JP2018132536A - Charging device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the size of a charging device using a contact charging roller, and prevent leak discharge from a core grid of the contact charging roller to an image carrier.SOLUTION: A charging device comprises: a charging roller 7 that is in contact with a photoreceptor 20 to charge the surface of the photoreceptor 20; a support member 8a that rotatably supports the charging roller 7; and a compression coil spring 61 that urges the charging roller 7 in the direction of the photoreceptor 20. The charging roller 7 includes a conductive bar-like core grid 71, and a surface layer 72 that covers an outer peripheral surface of the core grid 71 and at least part of end faces at both ends in the longitudinal direction of the core grid 71. A concave part 711 concave toward the center part in the longitudinal direction is formed in each of the end faces at both ends in the longitudinal direction of the core grid 71; the support member 8a has a convex part 82 that is insulative and engaged with the concave part 711; upon engagement of the convex part 82 of the support member 8a with the concave part 711 of the core grid 71, the charging roller 7 is rotatably supported by the support member 8a.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a charging device and an image forming apparatus including the charging device.

  As a charging device for an image forming apparatus, a conductive charging member is placed in contact with the photosensitive member and charged with a charging voltage for charging the photosensitive member for reasons such as ozonelessness, downsizing of the device, and reduction of charging current. Contact charging is often used. 9 to 11 show an example of a conventional contact charging device. 9 is a side view of a conventional contact charging device, FIG. 10 is an assembled perspective view of the contact charging device, and FIG. 11 is a partial vertical sectional view.

  As shown in FIG. 11, the charging roller 70a of the contact charging device includes a cored bar 71 made of a metal shaft having a diameter of 6 mm, for example, and a conductive elastic having a thickness of about 2 mm provided on the outer periphery of the cored bar 71. A layer 73, and a conductive surface layer 72 having a thickness of about 10 μm to several tens of μm, which is provided on the layer 73, are formed. The surface layer 72 is provided in order to prevent a bleeding phenomenon in which the plasticizer oozes out from the elastic layer 73 and adversely affects charging, or to adjust the charging roller 70a to an appropriate resistance.

  Both ends of the cored bar 71 extend axially outward from both ends of the surface layer 72 in the axial direction. On the other hand, the pair of resin support members 80a are formed with through holes 801 into which the extending portions of the cored bar 71 can be fitted. By inserting both ends of the cored bar 71 into the through holes 801 of the pair of support members 80a, the charging roller 70a is rotatably supported by the support member 80a. Further, a compression coil spring 61 is in contact with the pair of support members 80a, and the charging roller 70a is pressed against the photoreceptor 20 via the support member 80a by the compression coil spring 61. The leaf spring-like electrode member 62 comes into contact with one end portion of the cored bar 71 exposed through the through hole 801 of the support member 80a, and a charging bias voltage is applied to the cored bar 71 from the power source.

  In a tandem color apparatus having a plurality of image forming units, it is desired to reduce the size of the apparatus. Along with this, downsizing of the charging device is also desired. One method for reducing the size of the charging device is to reduce the diameter of the cored bar 71 of the charging roller 70a. However, since the charging roller 70a is supported by the support member 80a at both ends in the axial direction and is urged toward the photoconductor 20, the core metal 71 is used to suppress uneven contact with the photoconductor 20 in the axial direction. Predetermined strength is required and there is a limit to reducing the diameter.

  Therefore, a charging roller (hereinafter sometimes referred to as a “coat roller”) having a configuration in which the elastic layer 73 in the charging roller 70a is eliminated and the surface layer 72 is directly formed on the cored bar 71 is conceivable. FIG. 12 is a partial perspective view showing the coating roller 70b and its supporting structure. The coating roller 70b is formed by directly forming the surface layer 72 on the cored bar 71, and the cored bar 71 protruding axially outward from both ends of the surface layer 72 is rotatably supported by the support member 80b. Further, the coating roller 70b is urged against the photosensitive member 20 by the compression coil spring 61 that contacts the support member 80b. The support member 80a has an arcuate shape whose upper section is perpendicular to the axial direction so as not to interfere with the photoreceptor 20. In the charging device having such a configuration, the distance d between the cored bar 71 to which the charging bias voltage is directly applied and the photoconductor 20 is very short as much as the thickness of the surface layer 72.

