Contact charging member, image forming unit including the contact charging member and electrophotographic image forming apparatus including the image forming unit

Abstract: A contact charging roller (11) which is in contact with and electrically charges a photosensitive drum (16) is provided on a center shaft (11-1), and includes a roller-shaped conductive foam material portion (11-2) covering the shaft, and a tubular-shaped conductive resin material portion (11-3) covering the foam portion. The foam portion has an electric resistance of 9 × 10 5 Ω or less, the resin portion includes conductive agents and has a surface electric resistance in a range from 1 × 10 5 Ω/□ or more to 9 × 10 7 Ω/□ or less, and a hardness of the charging roller as a whole is Asker F 90° or less. An image forming unit includes the charging member and the drum. An electrophotographic image forming apparatus supplies a recording paper to the unit, transfers an image from the drum to the paper, and discharges the image-transferred paper.

Description: This invention relates to a contact charging member which electrically charges a member, an image forming unit including the contact charging member, and an electrophotographically image forming apparatus including the image forming unit, the apparatus including a printer, a copier, and the like. A corona discharger have been widely used as an electric charger for charging an image carrier, for example a photosensitive drum of the image forming unit of the electrophotographically image forming apparatus, uniformly. Recently, the corona discharger, however, is not widely used because it generates a lot of ozone harmful to a working environment and has large outer sizes so that it increases outer sizes of the unit and finally those of the apparatus and obstructs to provide a compact office automation equipment which is a modern trend. Instead of the corona discharger, a contact charging brush and a contact charging roller have come into use. FIG. 7A shows a conventional charging brush 1 which is known from U.S.P. No. 5,294,962 and FIG. 7B shows a conventional charging roller 4 which is known from Japanese Patent No. 2,632,899. The charging brush 1 is formed by sticking a base cloth 1b to a conductive base plate 1d with a conductive adhesive 1c, and the base cloth 1b has a large number of conductive bristles 1a planted thereon. A high-voltage power supply 2 applies a high voltage, for example about 1 kv, to the conductive base plate 1d and the conductive bristles 1a of the base cloth 1b bring their tip ends into contact with a circular surface of a photosensitive drum 3 so that the surface of the drum is charged uniformly. The charging roller 4 is formed by providing a conductive rubber layer 4b being relatively hard on a metal rod 4a and further providing a relatively high-resistance surface layer 4c on an outer surface of the rubber layer 4b. The metal rod 4a is applied with a high voltage by the high-voltage power supply 2, and a bottom end of a circular surface of the charging roller 4 is pressed against the circular surface of the drum 3 so that the circular surface of the drum is charged uniformly. According to the above described Japanese Patent, the hardness of the charging roller 4 is indicated by Asker C 54.5°, the surface layer 4c is formed by coating nylon resin to 10 µ m in thickness, and a volume resistivity of the charging roller 4 is 10 9 to 10 10 Ω·cm. The charging brush has such a problem that it tends to become dirty with dust or remaining toner attached on the surface of the photosensitive drum, and the attached dust or toner is hard to be cleaned up. Further, the dirty charging brush sometimes presents an unstable charging characteristic. For this reason, the charging roller have now begun to be attracted attentions of those who work in this technical field. US Patent No. 5,475,473 discloses a roller-shaped contact charging member in which the surface of the conductive foam material portion has a volume resistivity of 10 to 10 10 Ω cm and is covered with conductive resin material However, this publication does not mention the hardness of the roller-shaped contact charging member. In order to obtain a good image by using the charging roller, pressure applied to both ends of the metal rod must be uniformly distributed among a whole of the rod along its longitudinal direction so that the roller is pressed against the photosensitive drum uniformly. If there is a slight nonuniformity of charging on the drum, it causes a distinctive irregularity in a toner image formed on the photosensitive drum and lowers a quality of the image because a resolution of an image to be formed have been improved in recent years and toner particles have been made much finer. In order to uniformly distribute the pressure applied to the both ends of the metal rod among the whole of it in such a charging roller having the relatively hard conductive rubber layer, an outer diameter of the metal rod must be made large. The largest image formed by a conventional desktop type image forming apparatus corresponds to a paper sheet of A4 size (210 mm × 297 mm) defined by JIS (Japanese Industrial Standard) P0138. Recently, however, it is strongly demanded to form an image corresponding to a paper sheet of A3 size (two times as large as the A4 size) defined by JIS P0138, by the desktop type image forming apparatus. To meet this demand, the photosensitive drum must be made longer and consequently the charging roller must be larger. This makes the outer diameter of the metal rod and that of the charging roller larger, further makes the outer sizes of the image forming unit and those of the image forming apparatus larger. As a result of this, in a modern desktop type image forming apparatus in which an image forming unit is rotatably separable from the remaining members of the apparatus in a vertical direction to perform maintenance of the apparatus easily, it becomes difficult to open the image forming unit. To prevent the outer sizes of the unit and those of the apparatus from becoming larger, the inventors of the present invention prepared such a small charging roller which had an outer diameter of 10 mm and a surface hardness of Asker C 45°, a metal rod of which had a diameter of 6 mm and a rubber layer of which had a thickness of 2 mm. And, the inventors operated an image forming apparatus provided with an image forming unit using the small charging roller. As a result of this operation, the small charging roller could not be substantially uniformly pressed against the photosensitive drum. Specifically, when a force pressing the charging roller against the circular surface of