationary. In the course of the movement of the top layer toner particles, the external additive having a relatively high hardness coating the toner mother particles gradually wears the surface of the surface layer  25   b  into a substantially flat state for a long period of time. 
     As  FIG. 3B ,  FIGS. 6A and 6B  are sectional views of the first and second grooves  29   a  and  29   b  taken along a line perpendicular to the running direction (slant angle) of the grooves. The partial sectional views of the development roller  25  are not aligned with the direction of rotation of the development roller  25 . Toner particles on the first grooves  29   a  thus move onto the flat surfaces  30   a  of the projections  30 , and then move to any of the first and second grooves  29   a  and  29   b  adjacent to the projections  30 . Furthermore, toner particles on the second grooves  29   b  move onto the flat surfaces  30   a  of the projections  30 , and then move to any of the first and second grooves  29   a  and  29   b  adjacent to the projections  30 . The toner movement is identical to the other examples of the development roller  25 . 
     A method of manufacturing the development roller  25  having the above-described structure is described below. 
     Referring to  FIG. 7A , the first and second base grooves  29   a ′ and  29   b ′ are formed on the base unit  25   a  through component rolling. Referring to  FIG. 7B , an amorphous surface layer  25   b  is formed through electroless plating on the base unit  25   a  having the first and second base grooves  29   a ′ and  29   b ′. The first and second grooves  29   a  and  29   b  are thus formed in accordance with the first and second base grooves  29   a ′ and  29   b ′. The projection  30  refers to the top portion  30   a  externally protruded from half the depth of each of the first and second grooves  29   a  and  29   b  and the recess refers to a portion of the base unit  25   a  (opposite to the top portion  30   a ) deeper than half the depth of each of the first and second grooves  29   a  and  29   b . Hardness of the surface layer  25   b  is set to be higher than hardness of the base unit  25   a.    
     Referring to  FIG. 7C , the surface layer  25   b  of the top portion  30   a  of the projection  30  is surface-crystallized by heating through ion beam or localized heating. A depth t of the surface-crystallized portion (high-hardness portion  30   a ″) of the surface layer  25   b  is set to be within the toner average particle diameter (D50 particle diameter) of the toner used in the development device  5 ′ containing the development roller  25 . The surface hardness of the surface-crystallized portion (high-hardness portion  30   a ″) of the surface layer  25   b  is set to be higher than surface hardness of the other area of the surface layer  25   b  covering the recess of the first and second grooves  29   a  and  29   b . A toner charging property of the area of the surface layer  25   b  excluding the surface-crystallized portion (high-hardness portion  30   a ″) is higher than a toner charging property of the high-hardness portion  30   a″.    
     Another method of manufacturing the development roller  25  is described below. 
     Referring to  FIG. 8B , an amorphous surface layer  25   b  is formed through electroless plating on the surface of the base unit  25   a . Hardness of the surface layer  25   b  is set to be higher than hardness of the base unit  25   a . Referring to  FIG. 8B , the amorphous surface layer  25   b  is fully crystallized through annealing. The annealing temperature then is 300° C. or higher, but equal to or lower than a thermal processing temperature of the base unit  25   a . Referring to  FIG. 8C , the first and second grooves  29   a  and  29   b  are thus formed on the crystallized surface layer  25   b  on the base unit  25   a  through component rolling. The projection  30  refers to the top portion  30   a  externally protruded from half the depth of each of the first and second grooves  29   a  and  29   b  and the recess refers to a portion of the base unit  25   a  (opposite to the top portion  30   a ) deeper than half the depth of each of the first and second grooves  29   a  and  29   b . The area of the first and second grooves  29   a  and  29   b  on the crystallized surface layer  25   b  is again set to an amorphous state through component rolling. Hardness of the crystallized surface layer  25   b  of the top portion  30   a  becomes higher than hardness of the base unit  25   a . The development roller  25  is thus produced. 
     The development roller  25  of one embodiment of the invention is specifically described below. 
     Before forming the roughness portion on the base unit  25   a , the base unit  25   a  of the development roller  25 , made of STKM steel having an Hv (Vickers hardness) of 150, was centerless machined in surface finishing. A base roughness portion having a depth of 6 μm was formed on the surface of the base unit  25   a  through component rolling. The base recesses  29   a ′ and  29   b ′ (the bottoms of the recesses of the projections  30 ′) were formed in a sinusoidal wave configuration. The base flat surface  30   a ′ of the base projection  30 ′ was formed in a quadrangular pyramid frustum. The four inclined walls of the quadrangular pyramid frustum are formed respectively in continuation with the four walls of the sinusoidal wave recesses  29   a ′ and  29   b ′. Points where the four side walls of the quadrangular pyramid frustum of the base projection  30 ′ meet the four side walls of the sinusoidal wave curved recesses of the first and second grooves  29   a ′ and  29   b ′ are at half the depth of the base roughness portion. 
