Abstract:
A contact charger in an electrophotographic image forming apparatus for electrostatically charging a photosensitive, latent image support surface of a rotatably supported photosensitive drum in contact therewith. The contact charger comprises a generally elongated carrier and a charging member carried at a first side edge by the carrier. This charging member has a second side edge opposite to the first side edge which is held in sliding contact with the photosensitive surface over a predetermined contact distance. The charging member has a curved surface of a radius of curvature of 0.02 to 1.0 mm at a side edge portion thereof at which contact of the charging member with the photosensitive surface terminates. A power source for applying a voltage to the charging member is also employed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a charger for use in an image forming apparatus such as, for example, a copier or a printer and, more particularly, to a contact charger for electrostatically charging a photosensitive member by bringing a charging member into contact with such photosensitive member. 
     2. Description of the Related Art 
     In an electrophotographic image forming apparatus such as a copier, a printer or the like, image formation is performed by charging a photosensitive member with the use of a charger; forming an electrostatic latent image on a charged region of the photosensitive member by subsequent exposure to imagewise rays of light; developing the electrostatic latent image into a visible powder image; transferring the visible powder image onto a transfer material; and fixing the transferred visible powder image. 
     Various types of chargers are known and are broadly classified into corona chargers, wherein corona discharge is utilized, and contact chargers wherein the surface of the photosensitive member is brought into contact with a charging member such as, for example, a charging brush, a charging roller, a charging blade or a rotatably arranged endless charging belt. 
     The corona chargers have the advantage of providing a stable charging process, but have the disadvantage of producing a large amount of ozone, which tends to cause deterioration of the photosensitive member and to adversely affect human bodies. Because of this, the contact chargers are noticed which produce considerably less ozone than the corona chargers. 
     The contact chargers, however, have some disadvantages. The chargers employing the charging brush have the disadvantage of requiring much time to make the brush. The chargers employing the charging roller have a complicated structure because they require means for driving the roller. In addition, because the roller is left in contact with a photosensitive drum even when the image forming apparatus is at a standstill, creep deformation strains remain on the contact portion, thus resulting in insufficient charging. The chargers employing the endless belt have the disadvantage of increasing the size of the image forming apparatus and requiring a complicated structure for driving the belt. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed to overcome the above-described disadvantages and has for its object to provide an improved highly reliable contact charger capable of stably charging a photosensitive member held in contact therewith. 
     Another object of the present invention is to provide the contact charger of the above-described type which has a simple structure and can be readily manufactured at a low cost. 
     In accomplishing the above and other objects, the contact charger according to the present invention is held in contact with an external peripheral surface of an electrostatic latent image support member to thereby charge the electrostatic latent image support member. The contact charger comprises a carrier, a charging member carried by the carrier and held in contact with the external peripheral surface of the electrostatic latent image support member over a predetermined length adjacent to a free side portion of the charging member, and a power source for applying a voltage to the charging member. The charging member has a curved surface of a radius of curvature of 0.02 to 1.0 mm at a contact edge portion thereof at which contact of the charging member with the electrostatic latent image support member terminates. 
     Conveniently, the free side portion of the charging member is bent or folded backwards at the contact edge portion, to thereby provide the curved surface. 
     Preferably, the charging member has a volume resistivity of not greater than 10 9  Ω.cm, and the predetermined length over which the charging member is held in contact with the external peripheral surface of the electrostatic latent image support member ranges from 1 to 10 mm. 
     Advantageously, the voltage applied to the charging member includes a DC voltage component of 800 to 1500 V. 
     In another aspect of the present invention, the contact charger comprises a carrier, a flexible charging member having a fixed side edge carried by the carrier and a free side edge positioned downstream from the fixed side edge with respect to the direction of rotation of the electrostatic latent image support member, and a power source for applying a voltage to the charging member. The charging member is held uniformly in contact with the external peripheral surface of the electrostatic latent image support member over a predetermined length adjacent to the free side edge by the action of electrostatic adsorption, and has a curved surface of a radius of curvature of 0.02 to 1.0 mm at a contact edge portion thereof at which contact of the charging member with the electrostatic latent image support member terminates. 
