US 3921042 A
An electrostatic reproduction machine has a DC biased AC corona generating device, the device having a metal shield which is grounded through an AC bypass capacitor which permits AC current to be shunted to the ground while preventing DC current from being shunted.
Claims available in
Description (OCR text may contain errors)
United States Patent Shenoy Nov. 18, 1975 [5 ELECTROSTATIC REPRODUCTION 3.8()O,154 3/1974 Tanaka 317/262 A MACIHNE WITH IMPROVED CORONA 3,860,857 1/1975 Namiki et al. 317/262 A GENERATING DEVICE Vittal U. Shenoy, Rochester, NY.
Xerox Corporation, Stamford, Conn.
Filed: Nov. 25, 1974 Appl. No.: 527,112
US. Cl 317/262 A Int. Cl. H01T 19/00 Field of Search 317/2 R, 2 A', 2 F, 262 A,
References Cited UNITED STATES PATENTS 9/l973 Silverberg 317/2 F Primary Examiner-J. D. Miller Assistant Exanziner-Harry E. Moose, Jr.
Attorney, Agent, 0r Firm--Bernard A. Chiama; Earl T. Reichert is?  ABSTRACT An electrostatic reproduction machine has a DC biased AC corona generating device, the device having a metal shield which is grounded through an AC bypass capacitor which permits AC current to be shunted to the ground while preventing DC current from being shunted.
3 Claims, 2 Drawing Figures U.S. Patent Nov. 18, 1975 SheetlofZ 3,921,042
US. Patent Nov. 18, 1975 Sheet 2 of2 3,921,042
ELECTROSTATIC REPRODUCTION MACHINE WITH IMPROVED CORONA GENERATING DEVICE BACKGROUND OF THE INVENTION I This invention relates to an improvement in an elec trostatic reproduction machine, but more particularly to a reproduction machine having an improved corona generating device.
In conventional xerography, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support latent electrostatic images. In the process, the xerographic surface is electrostatically charged, and the charged surface is then exposed to a light pattern of the image being reproduced to thereby discharge the surface in the areas where light strikes the surface. The undischarged areas of the surface thus form an electrostatic charge pattern conforming to the original pattern.
The latent electrostatic image is then developed by contacting it with a finely divided electrostatically attractable powder referred to hereinafter as toner. The toner is held on the image areas by the electrostatic charge on the layer. Where the charge is greater, the greater amount of toner is deposited. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. Generally, the developed image is then transferred to a suitable transfer member, and the image is affixed thereto to, form a permanent record of the original document.
In the practice of xerography, the transfer member, ordinarily copy paper, is caused to move in synchronized contact with the photoconductive surface. During this time, an electrical potential opposite from the polarity on the toner is appied to the side of the paper remote from the photoconductive surface to ,electrostatically attract the toner image from the xerographic surface to the copy paper.
The copy paper, which is an-insulator, retains the charge, while inducing a reverse charge in the nondischarged areas of the xerographic surface. This charge orientation creates an electrostatic bond between the paper and xerographic surface. Removal of the copy sheet and the toner image loosely adhering thereto has long been a problem in the xerographic art.
To detack the copy paper from the xerographic surface, an AC corona generating device biased by a DC voltage may be used. Similar devices are used elsewhere within the machine, e.g., before transfer and before cleaning. In utilizing such devices, it is highly desiristic curve which has a slope which is not as steep as that of a similar device having a conventional grounded metal shield. While the slopes for both of these types of corona generating devices can be varied somewhat, there is a range of curves in between these extremes which cannot be produced.
located near ametal surface, e.g., the paper transport,
within the reproduction machine, an AC capacitive coupling may be set up between the device and the metal surface which will result in varying the AC current to the surface being treated by the device. The latter may cause adverse effects such as mistripping if the corona device is being used for detacking, or poor copy quality. In addition, plastic shields are subject to large charge buildups which may result in a change in the set point of the DC bias, and if the DC bias changes too much in either direction, it will detrimentally affect the operation of the device, e.g., it may result in mistripping in the case of a corona device used for detacking. It has been discovered that a corona generating device having a conventionally grounded metal shield cannot have its DC bias satisfactorily controlled by varying a resistance as described above because another path is provided for DC current, i.e., through the shield to ground.