the photosensitive drum was increased, the charging roller bent to separate its longitudinal center portion from the surface of the photosensitive drum so that an image formed on a recording medium, for example a paper sheet, by the apparatus had a defect at its portion corresponding to a longitudinal center portion of the surface of the drum from which the longitudinal center portion of the bent small charging roller was separated. Contrary to this, when the force pressing the charging roller against the circular surface of the photosensitive drum was decreased, the image formed on the sheet by the apparatus had many white spots. This invention derived from the above described circumstances, and an object of the present invention is to provide a contact charging member which can not only surely charge a member uniformly but also be compact and has a long life time, an image forming unit including the contact charging member, and an electrophotographic image forming apparatus including the image forming unit. In order to achieve the above described object, a contact charging member of this invention is provided on a center member and comprises a conductive foam material portion covering the center member, and a conductive resin material portion covering the conductive foam material portion and being used to be in contact with a member to be charged, the conductive foam material portion having an electric resistance of 9 × 10 5 Ω or less, the conductive resin material portion including conductive agents and having a surface electric resistance in a range from 1 × 10 5 Ω/□ to 9 × 10 7 Ω/□ , and a hardness of the contact charging member being Asker F 90° or less. In the contact charging member of the present invention characterized by the above configuration, the conductive foam material portion may have a roller-shape, the conductive resin material portion may have a tubular-shape, and the contact charging member may have a roller-shape. The member to be charged may be an image carrier including a surface having an image formation area on which an image is formed and image-free areas arranged in both outsides of the image formation area, the tubular-shaped conductive resin material portion preferably has both end areas corresponding to the image-free areas of the image carrier and preferably is fixed at the both end areas to the roller-shaped conductive foam material portion by an adhesive. To achieve the above described object of the present invention, an image forming unit of the present invention comprises an image carrier and a contact charging member which is in contact with a surface of the image carrier and electrically charges the surface of the image carrier uniformly, the contact charging member being provided on a center member and including a conductive foam material portion which covers the center member, and a conductive resin material portion which covers the conductive foam material portion and is in contact with the surface of the image carrier, the conductive foam material portion having an electric resistance of 9 × 10 5 Ω or less, the conductive resin material portion including conductive agents and having a surface electric resistance in a range from 1 × 10 5 Ω/□ to 9 × 10 7 Ω/□, and a hardness of the contact charging member being Asker F 90° or less. In the image forming unit of the present invention characterized by the above described configuration, the conductive foam material portion may have a roller-shape, the conductive resin material portion may have a tubular-shape, and the contact charging member may have a roller-shape. The image carrier may includes a surface having an image formation area on which an image is formed and image-free areas arranged in both outsides of the image formation area, the tubular-shaped conductive resin material portion preferably has both end areas corresponding to the image-free areas of the image carrier and preferably is fixed at the both end areas to the roller-shaped conductive foam material portion by an adhesive. The image carrier preferably have a drum shape and is rotatably driven, and the roller-shaped contact charging member preferably bring the conductive resin material portion into contact with the surface of the image carrier so that the charging member may be rotated by the rotation of the image carrier. In the image forming unit of the present invention characterized by the above described configuration, the image carrier includes an aluminum drum having an anodized aluminum layer provided on an outer surface thereof and a photosensitive material layer provided on the anodized aluminum layer. To achieve the above described object of the present invention, an electrophotographic image forming apparatus of the present invention comprises an image carrier which includes a photosensitive drum having a circular surface, a roller-shaped contact charging member which is in contact with the circular surface of the photosensitive drum and electrically charges the surface of the drum uniformly, an exposure device which exposes the uniformly charged surface of the drum with a desired image, a developing device which develops the exposed desired image on the surface of the drum, a recording medium supply device which supplies a recording medium toward the developed desired image on the surface of the drum, a transfer device which transfers the developed desired image from the surface of the drum to the supplied recording medium, and a recording medium discharge device which discharges the recording medium with the transferred developed desired image from the surface of the drum, the roller-shaped contact charging member being provided on a center member and including a roller-shaped conductive foam material portion which covers the center member and has an electric resistance of 9 × 10 5 Ω or less, and a tubular-shaped conductive resin material portion which covers the conductive foam material portion, is in contact with the surface of the photosensitive drum, includes conductive agents and has a surface electric resistance in a range from 1 × 10 5 Ω/□ to 9 × 10 7 Ω/□, and the hardness of the contact charging member being Asker F 90° or less. In the electrophotographic image forming apparatus of the present invention characterized by the above described configuration, the surface of the photosensitive drum has an image formation area on which an image is formed and image-free areas arranged in both outsides of the image formation area, the tubular-shaped conductive resin material portion of the roller-shaped contact charging member may have both end areas corresponding to the image-free areas of the drum and may be