     By the above-described construction, when the electrostatic latent image support member is rotated with the voltage from the power source applied to the charging member, the charging member closely adheres to the support member by an electrostatic adsorptivity generated by a voltage difference between the charging member and the support member. At the same time, an electric discharge takes place at a region where the peripheral surface of the support member gradually approaches the charging member upon rotation thereof, with the external peripheral surface of the support member being charged. Furthermore, the charged portion of the support member is stabilized at a contact region where the support member and the charging member are held in contact with each other. 
     In addition, because the contact edge portion of the charging member has the curved surface, the extent to which an electric field is concentrated in the proximity of the contact edge portion is relatively low and, hence, the charged condition of the support member is reliably maintained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become more apparent from the following description of preferred embodiments thereof with reference to the accompanying drawings, throughout which like parts are designated by like reference numerals, and wherein: 
     FIG. 1 is a schematic perspective view of a contact charger according to the present invention; 
     FIG. 2 is a side view of a free end of a charging member mounted in the charger of FIG. 1; 
     FIG. 3 is a view similar to FIG. 2, but indicating a modification thereof; 
     FIG. 4 is a view similar to FIG. 2, but indicating another modification thereof; 
     FIG. 5 is a view similar to FIG. 2, but indicating a further modification thereof; 
     FIG. 6 is a view similar to FIG. 2, but indicating a still further modification thereof; 
     FIG. 7 is a view similar to FIG. 2, but indicating a charging member according to a comparative example; and 
     FIG. 8 is a perspective view of the charging member, showing the bending moment applied thereto. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, there is schematically shown in FIG. 1 a contact charger 1 embodying the present invention. 
     The charger 1 shown in FIG. 1 comprises a generally elongated carrier 2 extending parallel to a photosensitive drum 11 having a photosensitive surface on its outer periphery, a generally rectangular charging member 3 extending parallel to the photosensitive drum 11 and secured at one side edge portion to the carrier 2, and a charging power source 4 electrically connected with the carrier 2. The charging member 3 is a flexible strip sheet made of electroconductive material or material having a low resistance. A free side edge 5 of the charging member 3 remote from the carrier 2 has a contact edge portion 6 held lightly in contact with the photosensitive surface of the photosensitive drum 11 at a contact pressure close to substantially zero. 
     As shown in FIGS. 2 to 6, the contact edge portion 6 of the charging member 3 is formed with a curved surface (shown by 7a in FIG. 2, 7b in FIG. 3, 7c in FIG. 4, 7d in FIG. 5 or 7e in FIG. 6) of a radius of curvature within the range of 0.02 to 1.0 mm, at which the contact between the charging member 3 and the photosensitive drum 11 terminates. 
     The curved surface 7a at the contact edge portion 6 of the charging member 3 may be formed by grinding the free side edge of the charging member 3 or in any suitable manner. 
     Alternatively, the curved surface may be formed, as shown by 7b in FIG. 3, by folding the free side edge of the charging member 3 backwardly to overlap in contact with the remaining portion of the charging member 3. Again alternatively, the curved surface as shown by 7c in FIG. 4 may be formed by folding the free side edge of the charging member 3 backwardly to overlay the remaining portion of the charging member 3 with a strip-shaped spacer 83 intervening therebetween, or the curved surface as shown by 7d in FIG. 5 may be formed by curling the free side edge of the charging member so as to encircle a rod-like spacer 84. Also, as shown by 7e in FIG. 6, the curved surface may be formed by bending the free side edge of the charging member so as to extend radially outwardly away from the photosensitive drum 11. 
     The contact charger 1 of the above-described construction operates as follows. 
     When the photosensitive drum 11 is rotated in a direction shown by an arrow (a) with a voltage applied from the power source 4 to the charging member 3, the charging member 3 is attracted to slidingly contact the photosensitive surface of the photosensitive drum 11 by the action of an electrostatic adsorptivity generated by a voltage difference between the charging member 3 and the photosensitive drum 11. At this moment, an electric discharge takes place from the charging member 3 towards the photosensitive surface of the photosensitive drum 11 at a region 9 delimited between the charging member 3 and an upstream portion of the photosensitive surface of the photosensitive drum 11 on a trailing side with respect to the direction of rotation of the photosensitive drum 11, to thereby electrostatically charge the photosensitive surface of the photosensitive drum 11. After the surface of the photosensitive drum 11 has been charged to a predetermined voltage at this region 9, the charged area of the photosensitive surface of the photosensitive drum 11 is stabilized as it is, during a continued rotation of the photosensitive drum 11, brought to a contact region 10 where the photosensitive drum 11 is held in contact with the charging member 3. 
     It is preferred that the length of the contact region 10, that is, the angular length of the photosensitive surface of the photosensitive drum over which the free side edge 5 of the charging member is held in contact therewith, is chosen to be within the range of 1 to 10 mm. The charging member 3 can be held more stably in contact with the photosensitive surface of the photosensitive drum 11 if a voltage including a DC voltage component of 800-1500 V is applied to the photosensitive drum 11. 
     Because the free side edge 5 of the charging member 3 has the curved surface (shown by 7a in FIG. 2, 7b in FIG. 3, 7c in FIG. 4, 7d in FIG. 5 or 7e in FIG. 6), concentration of an electric field generated by the voltage applied to the charging member 3 which would take place considerably if an edge exists in the charging member is advantageously minimized in the presence of the curved surface confronting the photosensitive surface. 