Thus, what is needed is a DC biased AC corona gencrating device in which the DC bias can be satisfactorily controlled, and which overcomes the disadvantages of the prior art devices discussed above. The needed device should be capable of having a resistance in series with the DC bias source varied so as to produce a characteristic curve as desired, i.e., a curve which can be produced with the prior art devices, or a curve in between these extremes.
SUMMARY OF THE INVENTION The present invention relates to a xerographic reproduction machine employing a DC biased AC corona generating device having a metal shield and means comprising a bypass capacitor between the shield and ground for accurately controlling the DC current.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view of an electrostatic reproduction machine embodying the principles of the present invention;
FIG. 2 is a schematic diagram of the electrical circuit for energizing a DC biased AC corona generating device and for accurately controlling the DC current.
DESCRIPTION OF THE PREFERRED EMBODIMENT For a general understanding of an electrostatic processing system in which the invention may be incorporated, reference may be had to FIG. 1. As in all electrostatic systems such as a xerographic machine of the type illustrated, a light image of an original to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material comprising carrier beads and smaller toner particles triboelectrically adhering thereto 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 fixed by a fusing device whereby the toner image is caused permanently to adhere to the support surface.
In the illustrated machine, an original D to be copied is placed upon a transparent support platen P fixedly arranged'in an illumination assembly generally indicated by the reference numeral 10. While upon the platen, an illumination system flashes light rays upon the original, thereby producing image rays corresponding to the informational areas on the original. The image rays are projected by means of an optical system 11 to an exposure station A for exposing the photosensitive surface of a moving xerographic plate in the form of a flexible photoconductive belt 12. In moving in the direction indicated by the arrow, prior to reaching the exposure station A, that portion of the belt being exposed would have been uniformly charged by a corona generating device 13 located at a belt run extending between belt supporting rollers and 22. The exposure station extends between the roller 20 and a third support roller '21.
The exposure of the belt surface to the light image discharges the photoconductive layer in the areas struck by light, whereby there remains on the belt a latent electrostatic image in image configuration corresponding to the light image projected from the original on the supporting platen. As the belt surface continues its movement, the electrostatic image passes around the roller 21- and through the developing station B located at a third run of the belt and in which there is-positioned a developing apparatus generally indicated by the reference numeral 15. The developing apparatus-l5 comprising a plurality of brushes 16 which carry developing material to the adjacent surface of the upwardly moving inclined photoconductive belt 12. As the developing material is applied to the xerographic belt, toner particles in the development material are attracted electrostatically to the belt surface to form powder images.
The developed electrostatic image is transported by the belt 12 to a transfer station C located at a point of tengency on the belt as it moves around the roller 22 whereat a sheet of copy paper is moved at a speed in synchronism with the moving belt in order to accomplish transfer of the developed image. There is provided at this station a transfer roller 17 which is arranged on the frame of the machine for contacting the non-transfer side of each sheet of copy paper as the same is brought into transfer engagement with the belt 12. The roller 17 is electrically biased with sufficient voltage so that a developed image on the belt may be electrostatically transferred to the adjacent side of a sheet of paper as the same is brought into contact therewith.
There is also provided a suitable sheet transport mechanism adapted to transport sheets of paper seriatim from a paper handling mechanism generally indicated by the reference numeral 18 to the developed image on the belt as the same is carried around the roller 22. A programming device operatively connected to the mechanism 18 and the illumination device for producing an electrostatic latent image on the belt 12 is effective to present a developed image at the transfer station C in timed sequence-with the arrival of a sheet of paper.
As the sheet emerges from the transfer roller, it is influenced by a detacking corona generating device 23 in a manner to be hereinafter explained. The sheet is thereafter retained on the underside of a transport mechanism 24 by suitable means such as vacuum for movement into a fuser assembly generally indicated by the reference numeral 19 wherein the developed and transferred xerographic powder image on the sheet is permanently affixed thereto. After fusing, the finished 4 copy is discharged from the apparatus at a suitable point for the collection externally of the apparatus-The toner particles remaining as residue on the developed image, background particles, and those particles otherwise not transferred are carried by the belt 12 to a cleaning apparatus positioned on the run of the belt between the rollers; 20, 22 adjacent the charging device 13. Thecleaning device comprises a rotating brush 26 and a corona generating device 25, for neutralizing charges remaining on the particles.
