Electrophotographic copier and charging means used therefor

Abstract: The invention is directed to an apparatus for electrophotographic process, equipped with a charging device of contact type using a conductive roller or brush, and the charging device used therefor. The apparatus is characterized in the following constructions: roll-shaped fiber aggregation (5) that is rotated in contact with the photoconductive surface (1) through spacing member; (5b) use as applied voltage to the fiber aggregation of periodically oscillating voltage having a lower limit exceeding the surface potential of photoconductor in the aforementioned device; a brush-type charger that is vibrated while contacted with the photoconductor surface and regulated as required; a fiber roll-type charger (5a) wherein ventilation holes (E) are disposed on the roller substrate surface; the fiber roll-type charger with holes further including a closed container with exhausting means enclosing the charger; set-up of a product of planting fiber intervals on a fiber roller and a peripheral velocity ratio of the photoconductor to the roller such that the product is smaller the average particle size of the developer used; changing application voltage to a charger roller depending transfer-treated portion or portions otherwise on the photoconductor surface; and provision of dirt preventer, aligned with the longitudinal direction to the charging roller with conductive fiber, and shiftable between contact and spaced position. Any of these can present desired effect.

Description: SPECIFICATION BACKGROUND OF THE INVENTION (1) Filed of the invention The present invention relates to an electrophotographic process and more particularly to an improvement of an electrophotographic copier and charging means used therefore, according to the preamble of claim 1. (2) Description of the Related Art As is well known, there have been made a lot of proposals on duplicating machine and particularly, in recent years, most of these are proposed on electrophotographic copiers. Such an electrophotographic copier is typically, elementally constructed of a photoconductor drum, a charging unit, an exposure unit, a developing unit, an image transfer unit, an erasing unit and a cleaner, and all the elements are disposed around the photoconductor drum to effect a series of electrophotographic process. In addition, there are arranged elementally a paper feed tray, paper guides, paper feed rollers, the image transfer charging unit, a suction unit (for conveying), a fixing unit and a paper discharge rollers. With such configurations, an image transferred on a sheet is fixed to create a duplication. More specifically, as shown in Fig.1 of a schematic view, an image forming apparatus based on electrophotography comprises a photoconductor drum 1, in which a photoconductive film is formed on a conductive support, and a series of the following element disposed upstream to downstream of a rotational direction of photoconductor drum 1, that is, a charging unit 102, an exposure unit 103 for illuminating light on photoconductor drum 1 impressed at a charging potential by charging unit 102 to discharge the static charges on photoconductor drum 1 and create a desired electrostatic latent image, a developing unit 104 for supplying toner powder to photoconductor drum 1 having the electrostatic latent image, an image transfer unit 106 for transferring the toner powder image on photoconductor drum 1 onto a recording sheet 105, a fixing unit 107 for melt-fixing the tonered image transferred on recording sheet 105 by heating and/or pressing, an erasing unit 108 for erasing the static charges remaining on photoconductor drum 1 after light-irradiation on photoconductor drum 1 and image transfer, and a cleaner 109 for removing the residual toner on photoconductor drum 1. Of these, as the charging unit for charging the photoconductor at a desired potential a corona charger utilizing corona discharge phenomenon has been used in prior art. This means requires a high voltage, so that there has been a fear that the voltage gives influence over microcomputer, etc. To make the matter worse, upon the corona-discharging, a large quantity of ozone gas will be generated which does not only deteriorate resin material used for the cleaning blade, etc, but also gives unpleasant feelings, causing about environmental problems. To eliminate such problems, charging means which charges a photoconductor by an electro-conductive roller or fiber aggregation applied with a voltage has been proposed, for example, in Japanese Patent Application Laid-Open sho-55 No.29837. Fig.2 shows an oblique view of an example of such a prior art charging means. In the figure, reference numeral 1 designates a photoconductor drum, of which surface 1a is in contact with conductive fiber 5a planted brush-wise on a fiber substrate 5d made of aluminum or other conductive material. In this case, since the mechanism is constructed such that the conductive fiber 5a fixed is brought into contact with photoconductor surface 1a, the structure might be simple, but the developer and other foreign substances are easy to build up between fibers or tips of fibers, causing abnormal discharge resulting in a reduction of the fiber life, and/or causing changing unevenness. On the other hand, in order to improve the situation, there is disclosed in JP-A-60 205551 a charging device for an electrophotographic copier according to the preamble of Claim 1, which, obliquely shown in Fig.3, comprises, for example, a shaft 5c and conductive fiber 5a (as stated above) planted therearound to form a roll-shaped member. This roll-shaped member is rotated relative to the photoconductor drum 1 by a driver (not shown). As a result, the reduction of the fiber life and changing unevenness which are caused by the adhesion of foreign substances or other reason can be