  The charging bias voltage applied to the cored bar 71 is normally 0.8 to 1.4 KV in the case where only the DC bias voltage is applied, and is maximum in the case where the AC bias voltage is superimposed on the DC bias voltage. It is about 2KV. When such a high voltage is applied between the cored bar 71 and the photosensitive member 20 facing each other with a minute distance d, the cored bar 71 has a slight dent / cutting line that is not normally noticeable. Such a protrusion and a coating defect / unevenness that does not normally cause a problem in the photoconductor 20 are the starting points, and discharge occurs between the metal core 71 and the photoconductor 20 and an excessive current flows. In order to suppress the occurrence of such an overcurrent, the charging bias voltage must be suppressed to a low value of 1.5 KV / mm or less, preferably 1.3 KV / mm or less.

  As a measure for preventing discharge between the cored bar 71 and the photoconductor 20, it is conceivable to provide the surface layer 72 over the entire axial direction of the cored bar 71. FIG. 13 is a partial perspective view showing the coating roller 70c having the surface layer 72 provided in the entire axial direction of the cored bar 71 and its supporting structure. With the coating roller 70c having such a configuration, the discharge between the cored bar 71 and the photoconductor 20 can be suppressed, but the surface layer 72 and the support member 80b are rubbed and used with the surface layer 72. There is a problem that the metal core 71 is exposed due to wear.

  As another measure for preventing discharge between the cored bar 71 and the photoconductor 20, as shown in FIG. 14, the portion covered with the surface layer 72 of the cored bar 71, that is, in contact with the photoconductor 20. It is conceivable to increase the distance D between the cored bar 71 and the photoconductor 20 at the part where the cored bar 71 at both ends in the axial direction of the coating roller is exposed. However, in such a configuration, the charging roller 70d is increased in size and weight.

  Patent Documents 1 to 6 describe a technique for preventing discharge between the cored bar and the photosensitive member in the charging roller. In any of the disclosed techniques, an elastic layer is formed between the cored bar and the surface layer. However, there is no disclosure of a technique for preventing discharge between the cored bar and the photoconductor in a coating roller having no elastic layer.

JP 2003-156920 A JP 2002-049217 A JP 09-07211 A JP-A-10-186800 Japanese Unexamined Patent Publication No. 07-1221012 Japanese Patent Laid-Open No. 06-175465

  An object of the present invention is to reduce the size of a charging device and to prevent leakage discharge from a cored bar of a charging roller to a photoconductor.

  The charging device according to the present invention that achieves the above object includes a charging roller that contacts the image carrier and charges the surface of the image carrier, a support member that rotatably supports the charging roller, and the charging roller. A charging device having a biasing member that biases in the direction of the image carrier, wherein the charging roller includes a conductive bar-shaped cored bar, an outer peripheral surface of the cored bar, and both longitudinal ends of the cored bar. A surface layer covering at least a part of the end surface, and a recess recessed toward the center in the longitudinal direction is formed on both end surfaces of the metal core in the longitudinal direction, and the support member is insulative and is engaged with the recess The charging roller is rotatably supported by the support member by engaging the convex portion of the support member and the concave portion of the cored bar.

  In the above configuration, the length of the charging roller in the longitudinal direction is preferably shorter than the length of the image carrier in the longitudinal direction.

  The said structure WHEREIN: It is preferable that the layer thickness of the said surface layer is 15 micrometers or more and 300 micrometers or less.