     During the continued rotation of the photosensitive drum 11, the photosensitive surface thereof passes underneath the charging member 3 and progressively departs from the contact region 10 between the charging member 3 and the photosensitive surface of the photosensitive drum 11. Accordingly, even though one or both of the photosensitive drum 11 and the carrier 2 undergo vibration under the influence of vibrations generated by a driving system, the contact edge portion 6 is held stably in contact with the photosensitive surface of the photosensitive drum 11, thereby providing a generally uniform charging characteristic thereon. 
     Furthermore, where the curved surface at the contact edge portion 6 of the charging member 3 is formed by folding as described hereinbefore and as shown in FIGS. 3 to 5, the strength of the free side edge 5, particularly the contact edge portion 6, of the charging member 3 is advantageously increased enough to avoid the generation of wrinkles, which has hitherto been caused by stresses generated by the rotation or stop of the photosensitive drum 11. Accordingly, the photosensitive surface of the photosensitive drum 11 having passed the contact region 10 is charged to a predetermined voltage. 
     The charging member 3 is in the form of a flexible member having an electroconductivity and a low electric resistance, and the material, thickness or the like thereof is not limited if the charging member 3 closely adheres to the photosensitive surface of the photosensitive drum 11 over a sufficient angular distance by the action of electrostatic adsorption. In general, the electroconductivity or the low-resistivity is imparted by dispersing electroconductive material in a sheet of metallic material or synthetic resin, or by subjecting the surface of a synthetic resin sheet to a suitable treatment by which it can be rendered electroconductive. In view of the charging characteristic, it is preferred that the charging member 3 has a volume resistivity of not greater than 10 9  Ω.cm and, preferably, within the range of 10 3  -10 8  Ω.cm. 
     Various experiments were carried out in order to examine a change in electrostatic charge build up on the photosensitive surface of the photosensitive drum 11 and a change in image quality resulting from printing. 
     A polyethylene film in which a conductive carbon powder was dispersed and which had a thickness of 50 μm, a Young&#39;s modulus of 7.4×10 6  g/cm 2 , a bending moment (M) of 0.15 g.cm, and a volume resistivity of 10 3  Ω.cm was employed as the charging member 3. The free side edge of the charging member 3 was folded backwardly as shown in FIG. 3 so as to overlap the remaining portion of the charging member 3 over an overlapping distance of 2 mm, that is, with the backwardly folded portion having a length of 2 mm. 
     It is to be noted here that the bending moment M is a moment required for turning the charging member of 1 cm in width b around a rod A having a round section of 1 cm in diameter D as shown in FIG. 8. This moment M is given by M=EI/ρ(I=bh 3  /12), where E represents the Young&#39;s modulus (g/cm 2 ) of elasticity of the charging member; I represents the moment of inertia of the cross section (cm 4 ) of the charging member; ρ represents the radius of curvature (cm), that is, the distance between the center of curvature of the rod A and the neutral plane NS of the charging member; and h represents the thickness of the charging member. 
     The photosensitive drum 11 employed was of a type having an outer diameter of 30 mm and having an organic photosensitive surface, and was driven at a peripheral velocity of 35 mm/sec. Furthermore, the voltage applied to the charging member 3 from the power source 4 was chosen to be -1200 V. 
     In the experiments to evaluate the charging characteristic of the photosensitive drum 11, a surface potentiometer identified by model 360 and manufactured by TREK, Inc. was used to measure the initial voltage of the charged photosensitive drum 11 and the surface voltage of the photosensitive drum 11 after a 1×4 dot matrix pattern of black points in which the black points are spaced one dot from the neighboring black points was printed on the entire surface of 500 sheets of A4-size papers at an image density of 300 dpi. During the experiments, a change in surface voltage in a direction widthwise of the photosensitive drum 11 was also measured. The location at which the measurements were carried out was angularly displaced about 75° downstream from the contact region 10 between the charging member 3 and the photosensitive surface of the photosensitive drum 11 with respect to the direction of rotation of the photosensitive drum 11. 
     In the experiments to evaluate the image quality, with the use of a densitometer identified by model PDA-65 and manufactured by Konica Kabushiki Kaisha of Japan, the image density and a change in image density in the width direction of the photosensitive drum 11 were measured at the initial stage of the printing and after the 1×4 dot matrix pattern was printed on 500 sheets of A4-size papers. 
     For the purpose of comparison with the contact charger according to the present invention, the similar experiments were carried out to evaluate the charging characteristic and the image quality, using a contact charger of a structure wherein, as shown in FIG. 7, a charging member 300 had a sharp free side edge 301 held in contact with a photosensitive member 302 (this charger is hereinafter referred to as a comparison charger). As is the case with the contact charger according to the present invention, a polyethylene film was employed as the charging member 300. 
     Table 1 indicates results of the experiments carried out with the charger according to the present invention to evaluate the charging characteristic of the photosensitive member. 
     
