Image forming apparatus with corona discharge device for transferring toner image
FIELD OF THE INVENTION 
     The present invention relates to an image forming apparatus of the toner image transfer type, and more particularly to improvements in corona discharge devices for transferring to a toner image onto paper. 
     BACKGROUND OF THE INVENTION 
     Electrophotographic copying machines and the like usually include a corona discharge device for transferring toner images formed on a photosensitive member onto paper. The use of a corona discharge device nevertheless involves a problem in that ozone produced by the discharge exerts an adverse influence on the photosensitive member. 
     FIG. 1 shows a corona discharge device 1a already proposed to overcome this problem. This known corona discharge device 1a is disposed under a photosensitive drum 2a and has a discharge wire 11a extending axially of the drum 2a and a stabilizer plate 12a surrounding the discharge wire 11a on opposite lateral side and on a lower side. The stabilizer plate 12a is formed at its bottom with an ozone discharge opening 13a for drawing air therethrough to discharge ozone from the device. 
     With the device 1a, however, paper P close an hopening 14a in the upper side of the device 1a when passing over the opening 14a, consequently impeding the flow of air therethrough to produce a negative pressure within the device 1a. Accordingly, the device has the drawback that the rear end of the paper P, when passing between the device 1a and the drum 2a, is separated from the drum 2a, making it impossible for the device to transfer the toner image properly. 
     SUMMARY OF THE INVENTION 
     Accordingly, the primary object of the present invention is to provide an image forming apparatus with an improved corona discharge device for transferring toner images to paper. 
     Another object of the invention is to provide an image forming apparatus with an improved corona discharge device which is operable to properly transfer toner images to paper at all times and which is adapted to effectively exhaust ozone generated by corona discharge. 
     The foregoing objects can be well fulfilled according to the present invention by providing an image forming apparatus which has a corona discharge device for transferring a toner image forming on a toner image carrier onto paper, the corona discharge device having a wire electrode, and a shield member surrounding the wire electrode. The shield member includes a lower portion located opposite the toner image carrier with respect to the wire electrode, and a pair of side portions extending from the respective side edges of the lower portion toward the toner image carrier. The shield member has a first opening in the lower portion and a second opening in one of the side portions at a position near the toner image carrier. A cover member is located between the toner image carrier and the wire electrode and extends downstream from the upstream side of the wire electrode with respect to the direction of transport of the paper to cover a part of the upper side of the wire electrode. The remaining part of the upper side of the wire electrode is used for transferring the toner image by corona discharge. The image forming apparatus further has an apparatus for sucking air from the inside of the shield member through the first opening, thus permitting air to flow into the shield member through the second opening and also through the remaining part from outside. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects or features of the present invention will become apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a diagram showing a conventional corona discharge transfer device and its surroundings; 
     FIG. 2 is a diagram of an image forming apparatus of the toner image transfer type embodying the invention; 
     FIG. 3 is a perspective view partly broken away showing a corona discharge transfer device of the image forming apparatus of FIG. 2; 
     FIG. 4 is a diagram showing the transfer device of FIG. 3 and its surroundings; 
     FIG. 5 (a) and FIG. 6 (a) are, respectively, a diagram showing lines of electric force of the transfer device and a diagram of transfer current values in the transfer region; 
     FIG. 5 (b) and FIG. 6 (b) are, respectively, a diagram showing lines of electric force and a diagram of transfer current values in the transfer region in the case where the stabilizer plate of the transfer device has no opening in a side portion thereof; and 
     FIG. 7 is a diagram showing a modified corona discharge transfer device and its surroundings. 
    
    
     In the following description, like parts are designated by like reference numbers throughout the several drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention will be described below. 
     FIG. 2 is a diagram schematically showing an image forming apparatus of the toner image transfer type embodying the invention. 