  The said structure WHEREIN: The said surface layer is good also as a structure which has a resistance layer and the protective layer which coat | covers the surface of the said resistance layer.

  The said structure WHEREIN: The said supporting member is good also as a structure which incorporates the electroconductive member in which one end part contacts the end surface of the said metal core, and the other end part contacts the electrode member directly or indirectly.

  In the above configuration, the concave portion includes a first concave portion and a second concave portion having an inner diameter smaller than that of the first concave portion provided on the bottom surface of the first concave portion, and the convex portion of the support member; The charging roller may be rotatably supported by the support member by engaging with the second concave portion of the cored bar.

  The said structure WHEREIN: It is good also as a structure by which the opening peripheral part of the said recessed part is made into the inclined surface which inclines toward the longitudinal direction center part of the said metal core, and therefore toward a radial inside.

  Also according to the invention. The charging device according to any one of the above, an exposure device that exposes the image carrier to form an electrostatic latent image on a surface of the image carrier, and a developing device that develops the electrostatic latent image with toner. An image forming apparatus comprising: a transfer device that transfers the developed toner image to a transfer material.

  According to the charging device of the present invention, it is possible to reduce the size of the device and to prevent leakage discharge from the core metal of the charging roller to the photoconductor, and to normally charge the surface of the image carrier.

1 is a partial vertical sectional front view showing an example of an image forming apparatus according to the present invention. 1 is a left side view of a charging device according to the present invention. FIG. 3 is an assembled partial perspective view of the charging device of FIG. 2. FIG. 3 is an enlarged vertical sectional view of a circled region in FIG. 2. It is an expanded sectional view of a surface layer. FIG. 6 is a partial vertical sectional view showing another embodiment of the charging device according to the present invention. FIG. 6 is a partial vertical sectional view showing another embodiment of the charging device according to the present invention. FIG. 6 is a partial vertical sectional view showing another embodiment of the charging device according to the present invention. It is a side view which shows the conventional charging device. FIG. 10 is a partial perspective view of the conventional charging device of FIG. 9. FIG. 10 is a vertical sectional view of a circled region in FIG. 9. It is a fragmentary perspective view which shows the other example of the conventional charging device. It is a fragmentary perspective view which shows the other example of the conventional charging device. It is a fragmentary perspective view which shows the other example of the conventional charging device.

  Hereinafter, a charging device and an image forming apparatus using the same according to the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to these embodiments. In the present specification, the longitudinal direction of the core bar is sometimes referred to as the axial direction.

(Image forming device)
First, the image output operation of the image forming apparatus according to the embodiment of the present invention will be described while explaining the outline of the structure with reference to FIG. FIG. 1 is an example of a partial vertical sectional front view of an image forming apparatus. 1 are the vertical direction, the horizontal direction, and the depth direction of the image forming apparatus.

  The image forming apparatus S shown in FIG. 1 is a so-called tandem color printer. Of course, in addition to a printer, the present invention can also be applied to a copying machine having a scanner, a facsimile, or a multi-function machine having these functions combined. Further, the image forming method is not limited to the tandem method, and other methods, for example, four developing devices are arranged around the rotation shaft, and these are sequentially opposed to the electrostatic latent image carrier to be full color. A so-called four-cycle method of creating an image or a monochrome method of creating a monochrome image with one developing device may be used.

  The image forming apparatus S includes an endless intermediate transfer belt 33 having conductivity. The intermediate transfer belt 33 is hung on a pair of rollers 31 and 32 disposed on both the left and right sides in the drawing. The roller 32 is connected to a motor (not shown). When the motor is driven, the roller 32 rotates counterclockwise, whereby the intermediate transfer belt 33 and the roller 31 in contact therewith are driven to rotate. A secondary transfer roller 34 is in pressure contact with the outside of the belt portion supported by the roller 32. The toner image formed on the intermediate transfer belt 33 at the nip portion (secondary transfer region) between the secondary transfer roller 34 and the intermediate transfer belt 33 is transferred onto the conveyed paper P.