                       TABLE 1______________________________________Initial surface voltage (V.sub.1)                   -800 VChange in initial surface voltage (ΔV.sub.1)                   ±100 VSurface voltage after 500 sheets were                   -800 Vprinted (V.sub.500)Change in surface voltage after 500 sheets                   ±110 Vwere printed (ΔV.sub.500)______________________________________ 
    
     Table 2 indicates results of the similar experiments carried out with the comparison charger. 
     
                       TABLE 2______________________________________Initial surface voltage (V.sub.1 &#39;)                   -820 VChange in initial surface voltage (ΔV.sub.1 &#39;)                   ±250 VSurface voltage after 500 sheets were                   -800 Vprinted (V.sub.500 &#39;)Change in surface voltage after 500 sheets                   ±330 Vwere printed (ΔV.sub.500 &#39;)______________________________________ 
    
     Table 3 indicates results of the experiments carried out with the charger according to the present invention to evaluate the image quality. 
     
                       TABLE 3______________________________________Initial image density (ID.sub.1)                    0.42Change in initial image density (ΔID.sub.1)                    ±0.04Image density after 500 sheets were printed                    0.42(ID.sub.500)Change in image density after 500 sheets                    ±0.04were printed (ΔID.sub.500)______________________________________ 
    
     Table 4 indicates results of the similar experiments carried out with the comparison charger. 
     
                       TABLE 4______________________________________Initial image density (ID.sub.1 &#39;)                    0.40Change in initial image density (ΔID.sub.1 &#39;)                    ±0.12Image density after 500 sheets were printed                    0.39(ID.sub.500 &#39;)Change in image density after 500 sheets                    ±0.16were printed (ΔID.sub.500 &#39;)______________________________________ 
    
