US 3382360 A
Description (OCR text may contain errors)
May 7, 1968 J. YOUNG ET AL 3,382,360 XEROGRAPHIC CHARGING SYSTEM HAVING MEANS FOR PROVIDING AN AIR CUSHION BETWEEN THE CHARGING DEVICE AND THE XEROGRAPHIQDRUM Filed Sept. 10, 1965 5 Sheets-5heet 1 /I!Lllllli May 7, 1968 YOUNG ET AL 3,382,360
XEROGRAPHIC CHARGING SYSTEM HAVING MEANS FOR PROVIDING AN AIR CUSHION BETWEEN THE CHARGING DEVICE AND THE XEROGRAPHIC DRUM Filed Sept. 10, 1965 5 Sheets-Sheet 2 COMPRESSED AIR POTENTIAL SOURCE ATTORNEYS May 7, 1968 YQUNG ET AL 3,382,360
XEROGRAPHIC CHARGING SYSTEM HAVING MEANS FOR PROVIDING AN AIR CUSHION BETWEEN THE CHARGING DEVICE AND THE XEROGRAPHIC DRUM Filed Sept. 10, 1965 3 Sheets-Sheet 5 V V r v war-5mm 4 INVENTORS. JAMES E. YOUNG "RENT w. HEMPHILL BY RAYMONfiT WRIGHT W -&..M
A TTORN E Y5 United States Patent 3,382,360 XEROGRAPHIC CHARGING SYSTEM HAVING MEANS FOR PROVIDING AN AIR CUSHION BETWEEN THE CHARGING DEVICE AND THE XEROGRAPHIC DRUM James E. Young, Pittsford, Kent W. Hemphill, Rochester, and Raymond T. Wright, Penfield, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of NewYork Filed Sept. 10, 1965, Ser. No. 486,304
9 Claims. (Cl. 250-495) This invention relates to xerography and, more particularly, to an improved corona generating device for imposing a uniform electrostatic charge on a xerographic plate.
In the process of xerography, for example, as disclosed in Carlson Patent 2,297,691, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This process discharges the plate areas in accordance with the radiation intensity that reaches them, and thereby creates an electrostatic image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely-divided material, such as an electroscopic powder that is brought into contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means.
In automatic equipment employing the principles of xerography, it is common to employ a xerographic plate in the form of a cylindrical drum which is continuously rotated through a cycle of sequential operations including charging, exposure, developing and transfer. During the cycle, copy is reproduced onto a support surface and, after transfer, the plate is cleaned before reuse. It is usual to charge the xerographic plate with corona of positive polarity by means of a corona generating device having an electrode which when supplied with potential above the corona threshold produces an emission of corona ions that deposit onto the plate surface. Typical of the corona generating devices employed heretofore are those described in US. Patent 2,777,957 to Walkup and U5. Patent 2,836,725 to Vyverberg, each constructed generally of an electrode wire or wires supported relatively close to the surface to be charged. In the former, a suppressing grid is placed intermediate the electrode and the plate and maintained at a potential desired for uniformly charging the plate surface; while in the latter, a grounded metallic shield generally surrounds the electrode except for an opening through which charge is emitted whereby control over the uniformity of the charge on the plate surface is effected by the relative movement thereof with respect to the electrode.
Inherent in xerographic apparatus of the type described above is the difliculty in uniformly charging the surface of a xerographic drum that must be rotated at high speeds during prolonged, continuous operation. This lack of uniformity has been primarily attributed to non-uniform spacing between electrode and plate, variations in thickness and electrical characteristics of the sensitized layer, and dirt which accumulates on the corona wires. Attempts to improve the uniformity of corona charging by the generating devices mentioned above, have required the corona wires to be energized to a potential substantially in excess of the corona threshold potential thereby minimizing the effect of the foregoing in detracting from the 3,382,360 Patented May 7, 1968 uniformity of charge on the xerographic drum. At times this has resulted in overcharging the photoconductive layer of the xerographic drum. Notwithstanding the application of higher potentials to the corona wires, it has not been possible to achieve a high degree of uniformity for the charge on a xerographic drum prior to its exposure.
