Abstract:
A charging device includes a movable member to be charged; a charging member contactable to the member to be charged to electrically charge the member to be charged; a discharging device, disposed upstream of a contact portion between the charging member and the member to be charged with respect to a movement direction of the member to be charged, for electrically discharging the member to be charged at a portion adjacent to the contact portion.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus such as an electrophotographic machine, more particularly to a charging device which contacts a charging member to a surface to be charged of a member such as a photosensitive member, while charging the surface. 
     Heretofore, in an electrophotographic machine (a copying machine, a laser beam printer, for example) and an image forming apparatus such as an electrostatic recording machine, as for a device for electrically charging the surface to be charged such as a photosensitive member, a dielectric material or the like, functioning as the image bearing member, a corona charging device has been widely used. This corona charging device is charged using corona discharge which is air discharge. It is very effective as for the means for electrically charging the surface to be charged uniformly to a predetermined potential. On the other hand, it requires a high voltage source. Additionally, it produces a significant amount of ozone. As is different from such a corona charging device, a contact charging device is usable with a low voltage, and it has the advantage that the amount of the ozone production is small. 
     In a contact charging device, the charging member supplied with a voltage is contacted directly to the surface to be charged, while the surface is being charged. Because of the above described advantages, in the image forming apparatus, for example, it attracts attention as a charging device to be replaced with the corona charging device for the image bearing member or the like, and the development is carried out for practical use. 
     For example, Japanese Laid-Open Patent Application No. 51492/1987 and Japanese Laid-Open Patent Application 230334/1987, propose that by forming an oscillating electric field having a peak-to-peak voltage which is twice or more as high as the charge starting voltage when a DC voltage is supplied to the charging member (alternating electric field) between the charging member and the member to be charged such as the photosensitive member, and by using a charging member which is provided with a high resistance layer as the surface layer, the charge uniformity of the photosensitive member is assured. In addition, the leakage prevention of current attributable to the pin hole or the defects of the surface of the member to be charged is accomplished. 
     In another case, an electrically conductive member (electrically conductive potential maintaining member) such as an electrically conductive fiber brush or an electrically conductive elastic roller, is contacted to the member to be charged as the charging member, and a DC voltage is externally applied, by which the electric charge is injected directly into the surface of the member to be charged in the form of a photosensitive member, and the photosensitive member surface is charged to the predetermined potential. 
     FIG. 6 is a schematic view of an example of the contact type device, and the charging device shown in this Figure, is provided with a charging roller 2 and a voltage source 3. 
     The charging roller 2 functioning as the charging member comprises a central core metal 2c, a conductive layer 2b outside it and a resistance layer 2c further outside it. 
     The charging roller 2 is disposed in parallel with the drum-like photosensitive drum 1 functioning as the member to be charged. Simultaneously therewith, the right and left opposite end portions (opposite end portions of the longitudinal direction of the shaft) of the core metal 2c is supported for rotation about an unshown bearings. The charging roller 2 is press-contacted by an urging means (not shown) at predetermined urging force against the surface to be charged 1a of the photosensitive drum 1 surface. It is possible to forcedly drive by a motor with the provision of gear or the like rotating in a direction R2 following the rotation of the photosensitive drum 1 in the direction R1. The voltage source 3 is a bias voltage application voltage source for the charging roller 2. This voltage source 3 and the core metal 2c of the charging roller 2 are connected electrically with each other through a power supply sliding contact 3a. A predetermined bias voltage is applied by a voltage source 3 to the charging roller 2. 
     As for this bias, a DC bias voltage and an AC biased DC voltage, have been proposed. 
     When the photosensitive drum 1 is rotated in a direction indicated by arrow R1, the charging roller 2 is driven in the direction of the arrow R2. At this time, the predetermined bias voltage is applied from the voltage source 3 to the charging roller 2. By doing so, outer peripheral surface of the photosensitive drum 1, that is, the surface to be charged 1a is electrically charged to the predetermined polarity and to the predetermined potential. 
     After that, the charged photosensitive drum 1 is exposed to image light 5 in accordance with the image to be formed by this, an electrostatic latent image is formed. The electrostatic latent image is visualized with the toner in the developing device 6. The visualized toner image is transferred by a transfer charger 7 to a transfer material P. 