     A nickel-phosphorus (Ni—P) layer was electroless plated to a thickness of 3 μm as the surface layer  25   b  on the base unit  25   a . The surface hardness of the surface layer  25   b  was an Hv of 550. The surface layer  25   b  of the top portion  30   a  was crystallized to within a depth t of 1.5 μm from the top surface of the projection  30  by heating the surface layer  25   b  with an ion beam directed thereto. The crystallized surface layer  25   b  had an Hv of 1000. More specifically, the high-hardness portion  30   a ″ of the top portion  30   a  was higher in hardness than the remaining area of the surface layer  25   b  excluding the high-hardness portion  30   a″.    
     Tests were conducted to study a toner charging property and a surface potential of the development roller of one embodiment of the invention. The tests included a toner rubbing test to measure a toner charge amount and a surface potential test on a toner transport surface of the development roller. 
     A nickel-phosphorus (Ni—P) layer as a sample plate was electroless plated to a thickness of about 3 μm on an STKM development roller. Surface hardness of the sample plate was an Hv of 550. Another sample plate having the same specification was produced, and then the sample plate was annealed at 400° C. for two hours to crystallize the surface thereof. Surface hardness of the sample plate was an Hv of 1000. It was learned that the annealing process increased the hardness of the surface layer of the sample plate. 
     The first toner previously described was used here. A blade was produced of the same urethane rubber as the one used for the toner regulator blade  26 . The toner was then dispersed on each sample plate, and the urethane rubber blade was rubbed on the toner on each sample plate. An amount of charge of rubbed toner was measured using an electric charge measuring instrument. The rubbing operation was repeated. Each time a predetermined number of rubbing operations was completed, the amount of toner charge was measured.  FIG. 9A  illustrates the toner rubbing test results. As illustrated in  FIG. 9A , the sample plate with the plated layer not annealed provided a higher toner charging property. 
     In the surface potential test of the toner transport surface of the development roller, a test development cartridge was used together with the previously described printer model LP9000C as a test driver. The test development cartridge and the test driver were modified so that the surface of the development roller is viewed. The sample development roller having the 3 μm thick nickel-phosphorus (Ni—P) electroless plated surface layer was produced. Another sample development roller was also produced by performing a 2-hour annealing process at 400° C. 
     The first toner previously described was used here. The test driver with the test development cartridge mounted was operated in an idling mode. Part of the surface of the development roller was exposed by removing the toner on the circumference surface of the development roller. A surface potential meter was set on the development roller. A voltage difference between a toner removal portion and a toner non-removal portion on the development roller was measured with the development roller rotated. The recovery rate along the development roller was determined.  FIGS. 9B and 9C  illustrate the surface potential test results.  FIGS. 9B and 9C  illustrate that a peak indicating a low surface potential periodically appears from the start of driving of the development roller (DR). A portion corresponding to the low surface potential peak is where the toner is removed from a transport surface of the development roller. Generally, the development roller illustrated in  FIG. 9B  free from the annealing process is better in surface potential than the annealed development roller illustrated in  FIG. 9C . More specifically, the annealing process degrades the surface potential recovery property of the toner transport surface of the development roller subsequent to toner image development. 
     The test results show that the surface of the top portion of the projection  30  crystallized through the annealing process increases the hardness thereof, and that the surface of the recess, not annealed, becomes amorphous, and provides a higher toner charging property. 
     In the development roller  25 , the surface hardness of the high-hardness portion  30   a ″ of the top portion  30   a  of the projection  30  in the development roller  25  is set to be higher than the surface hardness of the recess forming the first and second grooves  29   a  and  29   b  excluding the high-hardness portion  30   a ″. In the long service life of image forming of the development roller  25 , the wear of the surface layer  25   b  of the top portion  30   a , typically likely to be worn, is not heavy. A wear difference between the projection and the recess is smaller than in the development roller in the related art. Even after the long service life of image forming, no large change results in the depth of the roughness portion of the development roller  25 . The amount of toner transported to the development roller  25  does not change greatly. An image density level is thus maintained at a generally constant level. The development roller  25  can thus perform the development process for a long period of time. 
     Since the surface hardness of the recess of the development roller  25  is low, filming that is likely to take place in the recess typically having a slow refreshing property is prevented. Although the recess tends to lower the toner in toner charging property because of the distance from the toner regulator blade  26 , the amorphous recess controls a decrease in toner charging property. By setting the toner charging property of the recess to be higher than the toner charging property of the projection, toner charging is effectively performed. Toner coverage and toner splashing are controlled, and excellent development characteristics are provided. 
     In a toner transport method in which toner is not transported to the surface of the projection  30  by the toner regulator blade  26 , a function of the recess for maintaining the toner charging property at the surface of the recess is separated from a function of the projection for maintaining wear proofness on the surface of the projection (maintaining the depth of the roughness portion). The two functions are thus separately performed. 
     The top portion  30   a  of the projection  30 , if crystallized, is lowered in toner charging property. A low toner charging property prevents chargeup from taking place between the toner regulator blade  26  and the projection  30  of the development roller, thereby improving development results. In a toner transport method, toner having a toner particle size smaller than a depth of the roughness portion of the development roller is transported to the recess of the development roller with a front edge of the toner regulator blade placed into contact with the development roller, and the toner is not transported to the projection. In such a toner transport method, the supply of the toner to the projection is more effectively controlled. Filming of the toner on a flat portion of the projection and chargeup of the toner are prevented. 