     With this apparatus, an original is placed on a document table 8 and illuminated with light from a light source (not shown). The light reflected from the original is transmitted to a photosensitive drum 2 through mirrors 51 to 54 and a projection lens 55. The drum 2 has on its surface a photosensitive layer, which is illuminated with eraser lamps 62 and 64, sensitized by chargers 63 and 65, and thereafter exposed to the light from the original to form an electrostatic latent image thereon. The latent image is converted to a visible image with toner by a developing unit 66. The toner image is transferred onto paper by the corona discharge transfer device 1, to be described below in detail. The paper bearing the toner image thereon is separated from the drum 2 by a separating claw 67 and transported by a conveyor belt 74 to a fixing unit 75, in which the toner image is fixed to the surface of the paper. The paper is then discharged from the apparatus by delivery rollers 76. FIG. 2 further shows a cleaner 61, paper cassettes 71 and 72, and a timing roller 73. The conveyor belt 74 is formed with a multiplicity of holes or apertures, through which air is drawn by a suction fan 77, whereby the paper is held attracted to the belt 74 for transport. 
     With reference to FIGS. 3 and 4, the corona discharge transfer device 1 comprises a discharge wire 11 and a stabilizer plate 12 surrounding the discharge wire 11. 
     The stabilizer plate 12 includes side portions 121 and 122 covering the discharge wire 11 on opposite lateral sides, a lower portion 123 interconnecting the side portions 121 and 122 at their lower ends for covering the wire 11 on the lower side, and an upper portion 124 extending from the upper end of the side portion 121 to partly cover the wire 11 on the upper side. The lower portion 123 is formed in the middle of its width with five rectangular lower openings 13 longitudinally thereof. The upper portion 124 and the side portion 122 define an upper opening 14 therebetween. The side portion 121 has five side openings 15 formed longitudinally thereof at a location within one-fourth of its entire height as measured vertically from its upper end. 
     The discharge wire 11 extends through the space surrounded by the stabilizer plate 12 and is fastened by setscrews 171, 172 to insulating blocks 161, 162 provided at the longitudinal opposite ends of the plate. 
     The corona discharge transfer device 1 is disposed under the photosensitive drum 2 and mounted on a duct housing 19 extending toward the suction fan 77, with a packing 18 provided between the device and the housing. Paper P is transported between the device 1 and the drum 2 from a guide plate 20 toward the conveyor belt 74 in the direction of arrow a shown. 
     With the above arrangement, air flows through the upper opening 14 opposed to the drum 2 and also through the side openings 15 into the discharge device 1 and then flows out through the lower openings 13 into the duct housing 19 by being drawn by the suction fan 77, as indicated by arrows b and c. Accordingly, even when the paper P passing over the upper opening 14 of the device 1 blocks the flow of air through the opening 14 (arrow b), no negative pressure will be produced within the device 1 owing to the presence of the flow of air through the side openings 15 (arrow c). This obviates the likelihood that the rear end of the paper P will be drawn toward the transfer device 1, assuring the proper transfer of the toner at all times. 
     The side openings 15 are formed in the side portion 121 of the stabilizer plate 12 in proximity to the drum 2, more specifically, as located within one-fourth of the entire height of the side plate 121 as measured vertically from its upper end. When thus located, the openings permit air to flow continuously from an upper position downward, whereby the ozone generated in the device 1 can be exhausted effectively. If the openings are located otherwise, air will now flow through the device smoothly, failing to fully discharge the ozone. It is undesirable to form an opening in the upper portion 124 since the upper portion 124 serves to preclude early transfer of the toner, that is, to prevent the transfer of the toner onto the paper P at a position other than the contemplated region of transfer. 
     Next, FIG. 5 (a) and FIG. 5 (a) respectively, show lines of electric force and transfer current values in the transfer region as produced by the present device having the side openings 15. These illustrated results are almost in no way different from the corresponding lines of electric force and transfer current values shown in FIG. 5 (b) and FIG. 6 (b), respectively, as observed when the device has no side opening 15. The present device is therefore also fully useful with respect to transfer efficiency. 
     Instead of the upper portion 124, an early transfer preventing plate 21 may be attached to a guide plate 20 which is provided upstream from a corona discharge transfer device 1&#39; with respect to the direction of transport of paper P, as seen in FIG. 7. The plate 21 may be made of an insulating material or an electrically conductive material. The device 1&#39; has a side portion 121&#39; which is also formed with openings 15&#39; proximate to the photosensitive drum 2, whereby the same effect as already described is available. It is of course desirable to locate the openings 15&#39; within one-fourth of the height of the device as measured from its upper end. Further in the case where the guide plate 20 extends beyond the position opposed to the openings 15&#39;, the guide plate 20 may be formed with openings 22 opposed to the openings 15&#39; for passing air therethrough. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.