fixed at the both end areas to the roller-shaped conductive foam material portion of the roller-shaped contact charging member by an adhesive. In the electrophotographic image forming apparatus of the present invention characterized by the above described configuration, the photosensitive drum is rotatably driven, and the roller-shaped contact charging member may be rotated by the rotation of the photosensitive drum through friction produced between the surface of the conductive resin material portion of the roller-shaped contact charging member and that of the photosensitive drum. In the electrophotographic image forming apparatus of the present invention characterized by the above described configuration, the photosensitive drum preferably includes an anodized aluminum layer provided on an outer surface thereof and a photosensitive material layer provided on the anodized aluminum layer. The invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which; FIG. 1A is a side view schematically showing a whole structure of an image forming unit according to an embodiment and provided with a contact charging roller as a contact charging member according to an embodiment of the present invention; FIG. 1B is an enlarged cross sectional view schematically showing a cross section of the contact charging roller and that of a portion of a photo-sensitive drum against which the roller is pressed, the drum being an image carrier as a member to be charged by the contact charging member of the image forming unit shown in FIG. 1A; FIG. 2 is a schematic view showing an apparatus for measuring a surface resistance of the contact charging roller of FIG. 1A; FIG. 3A is an enlarged longitudinal sectional view schematically showing an interengagement relationship between the contact charging roller and the photo-sensitive drum, the roller and the drum being provided in the image forming unit of the embodiment, and the roller being used as the contact charging member according to one embodiment of the present invention; FIG. 3B is a schematic side view of the roller and the drum shown in FIG. 3A; FIG. 3C is an enlarged longitudinal sectional view schematically showing a contact relationship between the contact charging roller and the photosensitive drum, the roller and the drum being provided in an image forming unit according to a modification of the present invention, and the roller being used as the contact charging member according to one embodiment of the present invention; FIG. 4A is a table in which changes in an image forming quality performed by the image forming unit using the contact charging roller as the contact charging member according to one embodiment of the present invention, and those performed by the conventional image forming unit using the conventional contact charging roller as the contact charging member, are shown, those changes being caused by changing an amount of bite of the contact charging roller to the photosensitive drum in each of the unit of the present invention and the conventional unit; FIG. 4B is a table in which changes in the image forming quality performed by the image forming unit using the contact charging roller as the contact charging member according to one embodiment of the present invention is shown, those changes being caused by changing a resistance of a conductive foam material portion covering a center rod member in the contact charging roller and by changing a surface resistance of a tube of a conductive resin material portion covering the foam material portion in the contact charging roller; FIG. 5 is a schematic longitudinal sectional view showing a method for fixing the tube-shaped conductive resin material portion to the roller-shaped conductive foam material portion covering the center rod in the contact charging roller as the contact charging member according to one embodiment of the present invention; FIG. 6A is a schematic longitudinal sectional view of an electrophotographic image forming apparatus, the apparatus being provided with a plurality of image forming units each including the contact charging roller as the contact charging member according to one embodiment of the present invention; FIG. 6B is an enlarged schematic longitudinal sectional view of one of the image forming units shown in FIG. 6A; FIG. 7A is an enlarged schematic sectional view of a conventional brush-shaped contact charging member in a state that it is in contact with a circular surface of a photosensitive drum as a member to be charged by the brush-shaped contact charging member; and FIG. 7B is an enlarged schematic sectional view of a conventional roller-shaped contact charging member in a state that it is in contact with a circular surface of a photosensitive drum as a member to be charged by the roller-shaped contact charging member. A contact charging roller as a contact charging member according to one embodiment of the present invention, image forming units using the contact charging roller of the one embodiment of the present invention and using that of a modification thereof, and an image forming apparatus using a plurality of the units will be described in detail with reference to FIGS. 1A to 6B in the accompanying drawings. At first, the electrophotographic image forming apparatus and the image forming unit will now be described in detail with reference to FIGS. 6A and 6B. FIG. 6A schematically shows a longitudinal sectional view of an electrophotographic color printer as the image forming apparatus according to one embodiment of the present invention. The printer 31 includes a manual paper sheet feed tray 32a provided to swing between a vertical closed position and a horizontal open position in a front wall of an outer housing of the printer 31, and an additional paper sheet discharge tray 32b provided to swing between a vertical closed position and a horizontal open position in a rear wall of the outer housing. A paper sheet cassette 33 is detachably provided in a bottom portion of the housing, and a large number of paper sheets P, each having a predetermined size, are stacked up on a bottom plate 33-1 urged upward by an urging member in the cassette. An upper wall of the housing is structured as a top cover 34 which is swingable upwardly and downwardly around its rear end between a horizontal closed position shown in FIG. 6A and a vertical open position. On a front end portion of an upper surface of the top cover 34, a power switch, a liquid-crystal display device, input keys, etc. (those of which are not shown) are provided. A rear end portion of the top cover 34 constitutes a main paper sheet discharge tray 35 which is inclined downwardly toward