  A cleaning blade 35 for cleaning the surface of the intermediate transfer belt 33 is provided outside the belt portion supported by the roller 31. The cleaning blade 35 removes and collects untransferred residual toner at a contact portion with the intermediate transfer belt 33.

  Below the intermediate transfer belt 33 suspended between the rollers 31 and 32, yellow (Y), magenta (M), cyan (C), black ( K) four image forming units 2Y, 2M, 2C, and 2K (hereinafter may be collectively referred to as “image forming unit 2”) are detachably arranged with respect to the apparatus main body 10. In these image forming units 2, corresponding color toner images are created using the respective color developers.

  The image forming unit 2 includes a photoconductor 20 as an electrostatic latent image carrier. A charging device 6, a developing device 23, a primary transfer roller 24, and a cleaning blade 25 are arranged around the photoconductor 20 in order along the rotation direction (clockwise direction). Then, one end side of the cleaning blade 25 abuts on the outer peripheral surface of the photoconductor 20, and the toner remaining on the surface of the photoconductor 20 is removed and collected. The primary transfer roller 24 is in pressure contact with the photoconductor 20 with the intermediate transfer belt 33 interposed therebetween to form a nip portion (primary transfer region). An exposure device 22 is disposed below the image forming unit 2. As will be described later, the charging device 6 is a roller contact charging system.

  Above the intermediate transfer belt 33, hoppers 4Y, 4M, 4C, and 4K (hereinafter sometimes collectively referred to as “hopper 4”) containing toner to be supplied to the developing devices 23 of the respective colors are arranged. A paper feed cassette 50 is detachably disposed as a paper feed device below the exposure device 22. The sheets P stacked and accommodated in the sheet cassette 50 are sent out one by one to the conveyance path in order from the uppermost sheet by the rotation of the sheet feed roller 51 disposed in the vicinity of the sheet cassette 50. The paper P sent out from the paper feed cassette 50 is conveyed to the registration roller pair 52 and is sent out to the secondary transfer area at a predetermined timing.

  The image forming apparatus S can be switched between a monochrome mode in which a monochrome image is formed using one color toner (for example, black) and a color mode in which a color image is formed using four color toners.

  An image forming operation example in the color mode will be briefly described. First, in each image forming unit 2, the outer peripheral surface of the photoreceptor 20 that is rotationally driven at a predetermined peripheral speed is charged by the charging device 6. Next, light corresponding to image information is projected from the exposure device 22 on the surface of the charged photoconductor 20 to form an electrostatic latent image. Subsequently, the electrostatic latent image is made visible by toner as a developer supplied from the developing device 23. When the toner image of each color formed on the surface of the photoreceptor 20 in this way reaches the primary transfer region by the rotation of the photoreceptor 20, the toner image is transferred from the photoreceptor 20 to the intermediate transfer belt in the order of yellow, magenta, cyan, and black. 33 is transferred (primary transfer) and superimposed.

  Residual toner remaining on the photoconductor 20 without being transferred to the intermediate transfer belt 33 is scraped off by the cleaning blade 25 and removed from the outer peripheral surface of the photoconductor 20.

  The superimposed four color toner images are conveyed to the secondary transfer region by the intermediate transfer belt 33. On the other hand, the paper P is conveyed from the registration roller pair 52 to the secondary transfer area in accordance with the timing. Then, the four color toner images are transferred (secondary transfer) from the intermediate transfer belt 33 to the paper P in the secondary transfer region. The paper P on which the four color toner images are transferred is conveyed to the fixing device 1. In the fixing device 1, the paper P passes through the nip portion between the fixing roller 11 and the pressure roller 12. During this time, the paper P is heated and pressurized, and the toner image on the paper P is melted and fixed on the paper P. The paper P on which the toner image is fixed is discharged to a paper discharge tray 54 by a discharge roller pair 53.