The description of the present invention will be in re-' lation to the detacking corona generating device although it is to be understood that this invention applies to any DC biased AC corona generating'device.
' Referring now to FIG. 2, the detacking corona generating device 23 is continuously supplied with high voltage alternating current through a DC to AC converter 28 and a transformer 30. The discharge device is biased with a DC voltage from a suitable source such as battery 32. A suitable voltage dropping resistor 34 may be employed to provide the desired voltage level atthe discharge device 23. An AC bypass capacitor 36 is employed in the DC circuit to-shunt the AC component to ground. An AC bypass capacitor 38 is also employed in grounding the metal shield 40, the capacitor providing a low impedance path for AC current, but serving to block all DC current. The resistor 34 may be varied to produce the desired characteristic curve, and thus accurately control the DC bias since all of the DC current is, now through the belt 12, transformer 30, and DC source 32.
As stated above, it is highly desirable that the DC currentbe accurately controlled in a DC biased AC corona charging device to maintain optimum performance. In the detacking device 23 for example, if the DC current shifts too much one way or the other from 'its set-up value, stripping will not occur. Thus, the present invention provides for accurate control of the DC current.
- For purposes of illustration only, it is to be assumed that the charging device 13 imparts a positive charge on the xerographic surface and the toner has a negative charge thereon.
Uponexposure, the image areas will retain a high positive charge while the background areas will be discharged to a low, residual positive charge. At the development station, the toner particles will be attracted to the positively charged image areas. At thetransfer station, the copy sheet will be provided with a high positive charge to attract the negatively charged toner from thepositively charged xerographic surface.
The 'high positive charge on the paper and the minimal positive charge in the background areas of the xerographic surface provide an attractive force between the paper and the surface of the same magnitude as the attractive force between two surfaces of opposite polarity having the same potential difference.
If the reproduction machine is designed such that the xerographic surface is curved at the stripping station, the beam strength of the sheet of paper will cause the lead edge thereof to lift from the xerographic surface combined with the proper charge being placed upon the sheet.
Thus, if the xerographic surface is initially provided with a positive charge, the transfer roller will be biased to provide a positive charge on the copy paper to attract the negatively charged toner from the xerographic surface to the copy paper. In order to separate the copy paper from the xerographic surface, the DC voltage may be positive or negative depending on the magnitude of charge on the copy paper after it has passed through the transfer station. For optimum copy quality, a positive charge is desired on the body of the copy paper at the stripping station. If the positive charge on the paper is too high, however, contact between the paper and grounded elements of the xerographic machine' such as paper transports, fuser, etc., may cause an electrical discharge from the copy paper to the grounded elements. This electrical discharge may disturb the loosely adherent toner pattern on the copy paper and result in poor copy quality. Therefore, if the copy paper leaving the transfer station has a high positive charge thereon, the copy paper would be provided with a slight negative charge by the DC biased AC corona generating device 23 to provide a resultant positive charge on the copy paper insufficient for discharge to the paper transports or other grounded elements of the machine but great enough to hold the toner on the paper.
If the body of the copy paper leaving the transfer station has a low positive charge thereon, the corona generating device 23 would be biased by a positive DC u source to provide sufficient positive charge on the paper for good copy quality but sufficiently low to prevent subsequent discharge therefrom.
While the invention has been described with reference to the structure disclosed, it is not confined to the details set forth but is intended to cover such modifications or changes as may come within the scope of the following claims.
What is claimed is:
1. An improved electrostatic reproduction machine having means including a transfer station and a stripping station for producing a copy of an original document, the improvement comprising a DC biased AC co rona generating device having a metal shield, and means for grounding the metal shield, the grounding means including means for allowing AC current to be grounded while simuntaneously blocking DC current.
2. An improved electrostatic reproduction machine according to claim 1, wherein the grounding means includes an AC bypass capacitor.
3. An improved electrostatic reproduction machine according to claim 2, wherein the stripping station includes the DC biased AC corona generating device.