the rear end. A paper sheet conveyor belt 36 extending substantially horizontally in a back and forth directions is arranged in a center of the inner space of the housing. The conveyor belt 36 is stretched between a driving roller 37 and a driven roller 38 arranged at two predetermined positions spaced apart from each other in the back and forth directions in the inner space, and is driven by the driving roller 37 to be circulated in an anti-clockwise direction as shown by an arrow D in FIG. 6A. Four image forming units PU are arranged at equal intervals along an upper extending portion of the belt 36 in the inner space of the outer housing. The four image forming units PU are the same in their structures as to each other, and each of the units includes a photosensitive drum 16 which is an image carrier as a member to be charged uniformly by a contact charging roller 11 as a contact charging member described later. FIG. 6B shows an enlarged longitudinal sectional view of one of the image forming units PU. As shown in FIG. 6B, the image forming unit includes a cleaner 41, a contact charging roller 11, a writing head 43, and a developer 44 those of which are arranged around the drum 16 with excepting a position corresponding to a lower end of the circular surface of the drum 16. The cleaner 41, the contact charging roller 11, the writing head 43, and the developer 44 are arranged in this order around the drum 16 in the anti-clockwise direction in FIG. 6B, and are, excluding the writing head 43, supported by a common unit frame, together with the photosensitive drum 16. The four writing heads 43 of the four image forming units PU are fixed at four predetermined positions on a lower surface of the top cover 34 of the housing, and move upwardly and downwardly by the swing of the top cover 34 upwardly and downwardly. When the top cover 34 is swung upwardly from the closed position shown in FIG. 6A to the open position, the four writing heads 43 are moved upwardly from their predetermined positions in the four units, each of the predetermined positions being located between the contact charging roller 11 and the developer 44 around the drum 16 corresponding thereto, as shown by two dots-chain line in FIG. 6B. With such a structure, the four image forming units PU excluding the four writing heads 43 can be removed from the four predetermined positions in the housing easily to be performed a maintenance, a repair, and a replacement. In operation of each image forming unit, while the photosensitive drum 16 is rotated in one time, the cleaner 41 clears the circular surface of the drum, the contact charging roller 11 electrically charges the surface of the drum uniformly with high minus electric charge, the writing head 43 projects light beam on the electrically charged surface of the drum to form an electrostatic latent image with low minus electric charge in accordance with a writing data inputted in a control unit (described later) of the printer by the above described input keys on the top cover 34 or by a host computer (described later) connected to the control unit of the printer, and the developer 44 develops the latent image on the surface of the drum with a toner into a toner image. The four developers 44 of the four image forming units contain toners 51 of different colors, and particularly in this embodiment the developers 44 contain the toners 51 of magenta M, cyan C, yellow Y, and black K in an order that the developers 44 are arranged along the upper extending portion of the conveyor belt 36 in its predetermined moving direction "D". Each of the developers 44 includes a toner stirring device 44a, a toner supply roller 44b, a developing roller 46 and a doctor blade 44c. The toner stirring device 44a stirs the toner 51 contained in the developer 44, the toner supply roller 44b supplies the stirred toner to the developing roller 46, the developing roller brings the toner into contact with the circular surface of the drum 16 to develop the latent image formed on the surface as described above, and the doctor blade 44c removes a residual toner sticked on the developing roller 46. As shown in FIG. 6A, the paper sheet conveyor belt 36 always keeps its tension at a suitable value because a lower extending portion of the belt is urged downwardly by tension rollers 48, so that the upper surface of the upper extending portion of the conveyor belt 36 is stretched between the driving and driven-rollers 37 and 38 without any waves to be always in contact with the lower ends of the photosensitive drums 16 of the four image forming units. Further, the belt 36 is pressed upwardly at four positions on the lower surface of its upper extending portion by four elastic transfer members 45, the four positions being corresponding to the lower ends of the four photosensitive drums 16. With such a structure, the four positions on the upper surface of the upper extending portion of the conveyor belt 36 which corresponding to the lower ends of the four photosensitive drums 16 are in contact with the lower ends of the four drums and make four image transfer portions in the printer 31, and the four elastic transfer members 45 constitute four image transfer devices. The electrophotographic color printer 31 shown in FIG. 6A further includes a recording medium supply device which supplies the paper sheet P as one kind of the recording medium from the paper cassette 33 in the bottom of the inner space of the outer housing to an upstream end of the upper extending portion of the conveyor belt 36, the upstream end being corresponding to the driven roller 38, in the center of the inner space, and further supplies the paper sheet P to the above described four image transfer portions on the upper surface of the upper extending portion of the belt 36 by using the belt. The recording medium supply device includes a recording medium supply path 53 extending upwardly from a front end of the paper cassette 33 to the upstream end of the upper extending portion of the conveyor belt 36 along a curved trace directing a top of its curve toward the front wall of the housing. A pickup roller 55 is arranged near to an entrance of the supply path 53 to correspond to the front end of the paper cassette 33. In a downstream side of the pickup roller 55 along the supply path 53, a pair of feed out rollers 54 are arranged. In a downstream side of the paired feed out rollers 54 along the supply path 53, that is an exit of the supply path 53, a pair of waiting rollers 52 are arranged. Further, a press roller 49 arranged near to the paired waiting rollers 52 is pressed on the upstream end of the upper extending portion of the conveyor