     The roughness portion of the surface layer  25   b  is constructed of the same material and the degree of crystallization is differentiated between the projection and the recess (for example, the projection is set to be higher in the degree of crystallization than the recess). With this arrangement, the surface hardness and electrical resistance of the projection and recess can be controlled. The surface layer  25   b  at the recess and the projection is not fully crystallized (whether the surface layer  25   b  is fully crystallized or not is determined through x-ray diffraction). The surface composition of the development roller is thus easily set up. Filming (fusion of toner) takes place if the wear of the projection is too small as a result of high hardness thereof. By controlling the degree of crystallization, the generation of filming is controlled. 
     By allowing the surface layer  25   b  at the projection  30  to be heated in a localized fashion, the base unit  25   a  is almost free from crystallization. The base unit  25   a  is thus free from release of stress, and bowing and bending responsive to variations in the degree of crystallization. 
     An area of the projection  30  where crystallization advances is limited to within the range of an average particle diameter (D50 particle diameter) of toner in use from the top surface of the projection  30 . The toner particles transported to the recess that is subject to a decrease in charging property are thus allowed to be in contact with an amorphous recess. This arrangement prevents the toner from being lowered in the charging property. 
     Before forming the roughness portion on the base unit  25   a , the surface layer  25   b  is formed on the base unit  25   a  through electroless plating. Even if a material relatively hard to machine is used for a base unit  25   a , the configuration stability of the roughness portion is improved by the plated surface layer  25   b . The roughness portion has an increased surface smoothness, allowing the toner particles to be moved smoothly. Filming of the toner at the recess is thus controlled. The toner transportability and the toner charging property are excellently maintained. 
     Referring to  FIG. 11A , a mesh-like roughness pattern is formed on the circumference surface of the development roller  25  as on the development roller  25  disclosed in Japanese Unexamined Patent Application Publication No. JP-A-2007-121948. This development roller  25  includes grooves  29  in a predetermined axial area on the circumference thereof as the roughness pattern. The grooves  29  include first grooves  29   a  of a predetermined number continuously spiraling at a predetermined angle with respect to the axial direction of the development roller  25  (the predetermined angle is 45° in  FIG. 11A , but not limited to 45°), and second grooves  29   b  of a predetermined number continuously spiraling at an angle opposite to the slant angle of the first grooves  29   a . The first and second grooves  29   a  and  29   b  are formed at the respective slant angles at a predetermined pitch p with regular interval of W along the axial direction of the development roller  25 . The first and second grooves  29   a  and  29   b  may be different from each other in slant angle and pitch. 
     With reference to  FIG. 11B , the development roller  25  includes a base unit  25   a  made of a metal providing a relatively high hardness, and a single surface layer  25   b  formed on the circumference surface of the base unit  25   a . The base unit  25   a  is a metal sleeve made of an aluminum based metal such as 5056 aluminum alloy or 6063 aluminum alloy, or an iron based metal such as STKM steel. The surface layer  25   b  is a nickel-based or chromium-based layer plated on the base unit  25   a.    
     Referring to  FIG. 11D , first and second grooves  29   a ′ and  29   b ′ for forming the first and second grooves  29   a  and  29   b  are formed on the circumference surface of the base unit  25   a  of the development roller  25  through component rolling. The machining method of forming the first and second grooves  29   a ′ and  29   b ′ may be any known method. The discussion of the machining method is thus omitted here. The base unit  25   a  has island projections  30 ′ of a predetermined number surrounded by the first and second grooves  29   a ′ and  29   b ′. In the specification, the base recess refers to a portion of the base unit  25   a  deeper than half the depth of each of the first and second base grooves  29   a ′ and  29   b ′ and the base projection  30 ′ refers to a projection of the base unit  25   a  externally protruded from half the depth of each of the first and second base grooves  29   a ′ and  29   b′.    
     With reference to  FIGS. 11D and 12A , the top of the base projection  30 ′ is formed at the flat surface  30   a ′. The flat surface  30   a ′ of each the projection  30 ′ is square if the first and second grooves  29   a ′ and  29   b ′ have a slant angle of 45° and the same pitch p, and is diamond if the first and second grooves  29   a ′ and  29   b ′ have a slant angle of other than 45° and the same pitch p. The flat surface  30   a ′ of each the projection  30 ′ is rectangular if the first and second grooves  29   a ′ and  29   b ′ have a slant angle of 45° and different pitches p, and is parallelogrammic if the first and second grooves  29   a ′ and  29   b ′ have a slant angle of other than 45° and different pitches p. Regardless of the type of quadrilateral of the flat surface  30   a ′, the flat surface  30   a ′ of the projection  30 ′ becomes a quadrangular pyramid frustum with four inclined walls. 
     Each of the first and second base grooves  29   a ′ and  29   b ′ has a curved recess surface in a sinusoidal wave configuration along an inclination direction. Each of the four side walls of the quadrangular pyramid frustum of the base projection  30 ′ is continued to the curved recess surface in a sinusoidal wave configuration. The four side walls of the quadrangular pyramid frustum are respectively continued to the four side walls of the sinusoidal wave curved recesses at half the depth of the roughness portion. 