  On the other hand, residual toner remaining on the intermediate transfer belt 33 without being transferred onto the paper P is scraped off by the cleaning blade 35 and removed from the outer peripheral surface of the intermediate transfer belt 33. Thereafter, the rotational drive of each photoconductor 20 and the intermediate transfer belt 33 is stopped.

(Charging device)
2 is a left side view of the charging device 6, FIG. 3 is an assembled partial perspective view, FIG. 4 is an enlarged vertical sectional view of a circled region in FIG. 2, and FIG. 5 is an enlarged sectional view of the surface layer 72. The charging device 6 includes a charging roller 7, a support member 8a that rotatably supports the charging roller 7 at both ends in the axial direction of the charging roller 7, and an urging force that presses the charging roller 7 against the photoconductor 20 via the support member 8a. A compression coil spring 61 as means and a power source for applying a charging voltage to the charging roller 7 are provided. The charging roller 7 and the photoconductor 20 are arranged so that their rotational axes are parallel to each other and press to contact each other to form a nip portion. The photosensitive member 20 is rotated by a rotating means (not shown), and the charging roller 7 is driven to rotate by the rotation of the photosensitive member 20. The charging roller 7 is applied with an oscillating voltage in which an AC voltage is superimposed on a DC voltage by a power source. As a result, the surface of the photoconductor 20 is uniformly charged to a predetermined potential.

  Here, the axial length of the charging roller 7 is preferably shorter than the axial length of the photoconductor 20. That is, it is preferable that both ends of the charging roller 7 in the axial direction are located closer to the center than both ends of the photosensitive member 20 in the axial direction. For example, when the photoconductor 20 is an organic photoconductor, an uncoated portion tends to be generated on the upper end side of the coating when the photosensitive layer is dip-coated, and both ends of the charging roller 7 in the axial direction are formed on the photoconductor 20. This is because if it is positioned outside both ends in the axial direction, the uncoated portion and the charging roller 7 come into contact with each other, and an excessive current flows to destroy the photosensitive layer. In addition, the charging roller itself is also increased in size.

  As shown in FIG. 4, the charging roller 7 includes a cylindrical cored bar 71 made of a highly conductive and strong metal, and a part of the outer peripheral surface of the cored bar 71 and end faces at both ends in the axial direction of the cored bar. And a covering surface layer 72. On both end surfaces in the axial direction of the cored bar 71, circular concave portions 711 that are recessed toward the central portion in the axial direction are formed. The central axis of the circular recess 711, the cored bar 71, and the central axis are the same axis. And the peripheral part except the recessed part 711 of the axial direction both end surfaces of the metal core 71 is covered with the surface layer 72. In other words, the cored bar 71 is covered with the surface layer 72 except for the bottom surface and the inner peripheral surface of the recess 711.

  The core bar 71 may be made of a conductive material, and examples thereof include metal materials such as stainless steel, iron, copper, aluminum, and nickel. Among these, stainless steel having high corrosion resistance and less fatigue is preferable. Further, the cored bar 71 may be subjected to a surface treatment such as plating that enhances scratch resistance and rust prevention as long as the conductivity is not impaired.

  When the maximum printing size of the image forming apparatus S is A4 size, the core bar 71 having a diameter of 6 mm is preferably used. At this time, the inner diameter of the recess 711 is preferably 5 mm to 5.4 mm in diameter and 5 mm in depth.

  As shown in FIG. 5, the surface layer 72 includes a resistance layer 721 formed with a uniform thickness and a protective layer 722 covering the surface of the resistance layer 721.