belt 36. The electrophotographic color printer 31 shown in FIG. 6A more further includes a recording medium discharge device which discharges the paper sheet from a downstream end of the upper extending portion of the conveyor belt 36, the downstream end being corresponding to the driving roller 37, in the center of the inner space, toward the additional paper sheet discharge tray 32b in the rear wall of the outer housing or the main paper sheet discharge tray 35 in the top cover 34 of the housing. The discharge device includes a recording medium discharge path 57 extending from the downstream end of the upper extending portion of the conveyor belt 36 to the main paper sheet discharge tray 35 through the additional paper sheet discharge tray 32b along a curved trace directing a top of its curve rearward. A paper sheet separation member (not shown) is arranged near to the downstream end of the upper extending portion of the conveyor belt 36, that is near to an entrance of the discharge path 57. An image fixing device 56 are arranged between the downstream end of the upper extending portion of the conveyor belt 36 and the additional discharge tray 32b along the discharge path 57, and a pair of pull out rollers 58 are arranged at an exit of the discharge path 57 to forcibly pull out the paper sheet from the exit of the path 57 to the main discharge tray 35. The electrophotographic color printer 31 shown in FIG. 6A more further includes a control unit 59 in the inner space of the outer housing between the paper sheet conveyor belt 36 located at the center of the inner space and the paper cassette 33 located at the bottom of the inner space. The control unit 59 has various electric and electronic circuits for controlling the operation of the printer 31. The electrophotographic color printer 31 starts its operation when the above described power switch (not shown) is turned on, and a printing data including a material of the paper sheet P to be used, the number thereof, a printing mode to be used, and the other information is inputted in the control unit 59 by the above described input keys on the top cover 34 of the printer 31 or by the host computer connected to the control unit 59. At first, the uppermost one paper sheet in the large number of paper sheets P on the bottom plate 33-1 of the paper cassette 33 is supplied into the entrance of the recording medium supply path 53 by the pickup roller 55. The paired paper feed rollers 54 at the entrance of the path 53 pinches the paper sheet P picked up from the cassette 33 and feeds the paper sheet P toward the paired waiting rollers 52. The paper sheet P from the feed rollers 54 collides at its leading end with a contact line between the waiting rollers so that a skew thereof is amended, and the feeding of the paper sheet P by the feed rollers 54 is stopped temporally. While temporally stopping of the feeding of the paper sheet P, the circulation of the conveyor belt 36 by the driving roller 37 at a predetermined speed in the anti-clockwise direction as shown by an arrow D in FIG. 6A is started. Further, the photosensitive drum 16 and the developing roller 46 both of which are provided in each of the four image forming units PU as shown in FIG. 6B also start their predetermined rotations in the clockwise direction at a timing that the toner image formed on the drum 16 of each of image forming units PU will be transferred to the paper sheet P at each of the four image transfer portions while the paper sheet P will be conveyed on the conveyor belt 36. After the waiting rollers 52 start their rotation, the paper sheet P is pressed on the upstream end of the upper extending portion of the conveyor belt 36 by the press roller 49, and the press roller 49 applies a paper attracting bias to the paper sheet P so that the paper sheet is electrostatically forcibly attracted on the upper surface of the upper extending portion of the belt 36. As a result of this, the toner images of four colors formed by the four image forming units will surely be transferred at the four transfer portions to predetermined four positions on the paper sheet P without positional errors. The paper sheet P on which at least one color toner image among the four color toner images have been transferred in accordance with the printing data is separated from the upper surface of the upper extending portion of the conveyor belt 36 at the downstream end by the separation member (not shown), and then is passed through the fixing device 56 at which the transferred at least one color toner image on the paper sheet P is fixed on the paper sheet P with heat applied to the toner image by the fixing device 56. The paper sheet P passed through the fixing device 56 is discharged from the discharge path 57 to the additional discharge tray 32b when the additional discharge tray is positioned at its horizontal open position (described above but not shown) or to the main discharge tray 35 when the additional discharge tray is positioned at its vertical closed position (described above and shown in FIG. 6A), and on the additional tray 32b the paper sheet P is laid with the at least one color toner image fixed thereon facing upward or on the main discharge tray 35 the paper sheet P is laid with the at least one color toner image fixed thereon facing downward. As shown in FIG. 1B, the contact charging roller 11 of each of the four image forming units PU of the electrophotographic color printer 31 shown in FIG. 6A includes a round shaped metal bar 11-1 as a center member, a roller-shaped conductive foam material portion 11-2 covering the round bar, and a tube-shaped conductive resin material portion 11-3 covering the conductive foam material portion. And, as shown in FIG. 1A, a high voltage power supply 12 is connected to the round bar 11-1. The round bar is formed of a stainless steel and has a diameter of 6 mm. The conductive foam material portion 11-2 is made of a polyurethane foam which is formed by a reaction of polyol with isocyanate and includes carbon particles as conductive particles to make the foam material portion have an electrical resistance of 10 3 Ω. The conductive resin material portion 11-3 is made of conductive nylon tube which is formed by an extrusion molding to have a thickness of 100 µ m and a surface resistance of 5 × 10 6 Ω/□. The contact charging roller 11 as a whole has an outer diameter of 10 mm, a hardness of Asker F 80°, and an electrical resistance of 1 × 10 6 Ω. FIG. 2 shows an apparatus for measuring the surface resistance of the contact charging roller 11. In this measuring apparatus, a load of 500g is applied to each of both ends of the metal rod 11-1, the