Now in accordance with the instant invention, there has been discovered a novel corona generating apparatus in which a moving air cushion is provided for supporting an array of corona generating wires in floating overlying relationship with the surface to be charged. In this manner it is possible to pneumatically urge corona discharge onto the surface permitting substantially lower potential difference above the corona threshold on the corona Wire per unit of time than has been required previously. At the same time, any differences in corona wire output along the wire length caused by dust accumulations or output changes effected by ambient air conditions are obviated since the airflow delivered cleans the wire and insures a relatively uniform airflow around the wire. Moreover, the airflow passing between the corona generating device and chargeable surface spreads the charge uniformly over the surface and helps clean the surface from dust accumulations or excess developing material that has not been removed by the preceding cleaning step.
It is therefore an object of this invention to provide apparatus for uniformly charging a recipient surface moving relative thereto at high speeds in an efficient manner.
Another object of this invention is to provide charging apparatus capable of maintaining a substantially constant spacing between a corona generating wire and a recipient surface to be charged.
Still another object of this invention is to provide an improved construction and design for a corona generating device to be used for charging xerographic plates.
Still a further object of this invention is to provide xerographic charging apparatus having self'contained means to continuously clean a corona discharge wire and surface to be charged while charge is being applied thereto.
For a better understanding of the invention, as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:
FIGURE 1 is a schematic illustration of an automatic xerographic apparatus embodying the corona generator device of the invention;
FIGURE 2 is a perspective view of one embodiment of the corona generating device with parts of the device broken away to show details therein;
FIGURE 3 is a plan view of the bottom of the corona generating device of FIGURE 2;
FIGURE 4 is a section view along lines 4-4 of FIG- URE 3; and
FIGURE 5 is a partial sectional view of still another embodiment of a corona generating device in accordance with the invention.
For a general understanding of the xerographic processing system in which the invention is incorporated, reference is had to FIGURE 1 in which the various system components are schematically illustrated. As in all xerographic systems based on the concept disclosed in the above-cited Carlson patent, a radiation light image of copy to be reproduced is projected onto the sensitized surface of a xerographic plate to form electrostatic image thereon. Thereafter, the latent image is usually developed with an oppositely charged developing material to form a xerographic powder image corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface to which it may be fused by any suitable form of fusing device, whereby the powder image is caused permanently to adhere to the support surface.
The xerographic apparatus described herein typically may be of the typedescribed in Cerasani et al., U.S. Patent 3,076,392. In the apparatus referred to, copies to be reproduced, which usually are positive in form, are placed on a support tray from which they are fed onto a suitable transport mechanism designated 11. Suitable drive means are provided for the transport mechanism from motor 12 to endless belts 13 whereby the copy is moved past the optical axis of projection system 14 and is illuminated by projection lamp LMP-l. The image of the copy is reflected by mirror 15 through an objective lens 16 and then is reflected by mirror 17 downwardly through a variable slit aperture assembly 18 and onto the surface of the xerographic plate in the form of drum 19.
Xerogra-phic drum 19 includes a cylindrical member mounted in suitable bearings in the frame of the machine and is driven in a clockwise direction by a motor 24 at a constant rate that is proportional to the transport rate of the copy, whereby the peripheral rate of the drum surface is identical to the rate of movement of the reflected light image. The drum surface comprises a layer of photoconductive material 25, such as vitreous selenium, on a conductive backing 26, such as aluminum. The photoconductive material is sensitized prior to exposure by means of a corona generating device 30 in accordance with the invention, as will be described more fully below. Corona generating device 30 is energized from a suitable high potential source 31 and has connected to it a compressed air source 32.