     The toner image on the transfer material P having the toner image transferred thereto is fixed by a fixing device (not shown). Thereafter, it is discharged to an outside of the apparatus. On the other hand, the photosensitive drum after the image transfer is cleaned by a cleaning blade 9a of a cleaning device 9 so that the remaining developer (residual toner) is removed. Subsequently, it is discharged by a pre-exposure light device 10 in preparation for the following image formation. A nip N is formed at the contact portion between the surface 1a to be charged of the photosensitive drum 1 and the charging roller 2. On the basis of a movement direction R1 of the surface to be charged, in the upstream side and the downstream side of the nip N, a first gap G1 and a second gap G2 are formed, respectively. When the consideration is made as to the gaps G1 and G2, that is, the distance between the surface to be charged 1a and the charging roller 2, the first gap gradually decreases toward the nip N, whereas the second gap increases gradually away from the nip N. 
     However, as in the above-described conventional example, it is known that when the surface to be charged 1a of the photosensitive drum 1 is contacted to the charging roller 2, while charging the surface. Not only in the nip N, but also in the first and second gaps G1 and G2 upstream and downstream thereof, respectively, by the movement of the electric charge, particularly at the initial stage, that is, at the time of the state in which the exposed surface 1a of the photosensitive drum 1 is not scraped to a significant extent with the cleaning blade 9a or the like, if a halftone or solid black image is formed, the electric charge moved from the charging roller 2 to the surface to be charged of the photosensitive drum 1 in the first gap in the upstream, starts separation discharge and is moved from the surface to be charged 1a to the charging roller 2 conversely in the second gap in the upstream. Therefore, there is a liability that a part of electric charge on the charged surface 1a is removed. A white stripe having a length of 2-10 mm and a width of 0.5 mm or less is produced in a direction perpendicular to the movement direction of the surface to be charge 1a, so that the stabilized image formation is disturbed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a charging device and an image forming apparatus which is capable of charging the member to be charged in good order. 
     It is another object of the present invention to provide an image forming apparatus in which image defect in the form of a stripe is prevented. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view illustrating the structure of an image forming apparatus according to the present invention. 
     FIG. 2 is a sectional view illustrating a discharging device according to another embodiment of the present invention. 
     FIG. 3 is a sectional view of a discharging device according to another embodiment of the present invention. 
     FIG. 4 is a sectional view illustrating a charging roller according to a further embodiment of the present invention. 
     FIG. 5 shows an operation sequence of an image forming apparatus. 
     FIG. 6 is a sectional view of a conventional image forming apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, an embodiment of the present invention will be described in conjunction with the accompanying drawings. 
     FIG. 1 is a sectional view around an image forming station of an image forming apparatus according to an embodiment of the present invention. 
     In the image forming station, a cylindrical photosensitive drum 1 (member to be charged) is rotatably supported in the main assembly (not shown) of the apparatus. The photosensitive drum 1 is a drum type electrophotographic photosensitive member having an electrically conductive base layer 1b made of aluminum or the like, a laminated polymeric material formed thereon, that is, a photoconductive layer (surface to be charged) 1a. The OPC photoconductor in this embodiment is negatively chargeable. The entirety of the photosensitive drum 1 is rotated about a shaft 1d in a direction of an arrow R1 at a predetermined peripheral speed. 
     The charging member 2 is disposed above the photosensitive drum 1. The charging member 2 is contacted to the surface to be charged 1a of the photosensitive drum 1 to uniformly charging (primary charging) to a predetermined polarity and to the predetermined potential. In this embodiment, it is of a roller type (charging roller). 
     The charging roller 2 is provided with a core metal 2c, a conductive layer 2b thereon and a resistance layer 2a2 thereon. The longitudinal opposite end portions of the core metal 2c (in the direction perpendicular to the sheet of the drawing of FIG. 1) of the charging roller 2, are rotatably supported by unshown bearing members, and it is in parallel with the photosensitive drum 1. It is urged to the surface to be charged 1a of the photosensitive drum 1 by urging means (unshown) at a predetermined urging force. With the rotation of the photosensitive drum I in a direction R1, it is driven in a direction R2. To a core metal 2c of the charging roller 2, a power supply contact 3a is contacted. The bias voltage is applied from the voltage source through this contact 3a. By this, the surface to be charged 1a of the photosensitive drum 1 is charged to a predetermined polarity and potential. The charging roller 2 may be driven by the photosensitive drum 1. It may be positively rotated in the same direction as or in the opposite direction from the surface to be charged 1a of the photosensitive drum. 
     Furthermore, it may be fixed without rotation. The photosensitive drum which has been subjected to the charging process for uniform charging by the charging roller 2, is exposed to image information through exposure means 5 (laser beam scanning exposure, original image slit exposure or the like, and the latter is used in this embodiment). Then, an electrostatic latent image is formed on the surface 1a to be charged in accordance with the intended image formation. The electrostatic latent image is sequentially developed with the positively charged toner by a developing device 6 into a visualized image, sequentially. 