both ends being projected in the longitudinal direction of the roller 11 from the conductive foam and resin material portions 11-2 and 11-3, so that the circular surface of the roller is in contact with a measuring electrode 13. And, a power source 14 output of which is 10V is connected to the measuring electrode 13 and the metal rod 11-1 through an ampere meter 15. At this time, the ampere meter 15 shows the resistance of 1 × 10 6 Ω as that of the contact charging roller 11. FIG. 3A schematically shows an interengagement relationship between the contact charging roller 11 and the photosensitive drum 16 in each of the four image forming units PU of the electrophotographic color printer 31 shown in FIG. 6A, and FIG. 3B schematically shows a side view of the charging roller 11 and the drum 16 shown in FIG. 3A. As shown in FIGS. 3A and 3B, a drum supporting shaft 17 of the photosensitive drum 16 is rotatably supported at its both ends by a pair of bearings 18a on the unit frame 18 (see FIG. 6B) of the image forming unit PU. A drum gear 19 is fixed to the drum supporting shaft 17 at one end of the photosensitive drum 16 and is selectably rotatably driven by a driving system (not shown) of the printer 31. Both ends 21 of the contact charging roller 11 are also rotatably supported by a pair of bearings 22 on the unit frame 18 (see FIG. 6B) of the image forming unit PU, and a pinion 23 fixed to one of the both ends 21 of the contact charging roller 11 is in mesh with the drum gear 19 so that the charging roller 11 is rotatably driven by the rotation of the drum gear 19. Alternatively, as shown in FIG. 3C, the contact charging roller 11 may be rotatably driven by the rotation of the drum gear 19 through a friction produced between the surface of the roller 11 and the circular surface of the drum 16. In this modification, one of the both ends 21 of the contact charging roller 11 does not have the pinion 23 which is employed in the above described one embodiment, and the above described one of the both ends 21 of the contact charging roller 11 is only rotatably supported by a bearing 20 provided on the frame 18. A structure for a contact relationship between the contact charging roller 11 and the photosensitive drum 16 in the modification shown in FIG. 3C is simpler than that for an interengagement relationship therebetween in the above described one embodiment shown in FIGS. 3A and 3B. In each of the four image forming units PU of the printer 31 of the present embodiment, the contact charging roller 11 is pressed against the circular surface of the drum 16 so that the charging roller 11 bites its surface into the circular surface of the drum 16 by 0.4 mm. And the high voltage power source 12 applies a charging voltage which is produced by superposing a direct current voltage V DC (about -700V) on an alternating voltage V AC (a frequency of which is about 1300 Hz) having peak-to-peak voltage Vpp of about 1400V to about 1500V to the metal center rod 11-1, so that a charged potential of the circular surface of the drum 16 is set at about -650V. After the circular surface of the drum 16 is charged uniformly as described above by the contact charging roller 11, the inventors of the present invention try to form an image, for example halftone dots which is uniform in its whole area, on the recording paper, and a very fine quality of halftone dots image can be obtained. That is, there is no fault in the image on the recording paper. More specifically, not only a white belt or line will not be formed on a center portion of the image, the center portion corresponding to the longitudinal center portion of the circular surface of the photosensitive drum 16 from which the longitudinal center portion of the contact charging roller 11 is left by the bent of the charging roller, but also a lot of white spots will not be formed on a whole of the image, the white spots caused by an insufficient contact of the charging roller 11 to the circular surface of the photosensitive drum 16. In this embodiment, the photosensitive drum 16 includes a conductive drum base 16-1 formed of an aluminum, an anodized aluminum layer 16-2 formed on the conductive drum base 16-1 to have a thickness of 5 µ m, an under coat layer 16-3 of synthetic resin formed on the anodized aluminum layer 16-2 to have a thickness of 2 µ m, and a photosensitive layer 16-4 formed on the under coat resin layer 16-3. A whole resistance of the anodized aluminum layer 16-2 and the under coat resin layer 16-3 is so set high, for example in a range from 10 9 Ω to 10 10 Ω, that it does not lower characteristics (for example, a charging characteristic and a photoconductivity when the drum is exposed by light) of the photosensitive drum 16. The above described high resistance prevents electric charges with high voltage uniformly charged on the photosensitive layer 16-4 by the contact charging roller 11 and transportation charges with low voltage formed in the uniformly charged electric charges by exposing at least a portion of the uniformly charged electric charges with light, from escaping from the photosensitive layer 16-4. Further, since the anodized aluminum layer 16-2 has a very high hardness, even if the circular surface of the photosensitive drum 16 is damaged, the anodized aluminum layer 16-2 and the conductive drum base 16-1 will not be damaged. In the followings, how the above described arrangement and structure of the contact charging roller 16 according to one embodiment of the present invention are set will be described in details. At first, another contact charging roller having the same structure and size as those of the contact charging roller 11 according to one embodiment of the present invention, but being different from the charging roller 11 in their hardness is prepared. More specifically, the hardness of the another contact charging roller as a whole is Asker C 45° which is the same as that of the aforementioned conventional contact charging roller as a whole described in Japanese Patent No. 2,632,899, and the hardness of the contact charging roller 11 according to one embodiment of the present invention is Asker F 80° as described above. It is noted that the Asker C is used for designating a hardness of something which has a relatively high hardness, the Asker F is used for designating a hardness of something which has a relatively low hardness, and the hardness of something which has a relatively low hardness will not be measured by the Asker C. Next, the another contact charging roller is installed in an image forming unit of a printer, a structure of the image forming unit being the same as that of the image forming unit