Exposure of the drum to the light image discharges the photoconductive layer in the areas struck by light, whereby there remains on the drum a latent electrostatic image in image configuration corresponding to the light image projected from the copy. As the drum surface continues its movement, the electrostatic latent image passes through a developing station 35 at which a two-component developing material 36, which may be of the type disclosed in Walkup Patent 2,638,416, is cascaded over the drum surface by means of developing apparatus 37. In
the developing apparatus, developing material is carried upward by conveyor 39 driven by suitable drive means from motor 40 and is released onto chute 41 wherefrom it is cascaded down over the drum surface. Toner component 42 of the developer, which is consumed in developing, is stored in dispenser 43 and is released in amounts controlled by gate 44.
After developing, the xerographic powder image passes a discharge station 50 at which the drum surface is illuminated by a lamp LMP-Z, whereby residual charges on the non-image areas of the drum surface are completely discharged. Thereafter, the powder image passes through an image transfer station 51 at which the powder image is electrostatically transferred to a support surface web 52 by means of a second corona generating device 53 similar to corona generator 30 as will be hereinafter described. 1 1
The support surface to which the powder image is transferred may be of any convenient type, such as paper, and is obtained from a supply roll 54 to pass over guide rolls 55 and 56 and over suitable tensioning rolls being directed into surface contact with the drum in the immediate vicinity of transfer corona generating device 53. After transfer, the support surface is separated from the drum and is guided through a suitable heat fusing apparatus 57 whereby the powder image is permanently affixed to the support surface. Thereafter, the support surface is fed over a further system of guide and tensioning rolls and onto a take-up roll 58 that is driven by motor 59.
Following separation of the support surface from the drum, a corona generating device 65, likewise constructed in accordance with the invention hereof, as will be further described below, directs negative electrostatic charge to the residual powder image on the drum surface. The drum surface then passes through a cleaning station 66 at which the surface is brushed by cleaning brushes 67, rotated by a motor 68, whereby residual developing material remaining on the drum is removed. Suitable light traps are provided in the system to prevent any light rays from reaching the drum surface, other than the projected image, during the period of drum travel immediately prior to sensitization by corona generating device 30 until after the drum surface is completely passed through the developing station 35.
Since the construction of all the foregoing charging units is similar, a description of charging tmit 30 is deemed sufficient. Referring now specifically to FIGURES 24, there is shown corona generating device 30 constructed in accordance with a first embodiment of the present invention. The unit generally comprises a housing consisting of top wall 76 and sidewalls 77 made from any suitable electrically conductive material that is connected to ground. Formed in top 76 is an opening 78 to which is connected compressed air source 32 through a conduit 82 provided with a valve 84. Connecting walls 77 of housing 75 and coextensive therewith is an elec-' trically insulating frame 92 forming a plenum chamber 93 within the housing. There is embedded in frame 92 an array of corona discharge wires 95 to which is connected a potential source 31 ranging from about 3,000 to 10,000 volts D.C. Frame 92 and wire array 95 are arranged to lie in a plane generally parallel to the surface of advancing xerographic drum 19 and are supported above the surface of the drum on a cushion of air in a manner which will become apparent.
Extending from sidewalls 77 are pins 101 which are received through an elongated opening 103 in supporting brackets 105. Coincident with each opening 103 is a spring 107 for urging housing 75 toward the surface of drum 19 against the cushion of air which is desirably able to overcome the natural stiffness of the spring. A screw 109 is provided for adjusting the bias on spring 107. As can readily be appreciated, elongated openings 103 in supporting brackets allow housing 75 to be displaced vertically, but prevent lateral movement thereof with re- 5 spect to drum 19. Also, the brackets serve as means for maintaining the charging unit adjacent the surface of the drum especially where the force of gravity on the unit is away from the drum center as in the case of charging units 53 and 65. The number of biasing connections used will depend upon the size and shape of the unit and those shown are merely illustrative.