  The resistance layer 721 is formed of a thermoplastic resin composition in which a polymer type ionic conductive agent is dispersed. Examples of the thermoplastic resin composition include general-purpose resins such as polyethylene, polypropylene, polymethyl methacrylate, polystyrene, and copolymers thereof. Moreover, as the polymer type ion conductive material, a polymer compound containing a polyether ester amide component is preferable. Polyether ester amide is an ion conductive polymer material, and is uniformly dispersed and immobilized at a molecular level in a matrix polymer. Thereby, the dispersion | variation in resistance value accompanying the dispersion | distribution defect like the composition which disperse | distributed electronic conductive system agents, such as a metal oxide and carbon black, does not arise. In addition, since it is a polymer material, bleeding out hardly occurs.

  The resistance layer 721 is formed on the cored bar 71 by dissolving the material in an organic solvent to prepare a solution and applying the coating method such as dipping, spray coating, or roll coating. Further, the thickness of the resistance layer 721 is preferably 10 μm to 270 μm, more preferably 10 μm to 170 μm. As the resistance layer 721 is thicker, coating unevenness is more likely to occur, which leads to density unevenness.

  As a material for the protective layer 722, a resin material is preferable in that the film-forming property is good. As the resin material, a fluororesin, a polyamide resin, a polyester resin, or a polyvinyl acetal resin is preferable from the viewpoint of excellent non-adhesiveness and preventing toner adhesion. In addition, since the resin material generally has electrical insulation, if the protective layer is formed of the resin material alone, the characteristics of the charging roller are not satisfied. Therefore, the resistance value of the protective layer 722 is adjusted by dispersing a conductive agent such as a metal oxide or carbon black in the resin material. In addition, in order to improve the adhesiveness between the protective layer 722 and the resistance layer 721, a reactive curing agent such as isocyanate may be dispersed in the resin material.

  The protective layer 722 is formed by a coating method such as dipping as with the resistance layer 721. Further, the thickness of the protective layer 722 is preferably about 5 μm to 30 μm, more preferably 10 μm to 20 μm from the viewpoint of uniformity of the layer thickness.

  The total thickness of the surface layer 72 is preferably 15 μm to 300 μm, more preferably 15 μm to 200 μm, in total, the thickness of the resistance layer 721 and the thickness of the protective layer 722. As the surface layer 72 is thicker, layer thickness unevenness is more likely to occur, which leads to density unevenness. In addition, the surface roughness of the surface layer 72 is preferably 10 μm or less in terms of the 10-point average roughness Rz defined in JIS B 0601: 2001 from the viewpoint of charging uniformity.

  As shown in FIGS. 3 and 4, the support member 8 a includes a cubic main body 81 and a columnar protrusion 82 protruding from the side surface of the main body 81. The outer diameter of the convex portion 82 is slightly smaller than the inner diameter of the concave portion 711 formed on the end surfaces of both ends in the axial direction of the cored bar 71. Thereby, the convex part 82 of the support member 8a is engaged with the concave part 711 of the core metal 71, so that the support member 8a rotatably supports the core metal 71.

  The support member 8a is made of an insulating resin, such as a polyacetal resin (POM), a polyamide resin (PA), a polyterafluoroethylene resin (PTFE), a polyphenylene sulfide resin (PPS), a polyether ether ketone resin (PEEK), Polyamideimide resins (PAI) and resins to which additives are added for various purposes such as strength improvement are used.

  As shown in FIG. 4, a conductive member 84 for connecting the cored bar 71 and the compression coil spring 61 is built in one support member 8 a. The conductive member 84 has an L shape in which the horizontal portion 842 and the vertical portion 841 are connected, and the horizontal portion 842 and the vertical portion 841 are built in the convex portion 82 and the main body portion 81, respectively. The end portion of the horizontal portion 842 exposed from the tip of the convex portion 82 contacts the cored bar 71, and the end portion of the vertical portion 841 exposed from the lower surface of the main body portion 81 contacts the compression coil spring 61. The compression coil spring 61 not only serves as a biasing member but also has conductivity and serves as a conductive portion. As shown in FIG. 2, the compression coil spring 61 is in contact with an electrode member 62 connected to a power source, and a charging voltage is applied to the cored bar 71 from the power source via the compression coil spring 61 and the conductive member 84.