     With reference to  FIGS. 11B and 11C , and  12 A, the circumference surface of the base unit  25   a  has the grooves formed in component rolling. A high-hardness portion  25   a ′ on the circumference surface is hardened through component rolling. The high-hardness portion  25   a , is formed within a substantially constant thickness t 1  from the circumference of the base unit  25   a  and is higher in hardness than the remaining portion of the base unit  25   a.    
     The circumference of the base unit  25   a  having the first and second grooves  29   a ′ and  29   b ′ and the base flat surface  30   a ′ of the base projection  30 ′ (i.e., the surface of the high-hardness portion  25   a ′) is plated with an amorphous metal such as a nickel based electroless plate. The surface layer  25   b  is thus formed on the surface of the base unit  25   a . The surface layer  25   b  is lower in surface hardness than the high-hardness portion  25   a ′ of the base unit  25   a . The thickness t 1  of the surface layer  25   b  is set to be within the range of the toner average particle diameter (D50 particle diameter) of the toner in use. The recesses of the first and second grooves  29   a  and  29   b  and the projection  30  are formed on the surface layer  25   b  similar in shape to the base recesses of the first and second base grooves  29   a ′ and  29   b ′ and the base projection  30 ′. 
     A quadrilateral flat top portion  30   a  is formed on the projection  30 . With the surface layer  25   b  formed on the base unit  25   a , the top portion  30   a  continued to the first and second grooves  29   a  and  29   b  has a quadrangular pyramid frustum with four inclined side walls. The four side walls of the quadrangular pyramid frustum are respectively continued to the four side walls of the first and second grooves  29   a  and  29   b  having a sinusoidal wave configuration. 
     The top portion g of the development roller a is relatively heavily worn in the flat configuration while the surface layer c of the recess formation portion f of the first and second grooves is not worn in practice as illustrated in  FIG. 10B . The inventor of the invention has studied this phenomenon by conducting durability tests. The wear trace was measured using Keyence VK-9500 as a three-dimensional measuring laser microscope. The image forming apparatus used in the tests is printer model LP9000C manufactured by Seiko Epson. A development roller  25  to be discussed below was used instead of the original development roller in the printer model LP9000C. Printer model LP9000C was modified to employ the development roller  25 . Image forming conditions in the durability tests were the standard image forming conditions of the printer model LP9000C. 
     Before forming the roughness portion on the base unit  25   a , the base unit  25   a  of the development roller  25 , made of STKM steel, was centerless machined in surface finishing. The first and second base grooves  29   a ′ and  29   b ′ were formed on the base unit  25   a  through component rolling. A nickel-phosphorus (Ni—P) layer is electroless plated to a thickness of 3 μm as the surface layer  25   b  on the base unit  25   a . As illustrated in  FIG. 13A , the development roller  25  was machined as below. In the development roller  25 , the roughness depth (height from the bottom of the grooves  29   a  and  29   b  to the top surface of the projections  30 ) was 6 μm, the roughness pitch was 100 μm, the width of the projection  30  along a line extending at half the roughness depth was 60 μm, and the width of the recess along the half line was 40 μm. 
     The toner feed roller  24 , made of urethane foam, was installed to press against the development roller  25  by an amount of sink of 1.5 mm. The toner regulator blade  26  was made of urethane rubber, and installed to be pressed against the development roller  25  under a pressure of 40 g/cm. 
     Two types of toner were used. A first type of toner was produced by manufacturing polyester particles through a pulverizing process, and by internally dispersing proper amounts of a charge control agent (CCA), a wax, and a pigment with the polyester particles into toner mother particles. Then externally added to the toner mother particles were small silica particles having a size of 20 nm, median silica particles having a size of 40 nm, large silica particles having a size of 100 nm, and titania particles having a size of 30 nm. The process resulted in small size toner having an average diameter D50 of 4.5 μm, and smaller than the roughness depth of 6 μm. A second type of toner was produced by manufacturing styrene acrylate particles through a polymerization process, and by internally dispersing proper amounts of a wax, and a pigment with the styrene acrylate particles into toner mother particles. Then externally added to the toner mother particles were small silica particles having a size of 20 nm, median silica particles having a size of 40 nm, large silica particles having a size of 100 nm, and titania particles having a size of 30 nm. The process resulted in small size toner having an average diameter D50 of 4.5 μm. 
     Durability image forming tests were conducted on A4 size standard sheets using a text pattern having a monochrome image occupancy rate of 5% under the standard image forming condition of the printer model LP9000C. When the first type small size toner was used, the top four side edges of the top portion  30   a  of the surface layer  25   b  at the projection  30  having an initial profile denoted by a solid line in  FIG. 13B  tended to be worn into a curved profile denoted by a dot-and-dash chain line as the number of image forming cycles increased. When the second type small size toner was tested, the projections  30  tended to be worn into the curved profile similar to that when the first type toner was used. 