remedied and bettered remarkably. Another example of prior art is shown perspectively in Fig.4, in which there are provided a photoconductor drum 1, a photoconductive medium la made of a photoconductive dielectric layer, a charging member 5 comprising a roller shaft 5c covered with conductive rubber therearound. As shown in the figure, the charging mechanism of this kind has typically utilized elastic rollers as its changing means. In other words, a substance used for the member had to have a highly smooth surface and to be less changed or degraded with the passage of time, in order to afford uniform discharge. In addition, the means was required to be constructed such that, the charge supplying member should be prevented from damaging and the charge supplying member should not be voltage-dropped totally, in case where an abnormal current arose through the charge supplying member due to pinholes on the photoconductor, or other cause. Accordingly, in order to provide a charging member as described above, Japanese Patent Laid-Open hei-2 No.62563 discloses use of a charging brush that is planted with the fibers looped substantially perpendicular to a rotational direction of the image bearing medium (photoconductor) formed on the photoconductor drum surface. Fig.5 is an illustration showing the structure,and there are disposed photoconductor drum 1 with an image bearing medium 1a (photoconductor). Reference numeral 5 designates a charging member having charging brush which is formed with conductive fibers 5a looped shown in the figure. The looped fibers 5a are planted on a conductive substrate 5d with a 5g conductive adhesive to thereby form charging brush 5. In this case, the photoconductor drum 1 rotates in a direction shown by arrow R, while the conductive fibers 5a are planted so that the loop structure be perpendicular to the moving direction of the photoconductor drum surface. Using this means could reportedly inhibit stripe-like charging unevenness from occurring, compared to the conventional charging brush. Meanwhile, charging members using such conductive fiber can be conceivably classified into two kinds, one of which is constructed as shown in Figs.2 and 5 such that a charging member is formed like a brush and fixed stationary in sliding contact with the surface of photoconductive material 1a. The other type of the charging members is formed as a roll and the roll-shape member is brought into contact with photoconductive material 1a relatively with moving on the surface of photoconductive material 1a. The former one has a simple structure but exhibits a tendency that the fiber is built up with toner or other foreign substances, still likely causing charging unevenness. In the latter case, since the conductive fiber aggregation 5a moves, foreign substances is hard to build up, and an additional cleaning means might also be provided. Nevertheless, the structure becomes complicated, and when for example, the conductive fiber cloth is wound roll-shaped or belt-wise, the seam formed may cause charging unevenness. Causes of thus occurring charging unevenness were studied, and the following views were realized. First, it is generally known that the surface of photoconductor 1a will be charged when photoconductor 1a is brought into contact with conductive fiber aggregation 5a to which a voltage is applied. This electrification is conceivably caused by both discharge across the micro-clearance and by charge-injection from the contact points. The discharge across the micro-clearance starts to occur when the voltage across the clearance reaches a certain level. This voltage is determined by Paschen's rule of discharge, and an example of the relation is shown in Fig.6. Once the discharge occurs, charges transfer all at once from conductive fiber aggregation 5a to photoconductor 1a. This transfer causes the surface potential of photoconductor 1a to heighten and then the discharge stops. Even after completion of the discharge, photoconductor 1a is still elevated in its surface potential by the injection of charges from the contacts points. For this reason, portion which comes in touch with conductive fiber aggregation 5a in a longer time, or portion which contacts thereto at a higher possibility will bear higher potentials. This can be realized as to be the cause of charging distribution unevenness appearing in broomed traces or seams of conductive fiber aggregation 5a. On the other hand, charging unevenness of the stripe-type generated in brush-type charger is mainly attributed to long termed contact of the brush-like charging member made up of conductive fiber against the same contacting point on the image bearing medium. In addition, such a contact over a long period of time does not only rub certain points on the image bearing medium repeatedly causing possible scratches and wounds on the medium, but also wears the brush itself quickly. To make the matter worse, the developer may gradually be built up in the tips of the brush resulting in pollution. The adhesion of the developer to the ends of nap or fibers of the conductive fiber in the charger may deteriorate the fiber itself in its durability. Further, a long term contact of the charger onto the surface of the image bearing medium brings down the conductive fibers in a rotating direction of the medium, and the thus worn-out fiber cannot allow itself to keep uniform contact with the surface of the image bearing medium, causing ununiformity of charging to generate charge-distribution unevenness. Still, fibers are generally liable to absorb moisture, and fibers with dampness become too flexible, making it difficult for the fibers to stand upright. For this reason, once the fiber is exposed in a high humidity environment, the worn-out, or the state of being brought down of, the fiber cannot be cured. Meanwhile, used as a photoconductive material for the