As best shown in FIGURES 3 and 4, frame 92 has a plurality of orifices 116 which are in registry with a plurality of larger slots 118 recessed in the bottom above wire array 95 so that communication is provided between plenum chamber 93 and the exterior of charging unit 30. Slots 118 are elongated in a direction transverse to the path in which xerographic drum 19 is advanced past the charging unit, thereby exposing the drum surface to the corona current generated from wire array 95. Air from plenum chamber 103 is directed first through restricted orifices 116 before reaching larger slots 118 so that air at sufficient pressure is generated to lift charging unit 30 onto an air cushion formed between frame 92 and the about 10- to 20 pounds p.s.i.g. to float a unit weighing about a pound. In case of a loss in air pressure, a sensor switch 121 actuates two or more solenoids 123. which push upwardly against extensions 125 of housing 75 thereby preventing the charging unit from'contacting drum 19 during charging. Sensor switch 121 may be located in any convenient place, such as inside the air plenum.
Frame 92 is made from a light weight are resistant material that may suitably be formed of plastic in an injection mold wherein wire array 95 is placed prior to injecting the plastic material into the mold. Typical plastic materials suitable for this purpose are dialphthalate, epoxy and Rexolite, an acrylic plastic. Wire array 95 is a conductive copper that may have any suitable geometric arrangement, such as parallel wires connected at their ends, as best shown in FIGURE 3. Alternatively, the array may consist of a wire mesh. It has been found that wire diameters ranging from 2 to 5 mils work well as a corona discharge wire when used with potentials ranging from 3,000 to 10,000 volts D.C. Any suitable means may be used for uniting housing 75 with insulating frame 92, such as an adhesive.
In operation, a continuously moving air cushion is formed between the underside of the charging unit 30 and xerographic drum 19 supporting the charging unit in overlying floating relationship relative to the surface of the drum. The spacing between drum 19 and the bottom of the charging unit may be controlled by either regulating the pressure of plenum chamber 103 by valve 84 or adjusting the biasing on springs 107 by means of screws 109, or both. Not only does the air float the unit but it has been found that fluid flow across frame 92 also blows the corona charging current of ions or electrons, depending upon the polarity of charge, over the surface of drum 19, thereby more uniformly distributing the charge thereon. It is believed the airstream serves in this respect to accelerate the ionizing collisions that occur outwardly from wire array 95 to intensify the charging current. Because of this elfect, the number of ions or electrons available for charging dr-um 19 is believed increased many times due to the sweeping action of the positive airflow across the electric field surrounding wire array 95. Any dust that collects on either the framework 95 or the surface of xerographic drum 19 is blown free and clear by the fluid pressure. All of the foregoing enables alower potential to be applied for generating corona current at a given threshold potential.
In another embodiment of the present invention there is included a suppressor grid 152 which is spaced intermediate the array of corona discharge wires 155 and xerogra-phic drum 159, as shown in FIGURE 5. Grid 152 is in the form of a conductive screen having a potential substantially the same as the potential desired on the drum so that ions, in the case of positive charging, pass between the grid and continue onto the drum as long as a field exists between the grid and drum. Suitable potentials for wire array 155 and grid 152 are 3,000 volts and 400 volts, respectively.
As in the case of the first embodiment shown in FIG- URES 2 through 4, the charging unit contains a plenum chamber 161 which is formed by a housing 163 and a frame 165. Frame 165 is a composite structure made up of a conductive plate 171 containing orifices 172 and an insulating member 175 having larger openings 176. Orifices 172 and openings 176 are in registry so that comrnunication is provided between plenum chamber 161 and the exterior of the charging unit to form an air cushion for floating the charging unit in overlying relation to drum 159. Since weight is an important consideration, housing 163, as well as insulating member 175, which contains grid 152 and wire array 155, are desirably constructed from any suitable light weight dielectric material, such as plastic.
In accordance with the invention, fluid pressure from plenum chamber 161 is directed downwardly and across frame 165, as shown by the arrows in FIGURE 5, blowing the charging current uniformly over the surface of xerographic drum 159. Because of the close proximity of grounded plate 171 to corona wire array 155, a strong electrostatic field is formed therebetween enhancing the ion flow toward xerographic drum 159. Also, the potential applied to grid 152 permits greater selectivity of the charge desired on drum 159. In this manner, higher charge efficiency and uniformity are maintained at a given threshold potential. As already mentioned, an air cushion is formed between the charging unit and xerographic drum 159 for supporting the charging unit at a fixed distance from the surface of the drum and for serving as a self-contained cleaning medium during charging.