PU shown in FIG. 6B and according to one embodiment of the present invention, and the printer having only one image forming unit described above. And, by using the printer, image forming operations are conducted for many times with changing an amount of bite of the another contact charging roller into the circular surface of the photosensitive drum. FIG. 4A shows printing qualities obtained by the printer with changing the amount of bite. In order to compare to this, FIG. 4A further shows printing qualities obtained by a printer in which only one image forming unit is used and this image forming unit is the same as the image forming unit PU shown in FIG. 6B and using the contact charging roller 11 according to one embodiment of the present invention. In FIG. 4A, the printer using the contact charging roller 11 (Asker F-80°) can get good printing qualities in cases that the amount of bite is set at 0.2 mm, 0.4 mm, and 0.7 mm, but not at 0 mm because many white spots are formed in the image due to an insufficient contact of the contact charging roller 11 against the circular surface of the drum 16. In contrast to this, the printer using the another contact charging roller (Asker C-45°) can not get good printing qualities in all cases that the amount of bite is set at 0 mm, 0.2 mm, 0.4 mm, and 0.7 mm because many white spots are formed in the image due to the insufficient contact of the whole of the another contact charging roller against the circular surface of the drum 16 in the cases that the amount of bite is set at 0.1 mm and 0:2 mm, and a white belt or line is formed in the image due to the insufficient contact of the longitudinal center portion of the another contact charging roller against the circular surface of the drum 16 caused by bending of the another contact charging roller. In order to make the contact charging roller 11 have a good charging characteristic, that is to make the printer using that contact charging roller 11 have a good printing quality, both of the resistance of the conductive resin material portion 11-3 and that of the conductive foam material portion 11-2 must be set in suitable ranges, respectively. And this range is determined by an experiment performed by the inventors of the present invention. FIG. 4B shows a result of the experiment, and more specifically shows a relationship between the charging characteristic of the contact charging roller 11, that is the printing quality of the image formed on the recording paper by the printer, a change of the surface resistance of the tube-like conductive resin material portion 11-3, and a change of the resistance of the conductive foam material portion 11-2. As shown in FIG. 4B, a good charging characteristic of the contact charging roller 11, that is the good printing quality of the image formed on the recording paper by the printer, is obtained when the surface resistance of the conductive resin material portion 11-3 is in a range from 1 × 10 5 Ω/□ to 9 × 10 7 Ω/□ and the resistance of the conductive foam material portion 11-2 is in a range of 9 × 10 5 Ω/□, preferably 9 × 10 4 Ω/□. And, the conductive resin material portion 11-3 having the surface resistance of the 10 4 Ω/□ or less can not be realized because it must contain too much carbon particles to form the resilient conductive resin material portion 11-3. FIG. 5 shows a method by which the tubular-shaped conductive resin material portion 11-3 is fixed to the conductive foam material portion 11-2 not to influence the good charging characteristic of the contact charging roller 11. According to this method, both longitudinal ends of the tubular-shaped conductive resin material portion 11-3 are fixed to both longitudinal ends C of the circular surface of the conductive foam material portion 11-2 by an adhesive 24, and the both longitudinal ends C of the circular surface of the conductive foam material portion 11-2 are arranged in both outsides of a center portion B of the circular surface which corresponds to an image formation area on the circular surface of the photosensitive drum 16 (shown in FIG. 1B). That is, the both longitudinal ends C correspond to both longitudinal ends of the circular surface of the foam material portion 11-2 and further correspond to image-free areas arranged in both outsides of the image formation area on the circular surface of the drum 16. And, by preparing the tube-like conductive resin material portion 11-3 independent of the conductive foam material portion 11-2 on the center rod 11-1 and by fixing the tube-like conductive resin material portion to the circular surface of the conductive foam material portion 11-2 with the adhesive 24, even if the contact charging roller 11 having the two layers, that is the conductive foam material portion 11-2 and the conductive resin material portion 11-3, the contact charging roller 11 can be formed easily and cheaply. As describe above in detail, in the present invention, since the hardness of the contact charging roller as a whole is preferably set at Asker F 90° or less, the contact charging roller can be in contact with the circular surface of the image carrier uniformly even if the roller is formed to have a smaller diameter than that of the conventional one, so that the contact charging roller of the present invention can charge the circular surface of the image carrier uniformly to make the image carrier have the good image, and the image forming unit using the contact charging roller and the electrophotographic image forming apparatus can be formed to have a small outer size. And, since the image carrier of the image forming unit of the present invention preferably has the anodized aluminum layer as a blocking layer and the photosensitive layer provided on the anodized aluminum layer, further since the image carrier of the one embodiment of the present invention has an undercoat resin layer as an additional blocking layer provided on the anodized aluminum layer but under the photosensitive layer, withstand voltage of the image carrier is increased and a leak of charges from the image carrier is prevented, so that a durability of the image carrier is increased with maintaining the good image forming performance thereof. More further, in the present invention, since preferably the tubular-shaped conductive resin material portion has both end areas corresponding to the image-free areas of the image carrier outsides of its image-forming area and is fixed at the both end areas to the roller-shaped conductive foam material portion by an adhesive, a preparation of the conductive charge roller is easy and will not influence the image forming quality formed on the image carrier.