photoconductor drum are organic organic semiconductors, CdS, SeTe, As 3 Se 2 , etc, of which organic semiconductors are mostly used. Typically, N-type organic semiconductor bearing negative charges presents good attenuation characteristics in response to light exposure, but the same semiconductor bearing positive charges exhibits poor light-attenuation characteristics. For this reason, when a positive transfer voltage, that is, the same polarity with the charging voltage, is applied by the transfer roller to the surface of the photoconductor drum even through a recording sheet therebetween and thereafter the recording sheet is separated from the photoconductor drum, the potential of the photoconductor drum surface to which the transfer voltage is not applied receives some influence. Accordingly, when the charging voltage is applied by the roller to the photoconductor drum after the transfer, difference due to the aforementioned transfer voltage is caused to appears in the surface potential of the drum, by the electrificability of the charge roller. The difference in the surface potential has influence on image, causing fogs and density irregularity in the final image. There have been several proposals other than the above that use such charging means of contact type. For example, Japanese Patent Application Laid-Open sho-59 No.204859 discloses a means for preventing deterioration due to wear-out of a brush for use in a brush roller, planted with conductive fibers thereon as charging means, and contacted against a photoconductor. This mechanism is provided with a cam and a tracking roll in each end of the photoconductor and in each end of the brush roller, respectively, and the tracking rolls run on the cam surfaces and the tracking rolls step on respective projections disposed on the cams when the copier is out of operation, whereby the front ends of the brush is kept spaced from the surface of the photoconductor. However, such a structure does not only increase the number of parts for copier, but also requires control of the tracking rolls to step on the projections, and consequently the means cannot be realized as being very practical. Another publication in Japanese Patent Application Laid-Open sho-60 No.216361 discloses a means serving as both charging means and transfer means, comprising a roller or brush planted with conductive fibers to be brought in contact with a photoconductor, the means in which a first cycle performs charging operation while a second cycle effects transfer operation. In this case, a conductive member is applied by a combined voltage of a d.c.voltage and an a.c voltage of 20% or more of the d.c. voltage, where maximum and minimum values of voltage waveform for the a.c.-overlapped d.c.voltage are to be within ±200 to ±2000 volts. This measure requires no switching of the applied voltages at between charging operation and transfer operation, and improves uniformity of charging as well as achieves an excellent transfer efficiency. However, since this means is oriented to suffice a special usage for effecting both charging and transfer operations, the structure tends to be complicated. Another disclosure in Japanese Patent Application Laid-Open sho-64 No.73367 shows a charging means constructed such that, in charging a photoconductor by bringing a contact-type charging member, such as a conductive roll, which is applied with a combined voltage of d.c. and a.c. voltages, into contact with the photoconductor, portion by which the charging member is in contact with the photoconductor is formed with a resistance layer and a dielectric layer as a surface layer, and therefore a reactance of the charging member to a.c. voltage is smaller than the resistance of the charging means. Use of this means may prevent voltage-drop of the voltage supplying portion due to leak even though pin-holes may happen to occur on the surface of the photoconductor, and thus the image unevenness that would be caused by the voltage drop will not occur. Here, according to this publication, it is described that the frequency of the a.c. voltage used should be within a range of 50 to 2000 Hz. This proposal was made mainly to eliminate the lowering of image quality attributed to wounds such as pin-holes and other defects arising on the surface of the photoconductor, therefore, the concept on which the technology is based is quite different from what the present invention intends to achieve. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a charging method in the electrophotographic process and an image forming device using a conductive fiber aggregation in its image forming means, the method and device which can reduce charging unevenness occurring due to broomed traces and seams of conductive fiber aggregation and assure stable charging all the time, by inhibiting the partial elevation of the surface potential of the dielectric layer, caused by the charge injection in the contacting portion between the conductive fiber aggregation and a photoconductor or photoconductive dielectric layer. It is another object of the invention to provide an electrophotographic copier using as its charger a brush-type charging part composed of conductive fibers, the copier which can prevent unevenness caused in the paper feed direction, reduce wear-out of the brush tips and damage of the image bearing medium and further eliminate the accumulation of developer piled up in the brush tips that would bring about a pollution. It is a further object of the invention to provide an electrophotographic copier using a charger device composed of conductive fiber, the copier in which adhesion of residual developer that could not be removed from an image bearing medium surface by a cleaning blade onto the tips of conductive fibers of the charging device; wear-out of the conductive fibers of charging device being laid down in the rotational direction