US2879395A

US2879395A - Charging device

Info

Publication number: US2879395A
Application number: US513934A
Authority: US
Inventors: Lewis E Walkup
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1955-06-08
Filing date: 1955-06-08
Publication date: 1959-03-24
Anticipated expiration: 1976-03-24

Description

- L. E. WALKUP CHARGING DEVICE March; 24, 1959 2 Sheets-Sheet 1 Filed June a. 1955' @500 VAC. PEAK ill/ INVENTOR.

LEWIS E. WALKUP ATTORNEY March 24, 1959 1.. E. WALKUP CHARGING DEVICE 2 Sheets-Sheet 2 Filed June 8, 1955 oov CON

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INVENTOR. LEWIS E. WALKUP FIGS ATTORNEY CHARGING DEVICE Lewis E. Walkup, Columbus, Ohio, assignor, by mesne assignments, to Haloid Xerox Inc-., a corporation of New York Appli'cationJune 8,1955, Serial No. 513,934

Claims. (Cl. 250-495) This invention relates to apparatus for electrically charging an insulating layer to a potential below the maximum electrostatic potential that the insulating layer will hold without electrical breakdown, and has for a particular purpose thereof to afford apractical and eificient device for applying an electrostatic charge of predetermined potential to a photoconductive insulating layer, such as the photoconductive coating, of an electrophotographic plate.

In producing copies for electrophotography, it has been proposed to charge electrophotographic plates by using a corona charging device such as is disclosed in. the co pending application of I. J. Rheinfranlc, Serial Number 55,526, filed. October 20, 1948, entitled, Method and. Apparatus for Electrostatically Charging ImageLayers, andInow abandoned. Such a device has been used successfully to. charge electrophotographic plates. How ever, the. device is difiicult to control in, such a manner as to. place a predetermined potential. on the photoconductive'v insulating layer. An accurately timed drive mechanism is required to pass the plate under the charging device at a controlled. rate of speed in order that the amount of charge deposited on any area of the plate coating does not exceed the electrical breakdown strength of, the coating. Even with a controlled rate of travel of the. plate under the charging; device, potential variations. are not overcome since relatively small voltage variations in the power supply can produce relatively large changes in corona current with correspondingly large varations in charging rate. Variations in atmospheric pressure also affect the corona current. and charging rate. Different electrophotographic plates will. also have; difierent capacities due to difierentthicknesses of insulating layer or of dielectric constant, and, consequently, will require difierent quantities. of, charge to bring them to the same potential.

It electrophotographic plates are notcharged to a sufficient potential, the electrostatic latent image obtained uponexposure will be relatively weak and the resulting deposition of electrostatically atttractable material, such asa powder or a liquid mist, during development of the image will be small. If the plates are overcharged, other difliculties are encountered. Oneserious resultof. overcharging is the permanent damage tov the photoconductive layer by the creation. of small spotsor areas. on the plate which are so altered that they are. not thereafter capable of. holding charge evenv after. recharging the plate.

It has been further found that difierent areas on the same plate often become charged to-differentpotentials This latter irregularity of charge'is. dueperhapsto variations; inthe. photoconductive layer or perhaps in irregus larities in the ion spraying. capacity of different. portions of. the corona discharge device, and theresult of thisirregularity is astreaky effect when the photoconductive. layer is. dusted with an, electroscopicpowder to producev an image.

It. is disclosed. in myco-pendmg,apphcatromfienal No.

States Patent Patented Mar. 24, 1959 entitled, Charging Device, of which this application: is a continuation-in-part, that the introduction of an electrode such as a conductive grille between the source of ions, e.g. the. corona discharge electrode, and the insulating layer, such as the photoconductive insulating layer, to which the ions are flowing permits the flow of ions to be regulated and' the insulating layer can: be

charged to a potential below the maximum electrostatic:

potential that the insulating layer willv hold., As there pointed out, using this invention. the potential assumed by the insulating layer is also made relatively independentof. the chargingpotential, the timeof charging, and other:

characteristics of the charging unit.

If, in the apparatus of my application Serial1No.. 154,295, now US. Patent 2,777,957, the voltage. im pressed on the corona discharge electrode. is. an AC.

voltage sufiicient to cause corona discahrge, and a DC. voltage impressed on the shield or conductive grille which regulates the flow of ions from the corona discharge electrode, highly unusual and beneficial results are obtained.