     The possible reason why such a curved wear profile occurred is described below. As the development roller  25  rotates in  FIG. 6A , the toner feed roller  24  and the toner regulator member  26  are respectively pressed against the development roller  25 . Toner particles present on the flat surfaces  30   a  of the projections  30  move into the first and second grooves  29   a  and  29   b . Since the average diameter (D50 particle diameter) of the toner particles is smaller than the roughness depth, almost all the toner particles of the toner  28  having moved into the first and second grooves  29   a  and  29   b  are arranged in a plurality of layers. As the development roller  25  further rotates, toner particles present in the first and second grooves  29   a  and  29   b  move onto the flat surfaces  30   a  of the projections  30 . Since the top layer of toner particles is then about at the same level as the flat surface  30   a  of the projection  30 , mainly the toner particles at the top layer out of the toner particles in the first and second grooves  29   a  and  29   b  horizontally move, and most of the remaining toner particles at the lower layers remain stationary. In the course of the movement of the top layer toner particles, the external additive having a relatively high hardness coating the toner mother particles gradually wears the surface of the surface layer  25   b  into a substantially flat state for a long period of time. 
     As  FIG. 11B ,  FIGS. 6A and 6B  are sectional views of the first and second grooves  29   a  and  29   b  taken along a line perpendicular to the running direction (slant angle) of the grooves. The partial sectional views of the development roller  25  are not aligned with the direction of rotation of the development roller  25 . Toner particles on the first grooves  29   a  thus move onto the flat surfaces  30   a  of the projections  30 , and then move to any of the first and second grooves  29   a  and  29   b  adjacent to the projections  30 . Furthermore, toner particles on the second grooves  29   b  move onto the flat surfaces  30   a  of the projections  30 , and then move to any of the first and second grooves  29   a  and  29   b  adjacent to the projections  30 . The toner movement is identical to the other examples of the development roller  25 . 
     The development roller  25  is used with the surface layer  25   b  formed on the base flat surface  30   a ′ of the base projection  30 ′ as illustrated in  FIG. 12 . As the development roller  25  is used in image forming for a long period of time, the surface layer  25   b  on the base flat surface  30   a ′ is worn, and the base flat surface  30   a ′ of the base projection  30 ′ is then exposed as illustrated in  FIGS. 11C and 12B . The base flat surface  30   a ′ is set to be higher in surface hardness than surface layer  25   b  at the first and second grooves  29   a  and  29   b  (i.e., the recess of the surface layer  25   b ) through work hardening. If the base flat surface  30   a ′ of the base projection  30 ′ is exposed, the wear rate of the projection  30  of the development roller  25  against the toner regulator blade  26 , the toner feed roller, the toner external additive, etc. is decreased. The durability of the development roller  25  is increased. If the surface layer  25   b  at the base flat surface  30   a ′ is eliminated, the depth of the roughness portion of the development roller  25  changes slightly. However, since the wearing of the exposed base flat surface  30   a ′ is controlled, the wear rate of the projection  30  is reduced. As a result, a change in the depth of the roughness portion of the development roller  25  is controlled for a long period of time. 
     One method of manufacturing the development roller  25  is described below. 
     Referring to  FIG. 14A , the base unit  25   a  is component rolled to form the first and second base grooves  29   a ′ and  29   b ′. The high-hardness portion  25   a ′ is formed on the circumference of the base unit  25   a  through work hardening in the groove formation. Referring to  FIG. 14B , an amorphous surface layer  25   b  is formed through electroless plating on the surface of the base unit  25   a . The first and second grooves  29   a  and  29   b  are formed in accordance with the first and second grooves  29   a ′ and  29   b ′. The projection  30  refers to the top portion  30   a  externally protruded from half the depth of each of the first and second grooves  29   a  and  29   b  and the recess refers to a portion of the base unit  25   a  (opposite to the top portion  30   a ) deeper than half the depth of each of the first and second grooves  29   a  and  29   b . The high-hardness portion  25   a ′ of the base unit  25   a  is set to be higher in surface hardness than the surface layer  25   b . The surface hardness of the high-hardness portion  25   a ′ of the base unit  25   a  is set to be higher than the surface hardness of the surface layer  25   b . The development roller  25  of  FIG. 14A  having the surface layer  25   b  at the base flat surface  30   a ′ of the base projection  30 ′ thus results. As the surface layer  25   b  at the base flat surface  30   a , of the base projection  30 ′ is worn and exposed in the course of long service life of the development roller  25 , the base flat surface  30   a ′ of the base projection  30 ′ is also exposed as illustrated in  FIG. 12B . 
     The formation of the surface layer  25   b  on the base flat surface  30   a ′ of the development roller  25  illustrated in  FIG. 12A  is optional. The development roller  25  may be used with the surface layer  25   b  of  FIG. 12A  removed from the base projection  30 ′ and the base flat surface  30   a ′ exposed as illustrated in  FIG. 12B . The surface layer  25   b  on the base flat surface  30   a ′ may be removed through one of a known grinding process using a grinding machine and a known polishing process using a polishing machine. 
     The development roller  25  of one embodiment of the invention is specifically described below. 