of the image bearing medium; and wear-out of the fibers due to the change of environment; can be eliminated even when the charging device is brought into a long termed contact with the image bearing medium or the surface of the photoconductor, whereby the adhered developer to the conductive fibers can be prevented from contacting to the image bearing medium surface and therefore damaging the image bearing medium surface, charging unevenness can be inhibited, and the life of the conductive fiber itself can be increased. It is still another object of the invention to provide an image forming device in which, by providing a charging device of roll-shaped conductive fiber for an image forming device used in the electrophotographic process, failure of charging can be lessened and life of the fiber is improved and which is able to offer a final image with high quality free from defects due to charging unevenness, by properly limiting the condition of plating fibers and the relational ratio between the peripheral velocities of charger device and photoconductor drum. It is still further object of the invention to provide a method of applying charging voltage when the aforementioned conductive roller type charger and a fixing unit is used in a image forming device, the method by which the unevenness or difference in charged surface potential on an image bearing medium generated by transfer voltage can be reduced. To sum up, the object of the present invention is to solve the conventional problems such as occurrence of charging unevenness and or defects and to provide an image forming device and charging means therefore which is able to offer images with good quality as well as durable and inexpensive. The present invention has been performed in order to achieve the above object, and the object of the invention can be achieved by the features according to the characterizing portion of claim 1. An aspect of the invention lies in a charging means used in an electrophotographic copier, equipped with a conductive roller or a conductive brush as a charging device of contact type for effecting electrophotographic copying process, being constructed such that the charging device comprises an aggregation of conductive fiber, and, when a photoconductive dielectric layer is to be charged by bringing the conductive aggregation of fiber into contact therewith, the aggregation of conductive fibers is impressed with a periodically oscillating voltage having a lower boundary voltage higher than a desired surface potential of the photoconductive dielectric layer. When the photoconductive dielectric layer is charged by bringing the conductive aggregation of fiber into contact therewith, charges would be injected from the contact portion after completion of charging by discharge, causing charging unevenness and thus lowering quality of image. But with the arrangement described above, charging unevenness can be eliminated by the method since an oscillating voltage is generated by combining an a.c. voltage with a d.c. voltage required for charging so as to have a lower boundary voltage of a desired surface potential of the photoconductive dielectric layer and the thus generated oscillating voltage is applied to the aggregation of conductive fiber. In this case, it is effective that the oscillating voltage impressed to the aggregation of conductive fiber has a frequency of 100 Hz or more, and that the a moving velocity of the aggregation of conductive fiber is substantially equal to that of the photoconductive dielectric layer. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a schematic view showing basic configurations of an electrophotographic copier; Fig.2 is an oblique view showing an example of a conventional charging means; Fig.3 is an oblique view showing another example of a conventional charging means; Fig.4 is an oblique view showing a further example of a conventional charging means; Fig.5 is an illustrative view showing still another example of a conventional charging means; Fig.6 is a plot showing an example of characteristics of Paschen's discharge; Fig.7 is a schematic illustration showing an embodiment of an electrophotographic copier to which the present invention is applied; Fig.8 is an oblique view showing an example of a charging member used in a electrophotographic copier to which the present invention is applied; Fig.9 is an oblique view showing positional relation of a photoconductor drum and the charging member shown in Fig.8; Fig.10 is a characteristic chart showing time(T)-voltage(V) relations of waveforms of various signals (a driving signal to a photoconductor drum, an output indicating signal to a transfer roller, an charging voltage output indicating signal to a charging roller) and variation of surface potential of a photoconductor drum after transfer operation, in accordance with a prior art method; Fig.11 is a characteristic chart showing time(T)-voltage(V) relations of waveforms of various signals (a driving signal to a photoconductor drum, an output indicating signal to a transfer roller, an charging voltage output indicating signal to a charging roller) and variation of surface potential of a photoconductor drum after transfer operation, in accordance with the invention; DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, referring to the accompanying drawings, description in detail will hereinafter be made on an embodiment of an electrophotographic copier to which the present invention is applied. Fig.7 is a schematic illustration showing an embodiment of an image forming apparatus according to the present invention. First of all, configurations of the embodiment shown in Fig.7 will be explained. In the figure, a reference numeral 16 designates a controller for processing image-generating data transmitted from an unillustrated host computer, and another reference numeral 17 designates