Where DC. voltage or pulsed DC. voltage is used for" both the shield and the corona discharge electrode, the. polarity of the charge placed on the insulating layer is. determined by the polarityo-f the charge on the corona Thus, to reverse the polarity placeddischarge electrode. onv the insulating layer. requires switching the polarity placed. on, the corona discharge electrode. which is ofa: magnitude between 4,000 to 10,000 volts.

ofovercharging is reduced but not eliminated. In contrast to this, Where A.C. is usedf'for the corona discharge" In addition to determining the polarity of the charge: placed on the insulating layer, the shield also determines:

the maximum potential of that charge so that overcharging-is not merely improbable, but impossible. The charg ing device of the instant. invention also permits charging: even at low atmospheric, pressures without damage to.

the plates. Further, the instant invention eliminates the. necessity of placing the shield between the coronadischarge electrode and the insulating layer. shield may consist of conductive wires. spaced between the corona discharge electrode and the insulating plate as set forth in the said parent case, or, alternatively, may. consist of a rectangular or cylindrical trough with condnctive. back and sides but with noconductive member between the corona discharge electrode and the insulating.

layer to be charged, or. finally, the shield may consist simply of a flat conductive member positioned in. back of the corona discharge'electrode, ie on the side away from the insulating layer to be charged and extendedsufiiciently far laterally beyond the corona discharge electrode to prevent corona discharge-past the edge of the shield;

For a better understanding of this invention, reference is now directed to the following description taken in.con-

nection with the accompanying drawings, and the scope: of my invention will be pointed out in the. appended claims. In the drawings, Fig. l isa perspective view of? a cornoa. charging device having a control electrodein. accordance with a preferred form of my invention; and

In addition,., the DC potential placed. onthe shield does not abso-- lutely prevent overcharging of the insulating layer but; rather determines a. potential. above which therate of charging is substantially reduced sothat. the possibility Thus, the.

. 1 S, v I a 3 7 1 5 1 we out rrrr- 5O bstantially paraklel toi the: cor discharge wires 5. The belts run on pulleys 19 aflixed to

shafts

13 and 14, the shafts and pulleys being placed with their axes below the plane of guide plate 3 so that the belts and 11 slide along the top face of plate 3 to carry electrophotographic plate 9 under charging device 4 in predetermined spacing from grille electrode 7. The return paths of belts l0 and 11 are underneath guide plate 3. Plate 3 is supported on legs secured to the top of baseboard 1. The pulleys 19 are spaced apart to support electrophotographic plate 9 at two spaced regions across the plate. Insulating support blocks 6, formed of polystyrene or other good insulating material, comprise rectangular plates having their lower corners beveled oil? at an angle of degrees. The electrode 7 is made of conductive wire which is wound back and forth over support pins 21 mounted on the lower and beveled edges of blocks 6 so as to form a parallel wire screen or cage partially enclosing corona discharge wires 5, the electrode wires 7 being parallel to the corona wires. Preferably, the plane formed by a portion of the grille 7 and the plane formed by the upper surface of each of the

belts

10, 11 are parallel and at predetermined relative positions with each other, which distances will be later set out in more detail; but it is essential that the distances between the corona discharge wires 5 and the electrode grille 7 is great enough that no sparking will occurr at the voltages used to establish corona.

An electrically conducting metal plate or shield 22 extends between the spaced insulating members 6 above the corona discharge wires 5. For best operating results this shield is held at a low potential, preferably ground, with respect to the discharge wires 5 and is approximately the same distance from the discharge wires 5 as the grille 7.

5 in turn is connected to a source of DC. potential, as

a battery 32.

the a layer 2 is Mr".

".df bac'in meme. I ipr df te i i maimed against 1b w bla s,

The other end of secondary 30 is grounded.

Terminal 26 of control electrode 7 is connected by conductor 39 to a tap 38 on the potentiometer 34 which If desired, the center tap of the potentiometer 34 may be grounded so that the polarity of charge placed on the shield 7 may be varied as well as the magnitude of the potential.

A potential of at least about 4,000 volts between the corona electrode and the nearest conductor or electrodes is usually required in order to generate a useful corona discharge and it is preferable that the peak AC. voltage be no more than 10,000 volts in order to avoid sparking or excessive space charges in structures of practical dimensions.