     As discussed hereinabove, in the charger according to the present invention, the initial surface voltage (V 1  =-800 V) was identical with the surface voltage (V 500  =-800 V) after 500 sheets of A4-size papers were printed, and each of the change in initial surface voltage (ΔV 1  =±100 V) and the change in surface voltage (ΔV 500  =±110 V) was small and the difference therebetween was very small (10 V). 
     Furthermore, the initial image density (ID 1  =0.42) and the image density (ID 500  =0.42) after 500 sheets of A4-size papers were printed were both low, and the change in initial image density (ΔID 1  =0.04) and the change in image density (ΔID 500  =0.04) after 500 sheets of A4l -size papers were printed were both small. 
     On the other hand, in the comparison charger, there arose a voltage difference of 20 V between the initial surface voltage (V 1  &#39;=-820 V) and the surface voltage (V 500  &#39;=-800 V) after 500 sheets of A4-size papers were printed. The change in initial surface voltage (ΔV 1  &#39;=±250 V) and the change in surface voltage (ΔV 500  &#39;=±330 V) after 500 sheets of A4-size papers were printed were both considerable as compared with those (ΔV 1  =±100 V, ΔV 500  =±110 V) of the charger according to the present invention, respectively. In addition, the difference therebetween (|ΔV 1  &#39;|-|ΔV 500  &#39;|=-80 V) was considerably greater than that (|ΔV 1  |-|ΔV 500  |=-10 V) in the charger according to the present invention. 
     In the comparison charger, although no considerable change could not be found between the initial image density (ID 1  &#39;=0.40) and the image density (ID 500  &#39;=0.39) after the printing of 500 sheets of A4-size papers, the change in initial image density (ΔID 1  &#39;=0.12) and the change in image density (ΔID 500  &#39;=0.16) after the printing of 500 sheets of A4-size papers were both considerable as compared with those (ΔID 1 , ΔID 500  =0.04) of the charger according to the present invention, respectively. 
     It can be understood from the above that the charger according to the present invention in which the charging member 3 has the curved surface provides a remarkably stabilized charging characteristic over the overall length of the photosensitive drum 11, thereby maintaining the image density stable for a long time. 
     It is conceivable that unstable charging characteristic in the comparison charger is primarily due to the presence of sharp edges at the free side edge 301 of the charging member 300 at which contact thereof with the photosensitive member 302 terminates, and an electric field concentrated at the sharp edges brings about a discharge phenomenon which would become unstable by vibrations induced by the drive of the photosensitive member 302 or a microscopic change in the contact condition, or by the action of wrinkles produced by stresses generated by the rotation or stop of the photosensitive member 302. 
     Two kinds of experiments were carried out to evaluate the curvature of the curved surface of the charging member. 
     A charging member used in the first experiments was a polyethylene terephthalate film in which conductive carbon powder was dispersed and which had a thickness of 75 μm, a Young&#39;s modulus of elasticity of 10.2×10 6  g/cm 2 , a bending moment (M) of 0.71 g.cm, and a volume resistivity of 10 6  Ω.cm. The curvature of the curved surface of the charging member was changed by folding the free side edge of the charging member backwardly as shown in FIG. 4, and by changing the thickness of a strip-shaped spacer sandwiched between opposed portions of the charging member at the backwardly folded area within the range of 0 to 2.0 mm. The backwardly folded portion having the spacer of a thickness of 0 mm means the absence of the spacer as shown in FIG. 3, and the same is true for the second experiments. 
     A charging member used in the second experiments was made of polyethylene terephthalate and had one surface with a conductive coating. This charging member had a thickness of 25 μm, a Young&#39;s modulus of elasticity of 1.0×10 6  g/cm 2 , a bending moment (M) of 2.6×10 -3  g.cm, and a conductive coating surface resistance of 9×10 6  Ω/cm. The curvature of the curved surface of the charging member was changed by utilizing a rod-like spacer in the curved free side edge of the charging member as shown in FIG. 5 and by changing the diameter of the rod-like spacer. 
     With the use of these charging members, the change in initial surface voltage (ΔV 1 ), the change in initial image density (ΔID 1 ), the change in surface voltage (ΔV 500 ) after 500 sheets of A4-size papers were printed, and the change in image density (AID 500 ) after 500 sheets of A4-size papers were printed were measured, and each of them was evaluated by visually observing unevenness in density of the 1×4 dot matrix pattern. Moreover, the same experiments were carried out using the charging member shown in FIG. 7 and having sharp edges at the free side edge 301 thereof. 
     Table 5 indicates results of the first experiments wherein t represents the thickness (mm) of the spacer, φ the thickness (mm) of the spacer, r c  the radius of curvature (mm), N U1  results of evaluation with respect to the unevenness in initial image density, N U500  results of evaluation with respect to the unevenness in image density after 500 sheets of A4-size papers were printed, and Sp the spacer. 
     Symbols used in evaluating the image density are as follows. 
     O: Unevenness in density was slight and the density was relatively uniform. 
     Δ: Unevenness in density could be confirmed, but could be practically safely ignored. 
     x: Unevenness of density was deemed to be serious. 
     Table 6 indicates results of the second experiments. 
     