an engine controller for controlling an activation of the image forming apparatus in response to a signal dictating start of image forming, sent from the controller 16. A reference numeral 7 indicates a cassette for holding transfer material such as copy sheets. An arrangement is made such that a sheet is drawn out from cassette 7 by a paper feed roller 8 and conveyed by a series of conveyer rollers 9, 10 to a resist roller 11. A photoconductor drum 1 has a photoconductive dielectric layer thereon, and is rotated at a constant rate by driver means (not shown) in a clockwise direction in Fig.7. Disposed clockwise around the photoconductor drum 1 are a charger 5 made mainly of conductive fiber aggregation, an exposure-writing head or exposure unit 6, developing unit 2, a transfer unit 3 including a transfer roller, a cleaner 4. The developing unit 2 comprises a toner tank 2e having an agitating roller 2a therein, and a developer tank 2f having a magnet roller 2d for electrifying the toner and a mixing roller 2c for mixing the toner supplied by a supplying roller 2b from toner tank 2e. The cleaner 4 is provided in a form of a cleaning unit comprising mainly a cleaning blade 4a for scraping the toner from the surface of photoconductor drum 1 and toner conveying screw 4b for conveying the scraped toner to a container (not shown) for collecting the used toner. Meanwhile, a copy sheet that have passed through a place between transfer unit 3 and photoconductor drum 1 is fixed by a fixing unit 12 which comprises a heat roller 12a having a heater 12c built therein and a pressure roller 12b. Thus fixed copy material is conveyed by a conveying roller 13 and a paper discharging roller 14 to a stack guide 15. Next, description will be made on operation of the embodiment of the invention shown in Fig.7. First, data for image generation is sent from an unillustrated host computer to controller 16 to be processed therein. Then a signal dictating start of image formation is sent out to engine controller 17. From then on, the operation proceeds following a predetermined procedure. In the next, a transfer material such as copy sheets held in transfer material-holding cassette 7 is drawn out sheet by sheet by means of paper feed roller 8 to be conveyed through conveyer rollers 9, 10 up to the near side of resist roller 11. Photoconductor drum 1 is driven at a constant rate by the unillustrated rotating mechanism in a clockwise direction in Fig.7. At the time, charger 5 having conductive fiber aggregation 5a thereon is rotated such that the fiber aggregation 5a is in contact with photoconductor drum 1 with a constant bite (degree in which the fiber would cut into the drum) regulated by clearance keeping members 5b. In this while, charger 5 is applied with a combined voltage of, for example, -1000 V plus an a.c. voltage of 200 V (Vp-p), whereby the surface of photoconductor drum 1 will be charged uniformly at a desired voltage (for example, -600 V). Alternatively, a d.c. voltage of -1200 V, for example, may be impressed to uniformly charge the surface of photoconductor drum 1. In developing unit 2, in order to assure that magnet roller 2d may provide toner having a predetermined toner density, toner powder is supplied from toner tank 2e, as required, by supplying roller 2b to developer tank 2f, and the thus supplied toner powder is agitated by mixer roller 2c. During the agitation, the toner is electrified to bear charges of the same polarity with that of the voltage to be charged onto the photoconductor. In this state, when a voltage close to the charging voltage of the photoconductor is applied to the magnet roller, the toner powders adhere to portions that exposure unit 6 as an exposure writing head has irradiated, and thus the latent image is visualized. Next, resist roller 11 sends out a transfer material or copy sheet, etc. by measuring a timing so that the sheet may be positioned corresponding to an image on photoconductor drum 1. The transfer material is held between, and conveyed by, photoconductor drum 1 and transfer unit 3. During this, transfer unit 3 is impressed by a voltage of an opposite polarity to that of the toner. This is why the toner particles on photoconductor drum 1 move onto the transfer material. The toner particles on the transfer material is sandwiched between, and conveyed by, heat roller 12a with heater 12c incorporated therein and pressure roller 12b. In this while, the toner particles are molten and fixed on the transfer material. Then the transfer material is conveyed by conveying roller 13 and discharging roller 14 to stack guide 15. Meanwhile, toner that has not transferred and remains on the photoconductor drum 1 is scraped from photoconductor drum 1 by cleaning blade 4a of cleaner 4. Thus scraped toner is sent by toner conveying screw 4b to the used toner collecting container (not shown). This is a complete series of image forming process. In the invention, publicly known conductive fiber can be used as the conductive fiber constituting the charging member. An example of the conductive fiber is "REC", a product of UNITIKA or an equivalent that is made of a rayon fiber to which carbon particles are uniformly dispersed so as to have a desired resistance. An alternate example is "BELLTRON", a product of Kanebo, LTD. or an equivalent that is a conductive polyamide fiber. Besides these, any material can be selected and used properly. These conductive fibers can be formed into a padcloth, which in turn is adhered with, for example, a conductive adhesive to a conductive substrate to make a charging brush. The thus formed charging brush can be used as the charging member that is made in contact with the photoconductor drum. As an alternate embodiment, the thus formed conductive fiber cloth can be swathed spirally to form a conductive fiber member of roller type. Next, main aspects and features of