In the operation of the charging device of this invention, the relative spacing of the different elements has considerable effect on the results obtained. An example of spacings found to produce good results and to achieve the objects of this invention is such that the three corona discharge wires 5 are spaced /2" fro-m each other in a single plane but being threaded through aligned mounting holes in the insulating members 6. The shield 22 is spaced /2" above the wires 5, the wires of electrode 7 in a plane parallel to the plane of the discharge wires 5 are in a plane @1 below the wires 5, and the spacing between this plane and the plane of travel of insulating The turns or wires of the electrode 7 have produced good results when they are spaced from each other by approximately A suitable wire for corona wires 5 is a smooth stainless steel wire having a diameter of 0.0035 and a suitable wire for the strands of electrode 7 is 0.01" stainless steel.

Under these conditions, 8,000 volts AC. was applied to the discharge wires 5 and a negative 800 volts D.C.

was applied to the electrode 7, the shield 20 being at ground potential and a current was: found. to flow' from the wires- 5 to a metal plate passed under the charging devicein the position of plate 9.

It is, of course, obviousto those skilled. in theart' that the diameter of the corona wires 5 can be. made smaller or: larger provided the. voltage: applied to the wires. is suificient to generate. a. corona. discharge at a potential below that at. which sparking takes place. Likewise, the diameter. and spacing. of grille wires.7. can be varied over a rather wide: range although it is desirable. to use small wi'res so as to leave as. much open space as possible between. wires. while keeping the wire spacing small so. that the array of wires. 7 causeanelectric field which resembles the field from a. plate electrode positioned in the planes occupied, by the wires. It is also obviousthat. the charge ing. rate canhe increased. for any size of corona wires by increasing voltage on the corona; wires, although it is essential that: the voltage. between these wires and electrodes 7 and 22 be below the. spark breakdown potential. The potential onwire electrode 7 can also be ad'- justed to any desired valueto adjustthe potential which is applied to the insulating layer 2.

i Fig. 4' is a graph illustrating certain advantages of the charging device of this invention in preventing overcharging of the plate. Inobtaining this graph the charging device of Fig. 1' was modified'by. eliminating the shield 22 and. placing the control electrode 7 on the side of the corona 5 away from the plate 9. The control electrode 7 was constructed of sheet aluminum and had a rectangular cross section asshown in Fig. 5B. The. distance from the; corona wire 41 to each of the three sides of, the. control electrode 40 was A measured on a perpendicular line from the. corona to each surface. The distance from the corona wire. 4L to the surface of. the plate 9 was /2., A charge of, 8,000'volts A.C. was applied to the discharge wires 41, and a current was. found to flow from. the wires 41 to a metal plate passed under the charging device in the position of plate 9, the metal plate. being connected through a milliammeter to ground in order that the current, could be measured. Under. these conditions. the. DC. charge placed on electrode 40 was varied as shown. in the graph. The amount of plate currentwas. then determinedafor each setting of the potential on the. electrode 4.0.. This curve shows that the charging current to the plate passes through 0 for a particular determined. bias, i.e.. voltage between electrode 40 and the metal backing plate. When the plate is charged to. a potential equivalent to. the point where place current equals 0, no more current flows to the plate and, therefore, it isv impossible. for the plate. to. reach: a: voltage higher than the voltage at this point in the curve. In contrast to this, wherea DC. potential is placed on the corona electrode 41 and the D.C. bias placed on the shield 40, the plot of the plate current against the. differ.- ence in voltage between the shield and the plate, instead of being a straight line passing through 0 plate current, becomes asymptotic to the abscissa as plotted; that is, the plate current decreases markedly as a particular voltage level isreached but, nevertheless, continues charging; above this voltage although at an' appreciably slower rate: The curve obtainedfor the control electrode-corona electrode configuration shown in Fig. 1 has the same shape as here, but will cross the 0 plate current axis at a different point-the exact point depending on the spacings of the electrodes and the plate. Thus, it is impossible to overcharge a xerographic plate using the charging device of the instant invention whereas overcharging is possible when a DC. potential is placed on the corona electrodes.