  According to the charging device having such a configuration, the cored bar 71 is covered with the surface layer 72 over the entire axial direction, and the peripheral portions of the end faces of both ends in the axial direction of the cored bar 71 are also covered with the surface layer 72. When the diameter of the gold 71 is reduced, current leakage from the axial end of the core 71 to the photoconductor 20 that has been a problem in the past is reliably prevented.

(Second Embodiment)
In the first embodiment shown in FIGS. 2 to 4, the compression coil spring 61 as an urging member for bringing the charging roller 7 into contact with the photoconductor 20 is also used as a conductive portion for applying a charging voltage to the charging roller. The electrode member may be brought into direct contact with the conductive member built in the support member without using the coil spring 61 as the conductive portion. An example is shown in FIG. FIG. 6 is a partial vertical sectional view of one end side in the axial direction of the charging device. In addition, the same code | symbol may be attached | subjected to the same member and part as 1st Embodiment, and the description may be abbreviate | omitted.

  The charging device shown in FIG. 6 differs from the charging device of the first embodiment in the shape and structure of the support member 8b. The support member 8b in FIG. 6 includes a cubic main body 81, a columnar convex portion 82 protruding from the side surface of the main body portion 81, and a side surface opposite to the side surface from which the convex portion 82 of the main body portion 81 protrudes. And a second protrusion 83 protruding outward from a position away from the photoconductor 20 (downward position in FIG. 6). The convex portion 82, the main body portion 81, and the second convex portion 83 contain a conductive member 84 in which the first horizontal portion 842, the vertical portion 841, and the second horizontal portion 843 are continuously integrated. The end portion of the first horizontal portion 842 exposed from the tip of the convex portion 82 is in contact with the metal core 71, and the end portion of the second horizontal portion 843 exposed from the tip of the second convex portion 83 is the plate spring-like electrode member 62. To touch. Then, a charging voltage is applied from the power source to the metal core 71 through the electrode member 62 and the conductive member 84.

  On the other hand, the compression coil spring 61 abuts on the lower surface of the main body 81 of the support member 8 b and urges the charging roller 7 together with the support member 81 in the direction of the photoconductor 20.

(Third embodiment)
FIG. 7 shows another embodiment of the charging device according to the present invention. FIG. 7 is a partial vertical sectional view of one end side in the axial direction of the charging device. The other end side in the axial direction of the charging device has the same support structure. The same members and portions as those in the first embodiment are denoted by the same reference numerals and description thereof may be omitted.

  In the charging device shown in FIG. 7, the concave portion formed in the core 71 of the charging roller 7 has a first concave portion 712 and a second concave portion 713 having a smaller inner diameter than the first concave portion 712 provided on the bottom surface of the first concave portion 712. The main feature is that

  As a specific example, first concave portions 712 having an inner diameter of 5.4 mm are formed at both ends in the axial direction of the cored bar 71, similarly to the cored bar 71 shown in FIG. 4. A second recess 713 having an inner diameter of 5 mm is formed on the bottom surface of the first recess 712 (the right side surface in FIG. 7). The convex portion 82 of the support member 8a is inserted into and engaged with the second concave portion 713, and the charging roller 7 is rotatably supported by the support member 8a.

  In this embodiment, the inner peripheral surface of the first concave portion 712 is covered with the surface layer 72 as well as the peripheral portion of both end surfaces in the axial direction of the cored bar 71. As a result, creeping discharge from the both axial end surfaces of the cored bar 71 to the photoconductor 20 is more reliably prevented.