the present invention will be described with reference to the embodiments. Fig.8 is an oblique view of a charging member 5 used in an image forming apparatus of the invention. In this figure, a reference numeral 5c designates a shaft for rotatably supporting a roller body on which the fiber aggregation 5a is swathed. At the vicinity of both extremes of the shaft 5c, a clearance keeping members 5b having an outer diameter slightly smaller than that of the fiber aggregation are attached adjoining to the aforementioned fiber aggregation 5a. With respect to a material used to make the charger 5, a cloth of a synthetic fiber such as rayon, etc. onto which conductive granular material such as carbon powder is dispersed, can be used again as the conductive fiber aggregation 5a. In the embodiment shown in Fig.8, the thus formed conductive fiber is wound spirally on the shaft 5c to form a roll of the fiber aggregation 5a. As the clearance keeping members 5b, hard rubber materials can be used. The rubber material is shaped into a short-height cylinder having an outer diameter slightly smaller than that of the aforementioned fiber aggregation 5a, and the thus formed cylinders can be press-fit to the shaft 5. Fig.9 is an oblique view showing a positional relation between a photoconductor drum 1 and the charger 5 shown in Fig.8. In the figure, the photoconductor drum 1 comprises a metal drum 1b of aluminum as a substrate of the photoconductor drum and a photoconductive dielectric layer la disposed therearound. As is shown in the figure, charger 5 is disposed and supported such that conductive fiber aggregation 5a comes in contact with the dielectric layer la and clearance keeping members 5b are in direct contact with metal drum 1b, that is, the end portions of the photoconductor drum 1 on which no dielectric layer la is covered. As being thus constructed, the conductive fiber aggregation 5a can rotate following to the rotation of photoconductive dielectric layer la, as described above. Now, a specific example of charger 5 for use in an electrophotographic copier of the invention will be referred to. In the embodiment shown in Fig.9, a conductive roller shaft of 6 mm in diameter is used as the shaft 5c, around which a conductive fiber cloth made of a rayon cloth of 20mm wide with carbon powder dispersed thereon is swathed spirally to form a roll of conductive fiber aggregation 5a. Clearance keeping members 5b formed of a hard rubber material having an outer diameter of 10mm are pressingly fit in and fixed at both ends of the thus formed conductive fiber aggregation 5a. The keeping members 5b are in contact with metal exposed portions of the photoconductor drum 1 or the aluminum drum 1b to be driven thereby. Therefore, a smooth sliding can be performed and of course, no charging unevenness occurs. It should be noted that the apparatus of the invention is not to be limited to the above charges, charger 5 may be, for example, equipped with an individual driver means (not shown) such as a motor or the like. Besides, conductive fiber aggregation 5a can be made belt-typed. An embodiment of the invention will hereinafter be described. At the beginning of description of the embodiment, a mechanism of charging the photoconductive dielectric layer using the conductive fiber aggregation. In a portion where the dielectric layer is brought in contact with the conductive fiber or specifically the tips of fibers, charges move from places with a higher potential to places with a lower potential, while discharge occurs in accordance with the Paschen's discharge characteristics as exemplarily shown above in Fig.6, in a portion where the dielectric layer is spaced certain distances from the conductive fiber, specifically, for example, in the vicinity of the contact portion or on the side portion of the conductive fibers. The discharge will stop when charges on the conductive fibers move to the dielectric layer side and the potential difference across the clearance becomes lower than the discharge threshold level. After the completion of discharge, injection of charges still lasts, since the conductive fiber aggregation is in contact with the photoconductive dielectric layer, thus the surface potential in the contact portion increases, causing charging unevenness, as discussed above. Therefore, in the embodiment, during the charging process an a.c. voltage is overlapped to a d.c. voltage required for the charging so as to make a periodically varying voltage that has a lower limit higher than a desired surface potential of the photoconductive dielectric layer. Application of the thus created varying voltage to the conductive fiber aggregation can solve the problem of the above-described charging unevenness all at once. In this case, the oscillating voltage is preferably small, but if the lower limit of the varying voltage is lower than the desired surface voltage, charges might possibly be injected inversely from the photoconductive dielectric layer toward the conductive fiber aggregation. This is why the lower limit of the oscillating voltage should be higher than a desired surface voltage. Effective frequency of the oscillating voltage is 100 Hz or more, and in case of less than 100 Hz, it becomes quite difficult to inhibit appearance of charging unevenness caused by the varying voltage. In contrast, no limitation is particularly specified for the upper limit of the frequency, but since the charging system includes a capacitive component, an excessively high frequency makes the system unable to follow the oscillating voltage, only to lower the efficiency. Accordingly, 1,000 Hz or less frequency is suitable in practice. In charging, it is necessary to establish a secure contact between the fiber aggregation and the dielectric layer. Besides, it is preferable to reduce the mechanical rubbing between the both in view of improvement in