     Subsequently, the first toner image is transferred onto the transfer material P by a transfer device 7. The transfer device 7 is provided with a rotatable transfer roller 7a and a voltage source 7b. It is effective to charge the backside of the transfer material P to the polarity opposite from the toner by the roller 7a. By this, the toner image on the photosensitive drum 1 is sequentially transferred onto the upper surface of the transfer material P. Here, the transfer material P is conveyed from an unshown feeding device to the transfer station between the photosensitive drum 1 and the transfer device 7 at the proper timing in synchronism with the rotation of the photosensitive drum 1. 
     The transfer material having received the toner image P transferred thereto, is separated from the photosensitive drum 1, and it is conveyed to an unshown fixing device, and the toner image is fixed. Thereafter, it is discharged to an outside of the main assembly of the apparatus, or it is refed to the transfer station if it should receive an image on the backside, for example. 
     The photosensitive drum 1 after the image transfer, is cleaned by a cleaning blade 9a of the cleaning device 9, so that deposited contamination such as residual toner or the like is removed. Then, the residual charge is removed by the pre-exposure device 10. 
     Between the cleaning device 9 and the downstream charging roller 2, a charging device 11 of the present invention is disposed. The pre-exposure device 10 removes the residual charge from the surface to be charged 1a of the photosensitive drum 1 substantially to 0 V. The discharging device 11 is effective to provide the proper charging of the surface to be charged 1a by the charging roller 2. Before describing the charging device 11, the description will be made as to the charging roller 2. As shown in FIG. 1, the charging roller 2 of this embodiment comprises a core metal 2c, an electrically conductive rubber layer 2b comprising EPDM rubber (terpolymer of ethylene propylene diene) or the like and having a volume resistivity of 10 4  -10 5 , and an intermediate resistance layer 2a2 of hydrin rubber or the like having a volume resistivity of 10 7  -10 9  ohm.cm, and a blocking layer 2a1 of Toresin (available from Teikoku Kagaku Kabushiki Kaisha, trade mark) or another nylon material having a volume resistivity 10 7  -10 10  ohm.cm and having a hardness of 50-70 degrees (Asker C). The charging roller 2 is contacted to the photosensitive drum 1 at a total pressure of 1600 g so as to be rotated thereby. 
     Here, when the charging roller 2 is contacted to the surface to be charged 1a of the photosensitive drum 1, first and second small gap G1 and G2 are formed at the upstream and downstream side of the nip between them. The upstream gap G1 gradually decreases toward the nip N and toward the downstream, and the downstream gap G2 increases away from the nip N and toward the downstream. The nip extends through substantially the entire length of the charging roller (axial direction). Therefore, the first and second gaps G1 and G2 extend throughout the length of the charging roller 2. 
     The discharging device 11 comprises a light projection device comprising a fuse lamp. An illumination per unit time of the fuse lamp 11 is 5.0 Lux. It irradiates light to a portion forming the first gap G1 along the entire length in synchronism with application of the bias voltage from the voltage source 3 or for a longer period than the bias voltage application period. By the on and off of the fuse lamp 11, the surface 1a to be charged is electrically discharged in the portion of the first gap. If the discharging operation is not carried out, that is, when the image forming operation is carried out in the conventional manner shown in FIG. 6, a horizontal white stripe is produced from the first sheet to approx. 2000 sheet image forming operations upon forming half-tone and solid black image formations, because the charge movement between the charging roller 2 and the member to be charged 1a occurs in the first gap G1, nip N and the second gap G2. 
     Referring to FIG. 5, the description will be made as to the operation sequence of the image forming apparatus of FIG. 1 when the discharge is carried out. In response to a print start signal, the photosensitive drum which has been in the stand-by state is rotated, and a pre-rotation operation is carried out. Simultaneously with the start of the photosensitive drum 1, the fuse lamp (discharging device) 11 is actuated. In a period A1, the surface to be charged 1a of the photosensitive drum 1 is electrically discharged through more than one full-turn. 
     2. Subsequently, a DC bias voltage functioning as a primary charging bias to the charging roller 2 is actuated. 
     3. Then, the first image formation is carried out through a slid for an original. 
     4. After completion of the last sheet image formation, the photosensitive drum 1 is post-rotated. During the post-rotation period A2, the discharge exposure 15 is carried out by the fuse lamp 11 through not less than one full-turn of the photosensitive drum 1, and the rotation and the discharge exposure is stopped. The apparatus returns to the stand-by state until the next print starting signal is produced. 