     Before forming the roughness portion on the base unit  25   a , the base unit  25   a  of the development roller  25 , made of steel use stainless (SUS) steel having an Hv (Vickers hardness) of 250, was centerless machined in surface finishing. A base roughness portion having a depth of 8 μm was formed on the surface of the base unit  25   a  through component rolling. The base recesses  29   a ′ and  29   b ′ (the bottoms of the recesses of the projections  30 ′) were formed in a sinusoidal wave configuration. The base flat surface  30   a ′ of the base projection  30 ′ was formed in a quadrangular pyramid frustum. The four inclined walls of the quadrangular pyramid frustum are respectively formed in continuation with the four walls of the sinusoidal wave recesses  29   a ′ and  29   b ′. Points where the four side walls of the quadrangular pyramid frustum of the base projection  30 ′ meet the four side walls of the sinusoidal wave curved recesses of the first and second grooves  29   a ′ and  29   b ′ are at half the depth of the base roughness portion. Since the SUS steel as a material of the base unit  25   a  had a relatively large degree of work hardening, the surface hardness of the base unit  25   a  subsequent to component rolling was an Hv of 700. 
     A nickel-phosphorus (Ni—P) layer was electroless plated to a thickness t 1  of about 1.5 μm as the surface layer  25   b  on the base unit  25   a . The surface hardness of the surface layer  25   b  was an Hv of 500. The development roller  25  was thus obtained. 
     Durability tests similar to those described were conducted on the development roller  25 . The flat surface  30   a ′ made of the SUS steel was exposed as illustrated in  FIG. 7C , and it was verified that the wearing thereafter was controlled. 
       FIGS. 15A and 15B , respectively similar to partially expanded sectional views of  FIGS. 12A and 12B , illustrate a development roller  25  in accordance with another embodiment of the invention. 
     In the preceding example of the development roller  25  of  FIGS. 12A and 12B , the surface layer  25   b  is a single layer. Referring to  FIG. 15A , the development roller  25  includes a first surface layer  25   b ′ and a second surface layer  25   b ″. The first surface layer  25   b ′ is formed on the circumference of the base unit  25   a  and the second surface layer  25   b ″ is formed on the circumference of the first surface layer  25   b ′. A thickness of t 2  of the first surface layer  25   b ′ is set to be larger than a thickness of t 3  of the second surface layer  25   b ″. In this case, the thickness t 3  of the second surface layer  25   b ″ is set to be within the range of the toner average particle diameter (D50 particle diameter) of the toner in use. The surface hardness of the first surface layer  25   b ′ immediately inside the second surface layer  25   b ″ as the outermost layer is set to be higher than the surface hardness of the second surface layer  25   b ″. The toner charging property of the second surface layer  25   b ″ is set to be higher than the toner charging property of the first surface layer  25   b ′ immediately inside the second surface layer  25   b″.    
     It is not necessary that the base unit  25   a  of the development roller  25  be made of a metal having high hardness as a result of work hardening. Alternatively, as previously discussed, the base unit  25   a  may be made of a metal having high hardness. 
     The rest of the structure of the development roller  25  remains unchanged from the one previously discussed. The development roller  25  may be used in the development device  5 ′ and the image forming apparatus  1 . 
     The development roller  25  is used with the second surface layer  25   b ″ formed at the base flat surface  30   a ′ of the base projection  30 ′ as illustrated in  FIG. 15A . As the development roller  25  is used in image forming for a long period of time, the second surface layer  25   b ″ on the base flat surface  30   a ′ is worn, and the flat surface  30   a ″ of the first surface layer  25   b ′ at the base flat surface  30   a ′ is then exposed as illustrated in  FIG. 15B . The first surface layer  25   b ′ is higher in surface hardness than the second surface layer  25   b ″ at the first and second grooves  29   a  and  29   b  (i.e., the recess of the development roller  25 ). If the flat surface  30   a ″ of the first surface layer  25   b ′ at the base flat surface  30   a ′ is exposed, the wear rate of the projection  30  of the development roller  25  against the toner regulator blade  26 , the toner feed roller, the toner external additive, etc. is decreased. The durability of the development roller  25  is increased. If the second surface layer  25   b ″ at the base flat surface  30   a ′ is eliminated, the depth of the roughness portion of the development roller  25  changes slightly. However, since the wearing of the exposed the first surface layer  25   b ′ is controlled, the wear rate of the projection  30  is reduced. As a result, a change in the depth of the roughness portion of the development roller  25  is controlled for a long period of time. The surface layer  25   b  is not limited to two layers, but may include three or more layers. In such a case, the surface hardness of a layer immediately inside the outermost layer of the surface layer  25   b  is set to be higher in surface hardness than the outermost layer. 