durability of the both elements. As described above, in the invention, it is effective to use a charging member, as shown above, that is constructed such that a roll-shaped conductive fiber aggregation is rotatably supported by a shaft, and clearance keeping members having an outer diameter slightly smaller than that of the conductive fiber aggregation are fit in adjacent to the both ends of the fiber aggregation, whereby the fiber aggregation can come in secure contact with the dielectric layer and rotate at substantially the same rate with the rotation of the dielectric body, following the rotation thereof. The thus constructed means, upon charging process effected by the contact between the conductive fiber aggregation and the photoconductive dielectric layer, inhibits the partial elevation of the surface potential of the dielectric layer and therefore reduces charging unevenness occurring due to broomed traces and seams of conductive fiber aggregation, making it possible to assure a stable charging operation in a prolonged period of time. The specification of the oscillating voltage applied to charger 5 is not strictly limited to the above value, as long as the voltage has a lower limit higher than that of a desired surface potential and can generate a desired surface potential in total. Moreover, various kinds of waveforms such as chopping waves, pulsing waves, etc. other than alternating waves can be properly selected. It should be noted that the present invention is not limited to the embodiment described above, but any change and modification can be made within a range of the invention. Operation of the electrophotographic copier in accordance with the conventional driving manner, used to be effected such that, as shown in Fig.10, when photoconductor drum 1 is activated to drive by a driving signal "a", a charging output indicating signal "b" simultaneously activates charging roller 5, so that the charger impresses the surface of photoconductor drum 1 at a certain charging voltage in accordance with the signal "b" in Fig.10 (in which signals are expressed depending upon time T). Then, after a certain span of time, in accordance with an output indicating signal "c" to transfer roller shown in Fig.10, a predetermined voltage for transfer is impressed to transfer roller 3, so that toner powder image on photoconductor drum 1 is transferred onto recording sheet 5 sent out by conveyer roller 10 and resist roller 11. As a result, the surface potential of photoconductor drum 1 after transfer completed exhibits variation depending on time T as indicated by surface potential variation plot "d" in Fig.10. In accordance with the present invention, as shown in a waveform of charging output indicating signal to charging roller in Fig.11, since portion on the photoconductor corresponding to region subjected to the transfer voltage, is impressed by the charging roller with a charging application voltage higher than that applied to the region without the transfer voltage applied, the surface potential after completion of transfer operation of photoconductor drum 1 as an image bearing medium can be compensated. As a result, a uniform surface potential can be obtained as shown by the variation of the surface potential shown in Fig.11. As an experiment to confirm the effect of the invention, the surface potential of photoconductor drum 1 was measured. Upon the measurement of the surface potential of the photoconductor, a conductive roller mainly consisting of silicon rubber (resistance: 3.0 x 10 6 Ω, hardness JIS-A: 36 degrees, roller size: 11 mm ) was used as charging roller 5. When applied voltage to the charging roller was fixed at a constant value (for example, at -1180 V) in accordance with the conventional process shown in Fig.10, the resulting surface potential of the photoconductor was charged at about -600 V. The measurement of the surface potential of the photoconductor drum was carried out with respect to both the regions to which transfer operation had been effected and to which no transfer operation was effected. In the conventional process, the measurement of surface potential for the region with the transfer operation effected was -560 V, whereas the measurement of surface potential for the region with no transfer operation effected was -585 V. That is, the variation of surface potential was Δ25 V as shown in Table 1. Potential in Region with Transfer Potential in Region without Transfer Potential Variation -560 V -585 V Δ25 V As the same charging roller 5 is used, the surface potential of photoconductor drum 1 was measured by applying charging voltage in accordance with the present invention. That is, the voltage that was applied to charging roller 5 was adapted to change over between two levels. As shown in Table 2, the region on the photoconductor surface corresponding to the region without transfer was impressed by -1180 V as used to be. Applied Voltage Region without Transfer Applied Voltage Region with Transfer -1180 V (LOW) -1205 V (HIGH) The region on the photoconductor surface corresponding to the region with transfer was impressed by -1205 V. The surface potential of the photoconductor was measured by switching the two levels of charging voltage in accordance with this method of charging. The measurement of surface potential for the region with the transfer operation effected was -585 V, whereas the measurement of surface potential for the region with no transfer operation effected was -585 V. That is, the variation of surface potential was Δ0 V as shown in Table 3. Potential in Region with Transfer Potential in Region without Transfer Potential Variation -585 V -585 V Δ0 V As in the invention, when the photoconductor was charged by the charging roller having two levels of charging voltage, the resulting surface potential presented uniformity over the photoconductor surface as shown above, and