Figs. 2 and 3 illustrate difierent corona circuits. In Fig. 3 the switch 33 places the DC. bias from a DC. source, as a battery 32 on corona shield 40 through lead 42. The metal backing portion 8 of the xerographic plate 9 is grounded by contact 35. This is a preferred form of the circuit as, in this case, no high D.C. charge is placed on. the metalpla-te. which: is handled by personnel: However,v as the. essential factor is: the charge relation; ship between. the metal backing: plate 8 and. the shielfd; as shown. in; Fig:.2;. the: charge; if desired, maybe placed by means; of switch 33 on the metal backing. member 8. while. grounding'the shield 40;. Resistor 31,desirab1yhas. ahighvalue, say about 10 megohms, to reduce the current. in. case. of, an accidental grounding of backing mem-. her 8.,

Fig. 5v illustrates in: cross section various, forms; of. operable corona charging devices. As shown, the shape. of. the shield may take. awidely variant. number of terms; all of which are operable inthe process of the instant invention. and all of which cause the plate charging 'currenalto pass. through. 0 forv a particular bias. In. any event, the control-v electrode. should be. parallel lengthwise. both to the corona electrode and the insulating surfaceand should extend sufficiently far about the. corona electrode as to'preventcoronadischarge past the edges of the conductive electrode. When the control electrode is. positioned between the corona electrode and the surface to be charged, as in Fig. l, the. control electrode should: be an apertured grid as parallel spaced conductive wires. as shown, wire mesh, etc. When the control electrode. is positioned on the side of. the corona electrode away from. the surface to be charged, as in. any of the forms. shown in Fig. SA-E, the control electrode may also be: anapertured grid or maybe a continuous conductive surface or a continuous insulating surfacetwith parallel; spaced conductors imbeddedtherein.

It is, obvious that, while thesurface to bev charged is. a.

plane inthe examples. illustrated, it. can take other forms,

such as. cylindrical, for. example, in which event the in,- sulatinglayer may be coated on.a rotating belt or drum.. It is also obvious that the insulating layer may beheld. stationary in the. apparatus during charging, and the. ion. source and control electrode can. either be extensive. enough to cover. the entire surface or they can be. ad.- vanced over the stationary insulatingv surface.

I claim:

1. A device for. appl'yingan electrostatic charge. to. an insulating layer comprising, in combination, means for supporting said. insulating layer in a charging plane, an: elongated corona discharge electrode extending across. thewidth of said insulating layer. in spaced relation thereto,.means for producing relative travel. of said insulating layer and said corona discharge electrode in a direction substantially parallelto. the length of said. insulating layer, a control electrode extending across the width of said insulating layer and parallellengthwise bothtothe coronav electrode and the insulating layer and extending sufficiently far about the corona electrode as to prevent corona. discharge past the edges of. the control electrode, said: control electrode being spaced from said corona discharge electrode and from saidinsulating layer, an. A.C. sourcev connected to said corona discharge electrode for applying.

a corona generating A.C. voltage between said corona discharge electrode and said control electrode to producean AC; corona discharge from said corona discharge electrode, and a DC; potential source connected to said. control electrode for applying a DC. voltage which is smaller than the peak value of said A.C. voltage to create a DC. charging field through the insulating layer whereby said control electrode will function to limit the potential which is applied to said insulating layer by said device.

2. A device for applying an electrostatic charge to an insulating layer comprising, in combination, means for supporting said insulating layer in a charging plane, an elongated corona discharge electrode extending across the width of said insulating layer in spaced relation thereto, means for producing relative travel of said insulating layer and said corona discharge electrode in a direction substantially parallel to the length of said insulating layer, a control electrode extending across the width of said insulating layer and parallel lengthwise both to the corona electrode and the insulating layer and extending sumciently far about the corona electrode as to prevent corona discharge past the edges of the control electrode, said control electrode being spaced from said corona discharge electrode and from said insulating layer, an A.C. source connected to said corona discharge electrode for applying a corona generating A.C. voltage between said corona discharge electrode and said control electrode to produce an A.C. corona discharge from said corona discharge electrode, and a D.C. potential source connected to said control electrode for applying a D.C. voltage which is smaller than the peak value of said A.C. voltage to create a D.C. charging field through the insulating layer, and means for varying the magnitude and polarity of said D.C. voltage, whereby said control electrode will function to limit the potential which is applied to said insulating layer by said device.

3. A device for applying an electrostatic charge to an insulating layer comprising, in combination, means for supporting said insulating layer in a charging plane, an elonated corona discharge electrode extending across the width of said insulating layer in spaced relation thereto, means for producing relative'travel of said insulating layer and said corona discharge electrode in a direction substantially parallel to the length of said insulating layer, and a control electrode comprising a conductive grille between said corona discharge electrode and said insulating layer across the width of said insulating layer, said control electrode being spaced from said corona discharge electrode and from said insulating layer, an A.C. source connected for applying a corona-generating A.C. voltage between said corona discharge electrode and said control electrode to produce an A.C. corona discharge from said corona discharge electrode, and a D.C. potential source connected to said control'electrode for applying a D.C. voltage which is smaller than the peak value of said A.C. voltage to create a D.C. charging field through the insulating layer, whereby said control electrode will function to limit the potential which is applied to said insulating layer by said device.