                                           TABLE 5__________________________________________________________________________  t  r.sub.c        ΔV.sub.1           ΔID.sub.1               N.sub.U1                  ΔV.sub.500                      ΔID.sub.500                          N.sub.U500                              Struc.                                  Remarks__________________________________________________________________________Experiment 1  0  0.075         80           0.02               ∘                  150 0.05                          ∘                              FIG. 3                                  No SpExperiment 2  0.05     0.100        120           0.03               ∘                  140 0.04                          ∘                              FIG. 4Experiment 3  0.20     0.175        110           0.03               ∘                  100 0.02                          ∘                              FIG. 4Experiment 4  0.50     0.325        130           0.03               ∘                  120 0.03                          ∘                              FIG. 4Experiment 5  0.80     0.475        120           0.03               ∘                  120 0.03                          ∘                              FIG. 4Experiment 6  1.00     0.575        130           0.04               ∘                  140 0.04                          ∘                              FIG. 4Experiment 7  1.50     0.825        220           0.10               Δ                  200 0.09                          Δ                              FIG. 4Experiment 8  2.00     1.075        280           0.14               x  270 0.14                          x   FIG. 4Experiment 9  -- -- 250           0.15               x  300 0.18                          x   FIG. 7__________________________________________________________________________ 
    
     
                                           TABLE 6__________________________________________________________________________   φ      r.sub.c         ΔV.sub.1            ΔID.sub.1                N.sub.U1                   ΔV.sub.500                       ΔID.sub.500                           N.sub.U500                               Struc.                                   Remarks__________________________________________________________________________Experiment 10   0  0.025          80            0.02                ∘                   110 0.03                           ∘                               FIG. 3                                   No SpExperiment 11   0.10      0.075          80            0.02                ∘                   110 0.03                           ∘                               FIG. 5Experiment 12   0.50      0.125         100            0.02                ∘                   100 0.02                           ∘                               FIG. 5Experiment 13   1.00      0.575         120            0.03                ∘                   120 0.03                           ∘                               FIG. 5Experiment 14   1.50      0.775         160            0.07                ∘                   120 0.03                           ∘                               FIG. 5Experiment 15   2.00      1.025         200            0.10                Δ                   190 0.09                           Δ                               FIG. 5Experiment 16   3.00      1.525         340            0.18                x  300 0.17                           x   FIG. 5Experiment 17   -- -- 230            0.15                x  280 0.18                           x   FIG. 7__________________________________________________________________________ 
    
     As is clear from the results of the above experiments, the curvature r c  of the curved surface of the charging mender must be not greater than 1 mm. Furthermore, the provision of the curved surface having a curvature less than 0.02 mm not only results in an increase in manufacturing cost of the charging men, but, but also brings about the problem such that an electric field would be concentrated at the contact edge portion of the charging member. Accordingly, the curvature of the curved surface should be so chosen as to range from 0.02 to 1.0 mm. 
     From the foregoing, according to the present invention, because the contact edge portion of the charging member with respect to the photosensitive member has the curved surface, the extent to which the electric field is concentrated in the proximity of the contact edge portion is relatively low, and the condition in which the contact edge portion of the charging member is in contact with the photosensitive member is stabilized. Accordingly, not only uniform charge is imparted to the surface of the photosensitive member having passed the contact region with the charging member, but also a substantially constant voltage is maintained for a long time. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications otherwise depart from the spirit and scope of the present invention, they should be construed as being included therein.