(Other)
As shown in FIG. 8, the shape of the opening peripheral edge of the recess 711 (or the second recess 713) of the metal core 71 that engages with the convex part 82 of the support member 8 a is directed toward the axial center of the metal core 71. The inclined surface 714 may be inclined inward in the radial direction. Thus, by setting the shape of the peripheral edge of the opening of the recess 711 (or the second recess 713) as the inclined surface 714, the axial center of the recess 711 (or the second recess 713) formed at both ends in the axial direction of the cored bar 71. Even if the deviation occurs, the deviation can be absorbed by the inclined surface 714, and the photoreceptor 20 and the charging roller 7 can stably come into contact with each other in the entire axial direction.

(Example)
Leakage discharge was confirmed for the charging device of the first embodiment shown in FIGS. Specifically, bizhub C287 manufactured by Konica Minolta Co., Ltd. was used as an experimental machine, and the charging unit of the machine was replaced with the charging device of the first embodiment for experiments.
The thickness of the surface layer of the charging roller was 15 μm so as to be close to the photoreceptor, and the composition was adjusted so that the charging roller resistance (LogΩ) when a voltage of 500 V was applied was 4.5-6.
In this way, a voltage is applied to the charging roller until a maximum of −2 KV is obtained by superimposing the DC voltage on the AC voltage, and the presence or absence of discharge from the end of the charging roller in the axial direction is visually checked. Was confirmed by an image (half solid). As a result, no leak discharge or abnormal image was generated, which was good.

  The charging device of the present invention can reduce the size of the device and can prevent leakage discharge from the core 71 of the charging roller 7 to the photosensitive member 20, and can normally charge the surface of the photosensitive member 20. It is.

S Image forming device 6 Charging device 7 Charging roller 8a Support member 8b Support member 20 Photosensitive member (image carrier)
22 Exposure device 23 Development device 24 Primary transfer roller (transfer device)
61 Compression coil spring (biasing member)
71 Metal core 711 Concave portion 712 First concave portion 713 Second concave portion 714 Inclined surface 721 Resistance layer 722 Protective layer 62 Electrode member 72 Surface layer 82 Convex portion 84 Conductive member

Claims (8)

A charging roller that contacts the image carrier and charges the surface of the image carrier;
A support member for rotatably supporting the charging roller;
A charging device having a biasing member for biasing the charging roller in the direction of the image carrier,
The charging roller is
A conductive rod-shaped cored bar,
A surface layer covering at least a part of the outer peripheral surface of the cored bar and the end surfaces at both longitudinal ends of the cored bar,
Concave portions that are recessed toward the central portion in the longitudinal direction are formed on the end surfaces at both ends in the longitudinal direction of the metal core,
The support member has a convex portion that is insulative and engages with the concave portion,
The charging device is characterized in that the charging roller is rotatably supported by the support member by engaging the convex portion of the support member with the concave portion of the cored bar.   The charging device according to claim 1, wherein a length of the charging roller in a longitudinal direction is shorter than a length of the image carrier in a longitudinal direction.   The charging device according to claim 1, wherein the surface layer has a thickness of 15 μm to 300 μm.   The charging device according to claim 1, wherein the surface layer includes a resistance layer and a protective layer that covers a surface of the resistance layer.   The support member includes a conductive member having one end portion in contact with the end face of the core metal and the other end portion in direct or indirect contact with the electrode member. The charging device described. The recess is
A first recess;
A second recess having a smaller inner diameter than the first recess provided on the bottom surface of the first recess,
6. The charging according to claim 1, wherein the charging roller is rotatably supported by the support member when the convex portion of the support member is engaged with the second concave portion of the cored bar. apparatus.   The charging device according to any one of claims 1 to 6, wherein an opening peripheral edge portion of the concave portion is an inclined surface that is inclined toward a central portion in a longitudinal direction of the core metal and thus inward in a radial direction. A charging device according to any one of claims 1 to 7,
An exposure device that exposes the image carrier to form an electrostatic latent image on the surface of the image carrier;
A developing device for developing the electrostatic latent image with toner;
An image forming apparatus comprising: a transfer device that transfers a developed toner image onto a transfer material.