     As shown in the above-described operational sequence, the fuse lamp 11 illuminates the surface to be charged 1a in the first gap G1, by which the electric charge of this portion by the charging roller 2 is removed, and the charging is effected in the nip and the second gap G2. By doing so, the separation discharge which occurs by the surface to be charged 1a moving through the nip N and the second gap G2, and the white stripe having resulted at the time of halftone or solid black image formation can be avoided. 
     The time period in which the exposure by the lamp 11 is carried out may be the period in which the voltage is applied to the charging roller at the minimum. 
     Furthermore, the region of the drum 1 exposed to the lamp 11 light may be at least the region in which the toner image formation is possible for the next image formation. The region capable of toner image formation is a region in which a toner image can be formed on the drum 1 for any image information. 
     Since the surface of the phosensitive drum has been electrically discharged by the lamp 10 when the surface is going to reach the nip N, the lamp 11 functions to prevent the surface from being electrically charged by the charging roller 2. 
     Embodiment 2 
     Referring to FIG. 2, another embodiment of the charging device 11 will be described. The charging device 11 may also function as a pre-exposure device 10. 
     For example, a half mirror 12 is attached to a cleaning blade of the cleaning device 9, and the illumination light from the discharging device 11 (pre-exposure device 10) is divided into the light for discharging the surface to be charged 1a immediately after the cleaning station and the light for discharging the surface to be charged 1a at the first gap G1 for the charging. By doing so, there is no need of using special exposure device for the discharge of the first gap G1, and the entire structure is simplified, and in addition, the operational sequence can be simplified. In place of the half mirror 12, a total reflection mirror or a member finished to a mirror surface by evaporation or metal plating is usable. It is disposed for retraction so that the optical path of the illuminating light is switched between for the cleaning blade 9a and for the first gap G1. 
     It is preferable that a tangent line of the nip N is substantially in accord with the beam from the lamp, as shown in FIGS. 1 and 2, since then, the light reaches deep into the gap G1. 
     Embodiment 3 
     Referring to FIG. 3, a further embodiment will be described. As shown in this Figure, an outer circumference of the photosensitive drum 1, that is, the photoconductive layer 1a and the conductive base layer 1b, are formed with known transparent photosensitive material, and the discharging is effected with a discharging device 11 disposed inside the transparent photosensitive member with the light amount of 10 Lux.sec by the discharging device 11. The discharging device 11 thus electrically discharge the photosensitive member in the gap G1 along the entire length thereof in the width of 3 mm. The discharging device 11 may include an LED lamp. In embodiments 1 and 2, there is a liability that the light source or the mirror surface may be contaminated with the result that the amount of the light may be reduce by the scattered toner during use thereof or the like. However, in this embodiment, the light source is disposed inside the photosensitive drum, the discharge can be carried out, so that the horizontal white stripe can be avoided without the contamination and with a constant light quantity at all times, and with stability. The description will be made as to another charging member. In the foregoing embodiments, the charging roller comprises a core metal and three layers thereon. In this embodiment, has two layers on the core metal, as shown in FIG. 4. 
     The charging roller 2 shown in FIG. 4 comprises a core metal 2c, an electrically conductive layer 2b thereon and a high resistance layer 2a mainly comprising resin material thereon. By doing so, even if there are pin holes 1c and 1c or defects in the photoconductive layer 1a of the photosensitive drum 1, it can be avoided that the current leaks through these pin holes or the like. 
     The discharging device 11 having been described in Embodiments 1-4, both can be equally used when the bias voltage to be applied to the charging roller 2 by the voltage source 3 comprises only DC component, and when it is a DC biased AC. 
     Furthermore, the discharging device 11 is not limited to the light projection device such as the fuse lamp, but may be of another means, as long as the surface to be charged forming the first gap G1 can be effectively discharged. For example, a discharging brush is disposed in contact with the entire length of the surface 1a to be discharged. By this, substantially the same effects as in the light projection device can be provided. However, it is desirable to electrically isolate the ends of the discharging brush from the charging roller. It is desirable that when the charging roller is not urged to the photosensitive drum at all, it has a crowned roller, wherein the outside diameter gradually decreases from the longitudinal center of the roller toward the respective ends thereof, since then when the ends of the core metal of the charging roller are urged to press it to the photosensitive drum, the nip between the roller and the drum becomes straight in the longitudinal direction substantially with the constant width. If the nip is straight in the longitudinal direction in this way, the light by the discharging device (exposure lamp) becomes straight in the longitudinal direction, and therefore, it is easy to project the light to the drum at the upstream side of the nip. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.