By the above description there has been disclosed a corona generating device capable of charging a xerographic surface moving relative thereto uniformly and efficiently. Heretofore, corona charging could be accomplished satisfactorily only by imposing potentials on a corona wire or wires that greatly exceeded the threshold potential. With the arrangement described, the charging unit has been found to disseminate much higher charging currents to the xerographic surface per unit time at a given threshold potential allowing lower potentials to be used. At the same time, the positive airflow between the floating charging unit and the xerographic surface provides a uniform charge across the surface as well as a continuous cleaning medium. Hence, not only does increased corona current enable reducing the potential applied to the corona wire, but the uniformity of charge on the xerographic surface atfords higher quality reproduction.
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 i intended to cover such such modifications as will become apparent to those skilled in the art upon a reading of this disclosure; these are intended to be encompassed within the scope of this invention.
What is claimed is:
1. A corona generating device for uniformly charging a recipient surface moving relative thereto comprising:
(a) contiguous walls partially forming an enclosure, including an inlet opening in one of said walls for connecting a source of compressed air thereto;
(b) a frame coextensive with and connected to said walls to form a plenum chamber therebetween, said frame containing a plurality of air passages communicating therethrough between said plenum chamber to the exterior of said frame;
(c) an array of corona discharge wires supported in said frame at least partially exposed to the exterior thereof;
(d) a source of potential connected to said wires sufiicient to generate a corona charging current thereon; and
(e) means for supplying compressed air to said opening to effect during charging an air flow from said plenum through said passages providing support between said frame and the surface to be charged.
2. Apparatus according to claim 1 wherein said means for supplying compressed air to said opening includes a pressure control valve for maintaining a delivered pressure suflicient to control a desired spacing during charging between said frame and the surface to be charged.
3. Apparatus according to claim 1 including elastic means opposing said air cushion to urge said device toward the surface to be charged, the natural stiffness of said elastic means being less than the force of said air cushion.
4. Apparatus according to claim 1 including means for preventing said device from contacting a surface being charged when air pressure from said supply means is less than a predetermined minimum.
5. Apparatus according to claim 1 including a suppressor grid supported in said frame in the corona discharge path from said array of corona wires, said grid being connected to a potential substantially the same as that to be applied to a surface to be charged.
6. Apparatus according to claim 1 where in said frame consists of a dielectric module.
7. Apparatus according to claim 1 wherein said frame comprises a dielectric member having slots through which said array is partially exposed and a conductive plate containing apertures in registration with said slots.
8. In a xerographic apparatus including a xerographic drum comprising a photoconductive layer on a conductive substrate, and means to continuously rotate said drum sequentially past a plurality of processing stations, a corona generating device for applying a uniform electrostatic charge onto the surface of said drum comprising in combination:
(a) contiguous walls partially forming an enclosure, including an inlet opening in one of said walls for connecting a source of air thereto;
(b) a frame coextensive with and connected to said walls to form a plenum chamber therebetween being positioned generally parallel to the photoconductive layer, said frame containing a plurality of air passages communicting therethrough between said plenum chamber to the exterior of said frame;
(0) means for selectively urging said device against said air cushion;
(d) an array of corona discharge wires supported in said frame at least partially exposed to the exterior thereof;
(e) a source of potential connected to said wires sufficient to generate a corona charging current thereon; and
(f) means for supplying compressed air to said opening to effect during charging an air flow from said plenum through said passages providing an air cushion support between said frame and said xerographic drum.
9. A method for depositing electrical charge from an electrode unit onto a relatively moving insulating surface comprising the steps of:
(a) applying an air flow between the electrode unit and the insulator surface of pressure sufficient to float the unit relative thereto; and
(b) applying a charge generating potential to said electrode unit to impart charge therefrom onto said surface.
No references cited.
RALPH G. NILSON, Primary Examiner.
A. L. BIRCH, Assistant Examiner.