     In the manufacture of the development roller  25  having the above-described structure, an amorphous metal is electroless plated as the first surface layer  25   b ′ on the circumference of the base unit  25   a  having the roughness portion. The first surface layer  25   b ′ is annealed in a heat treatment process for crystallization. The hardness of the first surface layer  25   b ′ is thus increased. Crystallization is analyzed through x-ray diffraction. An amorphous metal or a crystallized metal is electroless plated on the circumference of the first surface layer  25   b ′ as the second surface layer  25   b ″. If an amorphous metal is used for the second surface layer  25   b ″, the second surface layer  25   b ″ is set to be more amorphous than the first surface layer  25   b ′ by varying the temperature of a plating bath and the composition of metals contained in the plating bath. The rest of the manufacturing method is substantially identical to the manufacturing method of the development roller  25  illustrated in  FIGS. 14A-14C . This the development roller  25  is also used with the second surface layer  25   b ″ formed on the base flat surface  30   a ′. When the second surface layer  25   b ″ at the base flat surface  30   a ′ of the base projection  30 ′ is worn and eliminated in the long service life of the development roller  25 , the base flat surface  30   a ′ of the base projection  30 ′ is exposed as illustrated in  FIG. 15B . 
     It is not necessary that the second surface layer  25   b ″ be formed on the base flat surface  30   a ′ of the base projection  30 ′ as illustrated in  FIG. 15A . More specifically, the development roller  25  may be used with the second surface layer  25   b ″ illustrated in  FIG. 15A  on the base flat surface  30   a ′ removed and with the first surface layer  25   b ′ illustrated in  FIG. 15B  on the base flat surface  30   a , exposed. The second surface layer  25   b ″ may be removed through one of a known grinding process using a grinding machine and a known polishing process using a polishing machine. 
     The development roller  25  of one embodiment of the invention is specifically described below. 
     Before forming the roughness portion on the base unit  25   a , the base unit  25   a  of the development roller  25 , made of STKM steel having an Hv (Vickers hardness) of 150, was centerless machined in surface finishing. A base roughness portion having a depth of 8 μm was formed on the surface of the base unit  25   a  through component rolling. The base recesses  29   a ′ and  29   b ′ (the bottoms of the recesses of the projections  30 ′) were formed in the same manner as previously discussed. 
     An amorphous nickel-phosphorus (Ni—P) layer was electroless plated to a thickness t 2  of 3 μm as the first surface layer  25   b ′. The first surface layer  25   b ′ was annealed at 400° C. for crystallization. The surface hardness of the first surface layer  25   b ′ was an Hv of 1000. An amorphous nickel-phosphorus (Ni—P) layer was electroless plated to a thickness t 3  of 1.5 μm as the second surface layer  25   b ″ on the first surface layer  25   b ′. The surface hardness of the second surface layer  25   b ″ was an Hv of 500. The development roller  25  was thus obtained. 
     Durability tests similar to those previously described were conducted on the development roller  25 . The flat surface  30   a ′ made of the SUS steel was exposed as illustrated in  FIG. 14C , and it was verified that the wearing thereafter was controlled. 
     Tests were conducted on the toner charging property and the surface potential of the development roller of one embodiment of the invention. The tests included a toner rubbing test to measure a toner charge amount and a surface potential test on a toner transport surface of the development roller. 
     A nickel-phosphorus (Ni—P) layer as a sample plate was electroless plated to a thickness of 3 μm on an STKM development roller. Surface hardness of the sample plate was an Hv of 550. Another sample plate having the same specification was produced, and then the sample plate was annealed at 400° C. for two hours to crystallize the surface thereof. Surface hardness of the sample plate was an Hv of 1000. It was learned that the annealing process increased the hardness of the surface layer of the sample plate. 
     The first type of toner previously discussed was used here. A blade was produced of the same urethane rubber as the one used for the toner regulator blade  26 . The toner was then dispersed on each sample plate, and the urethane rubber blade was rubbed on the toner on each sample plate. An amount of charge of rubbed toner was measured using an electric charge measuring instrument. The rubbing operation was repeated. Each time a predetermined number of rubbing operations was completed, the amount of toner charge was measured.  FIG. 9A  illustrates the toner rubbing test results. As illustrated in  FIG. 9A , the sample plate with the plated layer not annealed provided a higher toner charging property. 
     In the surface potential test of the toner transport surface of the development roller, a test development cartridge was used together with the previously described printer model LP9000C as a testing device. The test development cartridge and the test device were modified so that the surface of the development roller is viewed. The sample development roller having the 3 μm thick nickel-phosphorus (Ni—P) electroless plated surface layer was produced. Another sample development roller was also produced by performing a 2-hour annealing process at 400° C. in the same manner as previously described. 
     The first type of toner previously discussed was used here. The testing device with the test development cartridge mounted was operated in an idling mode. Part of the surface of the development roller was exposed by removing the toner on the circumference surface of the development roller. A surface potential meter was set on the development roller. A voltage difference between a toner removal portion and a toner non-removal portion on the development roller was measured with the development roller rotated. The recovery rate of the development roller was determined.  FIGS. 9B and 9C  illustrate the surface potential test results.  FIGS. 9B and 9C  illustrate that a peak indicating a low surface potential periodically appears from the start of driving of the development roller (DR). A portion corresponding to the low surface potential peak is where the toner is removed from a transport surface of the development roller. Generally, the development roller illustrated in  FIG. 9B  free from the annealing process is better in surface potential than the annealed development roller illustrated in  FIG. 9C . More specifically, the annealing process degrades the surface potential recovery property of the toner transport surface of the development roller subsequent to toner image development. 