upon printing, a sharp image with less fog was obtained by uniformalizing the surface potential over the photoconductor surface. As another experiment to establish the effect of the invention more confirmatively, the surface potential of photoconductor drum 1 was measured. Upon the measurement of the surface potential of the photoconductor, a conductive roller mainly consisting of urethane rubber (resistance: 1.2 × 10 5 Ω, hardness JIS-A: 35 degrees, roller size: 11 mm ) was used as charging member. When applied voltage to the charging roller was fixed at a constant value (for example, at -1140 V) in accordance with the conventional process, the resulting surface potential of the photoconductor was charged at about - 600 V. The measurement of the surface potential of the photoconductor drum was carried out with respect to both the regions to which transfer operation had been effected and to which no transfer operation was effected. In the conventional process, the measurement of surface potential for the region with the transfer operation effected was -555 V, whereas the measurement of surface potential for the region with no transfer operation effected was -580 V. That is, the variation of surface potential was Δ25 V. Therefore, the voltage that was applied to charging roller 5 was adapted to change over between two levels. That is, the region on the photoconductor surface corresponding to the region without transfer was impressed by -1140 V as used to be. On the other hand, the region on the photoconductor surface corresponding to the region with transfer was impressed by -1165 V. The surface potential of the photoconductor was measured by switching the two levels of charging voltage in accordance with this method of charging. The measurement of surface potential for the region with the transfer operation effected was -580 V, whereas the measurement of surface potential for the region with no transfer operation effected was -580 V. That is, the variation of surface potential was Δ0 V. As in the invention, when the photoconductor was charged by the charging roller having two levels of charging voltage, the resulting surface potential presented uniformity over the photoconductor surface as shown above, and upon printing, a sharp image with less fog was obtained by uniformalizing the surface potential over the photoconductor surface. Although description of this embodiment was made on the example in which a conductive roller-type transfer unit (or transfer roller) was used as its transfer unit 3, the charging voltage applying means should not be limited to such transfer rollers, but the invention can be applied to general electrophotographic copiers using a corona discharge type transfer unit. As described above, in the conventional charging voltage application method, the surface potential of photoconductor is influenced or caused to be different by the transfer operation. That is, the difference of surface potential is generated between whether the transfer voltage is applied to the image bearing medium or not. More specifically, in the reverse-developing process, taking the charged surface potential to be positive, the surface potential of a region that are subjected to the transfer voltage will be lowered since the polarity of the transfer voltage is opposite to that of the charging voltage. To deal with this, in accordance with the method of the invention for applying charging voltage to the image bearing medium, the voltage applied to the charging roller is increased by an increment corresponding to reduction of the surface potential caused by the transfer voltage, in order to uniformalize the surface potential after the charging. By this method, the portion of photoconductor having its surface potential lowered by the transfer can be supplied with more charges, so that the lowered surface potential is compensated to be uniform. As a result, it is possible to provide an image with sharpness and less fog.

Claims: An electrophotographic copier, equipped with a conductive roller or a conductive brush as a charging device (5) of contact type for effecting electrophotographic copying process, wherein said charging device (5) comprises an aggregation of conductive fiber formed like a roll (5a); and clearance keeping members (5b), disposed at least in both ends of said roll-like aggregation of conductive fiber (5a), and when a photoconductive dielectric layer (1a) is to be charged by bringing said conductive aggregation of fiber (5a) into contact therewith. said clearance keeping members (5b) come in contact with the surface of said photoconductive dielectric layer (1a) at both ends thereof for keeping a predetermined clearance, so that said roll-like aggregation of conductive fibers (5a) is rotated through said clearance keeping members (5b) following to the rotation of photoconductive dielectric layer (1a), characterized in that , when said photoconductive dielectric layer (1a) is to be charged by bringing said conductive aggregation of fiber (5a) into contact therewith, said aggregation of conductive fibers (5a) is impressed with a periodically oscillating voltage having a lower limit voltage higher than a desired surface potential of said photoconductive dielectric layer (1a). An electrophotographic copier according to claim 1, wherein said oscillating voltage impressed to said aggregation of conductive fiber (5a) has a frequency of 100 Hz or more. An electrophotographic copier according to claim I or 2. wherein a moving velocity of said aggregation of conductive fiber (5a) is substantially equal to that of said photoconductive dielectric layer (1a).

Inventors: HAYAKAWA TAKASHI, IRIHARA KOUICHI, NISHIO YUKIHITO, TANI KENJI, WADA TAKASUMI, HAYAKAWA, TAKASHI, IRIHARA, KOUICHI, NISHIO, YUKIHITO, TANI, KENJI, WADA, TAKASUMI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: G03G15
IPC codes: G03G15