Claims: A contact charging member (11) provided on a center member (11-1), characterized by comprising: a conductive foam material portion (11-2) covering the center member and having an electric resistance of 9 × 10 5 Ω, or less; and a conductive resin material portion (11-3) covering the conductive foam material portion and being used to be in contact with a member (16) to be charged by the contact charging member, the conductive resin material portion including conductive agents and having a surface electric resistance in a range from 1 × 10 5 Ω/□ to 9 × 10 7 Ω/□, and a hardness of the contact charging member being Asker F 90° or less. A contact charging member according to claim 1, wherein the conductive foam material portion (11-2) has a roller-shape, the conductive resin material portion (11-3) has a tubular-shape, and the contact charging member (11) has a roller-shape. A contact charging member according to claim 2, wherein the member (16) to be charged is an image carrier including a surface having an image formation area (B) on which an image is formed and image-free areas (C) arranged in both outsides of the image formation area, the tubular-shaped conductive resin material portion (11-3) has both ends corresponding to the image-free areas of the image carrier (16) and is fixed at the both ends to the roller-shaped conductive foam material portion (11-2) by an adhesive (24). An image forming unit (PU) comprising: an image carrier (16); and a contact charging member (11) which is in contact with a surface of the image carrier and electrically charges the surface of the image carrier uniformly, the image forming unit characterized in that the contact charging member (11) is provided on a center member (11-1) and includes a conductive foam material portion (11-2) which covers the center member and has an electric resistance of 9 × 10 5 Ω or less, and a conductive resin material portion (11-3) which covers the conductive foam material portion and is in contact with the image carrier, the conductive resin material portion including conductive agents and having a surface electric resistance in a range from 1 × 10 5 Ω/□ to 9 × 10 7 Ω/□, and a hardness of the contact charging member is Asker F 90° or less. An image forming unit according to claim 4, wherein the conductive foam material portion (11-2) of the contact charging member (11) has a roller-shape, the conductive resin material portion (11-3) of the contact charging member has a tubular-shape, the contact charging member has a roller-shape, and the image carrier (16) has a drum-shape. An image forming unit according to claim 5, wherein the image carrier (16) includes a surface having an image formation area (B) on which an image is formed and image-free areas (C) arranged in both outsides of the image formation area, the tubular-shaped conductive resin material portion (11-3) has both ends corresponding to the image-free areas of the image carrier and is fixed at the both ends to the roller-shaped conductive foam material portion (11-2) by an adhesive (24). An image forming unit according to claim 5, wherein the drum-shaped image carrier (16) is rotatably driven, and the roller-shaped contact charging member (11) is rotated by the rotation of the drum-shaped image carrier through friction produced between the surface of the conductive resin material portion (11-3) of the roller-shaped contact charging member and that of the drum-shaped image carrier. An image forming unit according to claim 5, wherein the image carrier (16) includes an aluminum drum having an anodized aluminum layer (16-2) provided on an outer surface thereof and a photosensitive material layer (16-4) provided on the anodized aluminum layer. An electrophotographic image forming apparatus (31) comprising: an image carrier (16) which includes a photosensitive drum having a circular surface; a roller-shaped contact charging member (11) which is in contact with the circular surface of the photosensitive drum (16) and electrically charges the circular surface of the photosensitive drum uniformly; an exposure device (43) which exposes the uniformly charged circular surface of the photo-sensitive drum with a desired image; a developing device (44) which develops the exposed desired image on the circular surface of the photosensitive drum; a recording medium supply device (33, 55, 53, 54, 52, 49, 37, 38, 36) which supplies a recording medium (P) toward the developed desired image on the circular surface of the photosensitive drum; a transfer device (45) which transfers the developed desired image from the circular surface of the photosensitive drum to the supplied recording medium; and a recording medium discharge device (57, 32b, 58, 5) which discharges the recording medium on which the developed desired image have been transferred, from the circular surface of the photosensitive drum, the apparatus characterized in that the roller-shaped contact charging member (11) is provided on a center member (11-1) and includes a roller-shaped conductive foam material portion (11-2) which covers the center member and has an electric resistance of 9 × 10' 3 Ω, or less, and a tubular-shaped conductive resin material portion (11-3) which covers the conductive foam material portion and is in contact with the image carrier (16), the conductive resin material portion including conductive agents and having a surface electric resistance in a range from 1 × 10 13 Ω/□ to 9 × 10 7 Ω/□, and a hardness of the roller-shaped contact charging member is Asker F 90° or less. An electrophotographic image forming apparatus according to claim 9, wherein the circular surface of the photosensitive drum (16) has an image formation area (B) on which an image is formed and image-free areas (C) arranged in both outsides of the image formation area, the tubular-shaped conductive resin material portion (11-3) of the roller-shaped contact charging member (11) has both ends corresponding to the image-free areas of the photosensitive drum and is fixed at the both ends to the roller-shaped conductive foam material portion (11-2) by an adhesive (24). An electrophotographic image forming apparatus according to claim 9, wherein the photosensitive drum (16) is rotatably driven, and the roller-shaped contact charging member (11) is rotated by the rotation of the photosensitive drum through friction produced between the surface of the conductive resin material portion (11-3) of the roller-shaped contact charging member and that of the photosensitive drum. An electrophotographic image forming apparatus according to claim 9, wherein the photosensitive drum (16) includes an anodized aluminum layer (16-2) provided on an outer surface thereof and a photosensitive material layer (16-4) provided on the anodized aluminum layer.

Inventors: ABE TETSUYA, KANASUGI MASAKI, KOUROKU MORIYUKI, ONO NORIKI, YORIFUJI TAKAO, ABE, TETSUYA, KANASUGI, MASAKI, KOUROKU, MORIYUKI, ONO, NORIKI, YORIFUJI, TAKAO
Applicants: CASIO COMPUTER CO LTD, CASIO ELECTRONICS MFG CO, CASIO COMPUTER CO., LTD., CASIO ELECTRONICS MANUFACTURING CO., LTD.
Application year: 1998
Main classification: G03G15
IPC codes: G03G15, F16C13