     The test results show that the surface of the top portion of the projection  30  crystallized through the annealing process increases the hardness thereof, and that the surface of the recess, not annealed, becomes amorphous, and provides a higher toner charging property. 
     If a single surface layer  25   b  is formed on the base unit  25   a  of the development roller  25 , the surface hardness of the base unit  25   a  is set to be higher than the surface hardness of the surface layer  25   b  as the outermost layer. If a plurality of surface layers  25   b  are formed on the base unit  25   a , the surface hardness of the first surface layer  25   b ′ immediately inside the second surface layer  25   b ″ is set to be higher than the surface hardness of the second surface layer  25   b ″. In the service life of image forming of the development roller  25 , one of the first surface layer  25   b ′ at the base flat surface  30   a ′ of the base projection  30 ′ and the second surface layer  25   b ″ at the base flat surface  30   a ′ is worn by the toner regulator blade  26 , the toner feed roller, the toner external additive, etc. When one of the base flat surface  30   a ′ and the first surface layer  25   b ′ is exposed, the wear rate of the projection  30  of the development roller  25  is decreased. The durability of the development roller  25  is thus increased. 
     If one of the surface layer  25   b  and the second surface layer  25   b ″ at the base flat surface  30   a ′ is eliminated, the depth of the roughness portion of the development roller  25  changes slightly. However, the wearing of one of the exposed base flat surface  30   a ′ and the exposed first surface layer  25   b ′ is controlled. As a result, a change in the depth of the roughness portion of the development roller  25  is controlled for a long period of time. The amount of toner transported to the development roller  25  does not change greatly. An image density level is thus maintained at a generally constant level. The development roller  25  can thus perform the development process for a long period of time. 
     Although the toner charging property is lowered by one of the exposed top portion  30   a  and the exposed first surface layer  25   b ′ at the projection  30 , toner particles pinched between the development roller  25  and the toner regulator blade  26  result in stronger frictional force than that at the recess. A decrease in the toner charging property is controlled accordingly. Toner coverage and toner splashing are controlled, and excellent development characteristics are provided. 
     In a toner transport method in which toner is not transported to the surface of the projection  30  with a toner regulator blade  26 , a function of the recess for maintaining the toner charging property at the surface of the recess is separated from a function of the projection for maintaining wear proofness on the surface of the projection (maintaining the depth of the roughness portion). The two functions are thus separately performed. 
     The thickness of one of the surface layer  25   b  and the second surface layer  25   b ″ is set to be within the range of an average particle diameter (D50 particle diameter) of the toner in use. The toner transported to the recess subject to a decrease in the charging property is placed into contact with the amorphous recess. A decrease in the toner charging property is thus controlled. 
     One of the surface layer  25   b  and the second surface layer  25   b ″ may be removed through a grinding process of a grinding machine or a polishing process of a polishing machine. If the development roller  25  having the exposed the base flat surface  30   a ′ of the base projection  30 ′ of the base unit  25   a  or the exposed first surface layer  25   b ′ at the base flat surface  30   a ′ is used from the start, the same operation and advantages previously described may be provided. 
     The development device  51  containing the development roller  25  can develop toner images on the latent image bearing unit in accordance with the electrostatic latent images for a long period of time. The image forming apparatus  1  containing the development device  5 ′ can provide stable and excellent-quality images for a long period of time. 
     The number and pitch of the second grooves  29   b  may or may not be identical to the number and pitch of the first grooves  29   a . The number of first grooves  29   a  may be 1 or more, and the number of second grooves  29   b  may be 1 or more. 
     The toner particles are coated with silica having a relatively high hardness as an external additive with the silica coverage ratio to the toner mother particles being 100% or more. Silica is abundant in the surface of the toner mother particles. This causes a relatively high wear rate in the surface layer  25   b  of the projection  30 . Even if the development roller  25  is used in the development device  5 ′ that uses the toner having a silica coverage rate of 100% or more, the durability of the development roller  25  is still effectively increased. 
     The base recesses of the first and second grooves  29   a ′ and  29   b ′ are not limited to the sinusoidal wave configuration. The base recesses may be curved or may be an inverted quadrangular pyramid frustum with a flat top surface. In such a case, the inverted quadrangular pyramid frustum may be continued to a quadrangular pyramid frustum of the base projection at inflection points thereof (at positions about half the depth of the base roughness). 
     In the above-described embodiments, the invention is applied to the image forming apparatus  1  containing the rotary development unit  5 . The invention is not limited to the image forming apparatus  1 . The invention is applicable to image forming apparatuses including a development device with the development roller having at least a roughness portion. Such image forming apparatuses include an image forming apparatus having an image forming units arranged in tandem, a four-cycle image forming apparatus, a monochrome image forming apparatus, and an image forming apparatus that directly transfers a toner image to a transfer material (transfer medium of one embodiment of the invention) from an image bearing unit (i.e., an image forming apparatus having no intermediate transfer medium). The invention is applicable to any image forming apparatus falling within the scope defined by the claims.