4. A device for applying an electrostatic charge to an insulating layer comprising, in combination, means for supporting said insulating layer in a charging plane, an elongated corona discharge electrode extending across the path of said insulating layer in spaced relation thereto, means for producing relative travel of said insulating layer and said corona discharge electrode in a direction substantially parallel. to the length of said insulating layer, a control electrode extending across the width of said insulating layer and partially surrounding said corona discharge electrode on the side away from said insulating layer, said control electrode being spaced from said corona discharge electrode and from said insulating layer, an A.C.source connected to said corona discharge electrode for applying a corona generating A.C. voltage between said corona discharge electrode and said control electrode to produce an A.C. corona discharge from said corona discharge electrode, and a D.C. potential source connected to said control electrode for applying a D.C.

1 8 voltage which is smaller than the peak value of said A.C. voltage to create a D.C. charging field through the insulating layer, whereby. said control electrode will function to limit the potential which is applied to said insulating layer by said device.

5. A device for applying an electrostatic charge to an insulating layer comprising, in combination, means for supporting saidinsulating layer in a charging plane, an elongated corona discharge electrode extending across the width of said insulating layer in spaced relation thereto, means for producing relative travel of said insulating layer and said corona dischargeelectrode in a direction substantially parallel to the length of said insulating layer, a control electrode extending across the width of said insulating layer and partially surrounding said corona discharge electrode on the side away from said insulating layer, said control electrode being spaced from said corona discharge electrode and from said in sulating layer, an A.C. source connected to said corona discharge electrode for applying a corona generating A.C. voltage between said corona discharge electrode and said control electrode to produce an A.C. corona discharge from said corona discharge electrode, and a D.C. potential source connected to said control electron for applying a D.C. voltage which is smaller than the peak value of said A.C. voltage to create a D.C. charging field through the insulating layer, and means for varying the magnitude and polarity of said D.C. voltage, whereby said control electrode will function to limit the potential which is applied to said insulating layer by said device.

6. A device according to claim 5 wherein the control electrode is semi-cylindrical.

7. A device according to claim 5 wherein the control 7 electrode has a rectangular cross section.

8. A device according to claim 5 wherein the control electrode is of uniformly-spaced parallel wires.

9. A device according to claim 5 wherein the control electrode has a continuous conductive surface.

10. A method of sensitizing a photoconductive insulating layer by applying electric charge to its surface comprising applying an A.C.corona generating potential to a corona discharge electrode extending across the width of the layer to be charged and extending substantially parallel thereto while spaced apart therefrom, creating a D.C. charging field through the photoconductive insulating layer, and moving said-electrode relative to said photoconductive insulating layer and in a direction substantially parallel to the length of the surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,132,707 Smith Oct. 11, 1938 2,143,214 Selenyi Jan. 10, 1939 2,543,051 Oughton et al Feb. 27, 1951 2,647,464 Ebert Aug. 4, 1953 2,693,416 Butterfield Nov. 2, 1954 2,701,764 Carlson Feb. 8, 1955 2,741,959 Rheinfrank Apr. 17, 1956 r 2,777,957 Walkup Jan. 15, 1957 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0. 2, 879,395 March 24, 1959 Lewis E. Walkup- It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

line 65, for "in", second occurrence, read to column ,000 volts" read 4,000 and 10,000 volts column 4., line 23, for "plate" read plates column '7, line 22, for "e'lonated" read elongated line 32, after "connected" insert to said corona discharge electrode line 47, for "path read Width column .8, line 25, for "electronn read electrode Column 1, 2, line 28, for "4,000 to 10 Signed and sealed this 22nd day'of" March 1960.

(SEAL) Attest:

KARL H. AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,879,395 March 24, 1959 Lewis E Walkup It is herebj certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 65, for "in", second occurrence, read to column 2, line 28, for "4,000 to 10,000 volts" read 4,000 and 10,000 volts column 4, line 23, for "plate" read plates 3 column '7, line 22, for "e-lonated" read elongated line 32, after "connected" insert to said corona discharge electrode line 47, for "path" read Width column 8, line 25, for "electron" read electrode Signed and sealed this 22nd day of March 1960.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Ofiicer Commissioner of Patents