CA1120530A

CA1120530A - Corona generating element

Info

Publication number: CA1120530A
Application number: CA000326156A
Authority: CA
Inventors: Manfred R. Kuehnle
Original assignee: Coulter Systems Corp
Current assignee: Coulter Systems Corp
Priority date: 1978-04-24
Filing date: 1979-04-23
Publication date: 1982-03-23
Also published as: DE2916330A1; LU81181A1; AT364602B; DK166679A; SE7903522L; GB2021871A; NL7903167A; FR2424649A1; ATA301479A; CH629626A5; IT1162627B; IT7948817A0; GB2021871B; IL57173A

Abstract

ABSTRACT OF THE DISCLOSURE

A corona generating element for use in applying a uniformed electrostatic charge over a surface when the element is connected to a source of high voltage is disclosed. The element comprises a strand of metal wire having a plurality of similar discontinuous unitary thin portions at uniformly spaced intervals alongs the length of the wire with each thin portion terminating in at least one knife edge. This knife edge constitutes a corona emission surface, whereby, when a source of high voltage is connected to the corona generating element, a corona will be generated along the knife edge of each of the thin portions along the length of the metal wire. The corona generating device is particularly useful in applying a uniform electrostatic charge of negative polarity to the surface of a photoconductive member.

Description

S3~

This lnvention relates to a corona generating element and more particularly to a corona generatiny element for use in electrophotography.
corona generatlng elements are used in electro--5 photography to apply a uniform electrostatic charge to the surface of a photoconductive member. The charye applied may be either positive or negative depending on the nature of the photoconductlve member. If the charye applied to the surface of the member is not uniform, the electrostatic image formed on the surface on exposure to a light pattern will not correspond exactly to the liyht pattern but will result partly from variations in the uniformity of the applied charge.
In Uniked States Patenk No. ~,025,339 to M. R. Kuehnle, there i~ described a photoconductive member of e~tremely hiyh sensitivity that is capable of reproducing or creatiny high resolution Images, that is, each pOint on the surface of the member is capable of selectively ~ discharying in accordance with the intensity of incident light so that an almost infinite scale of grey tones can be reproduced on the resultant image. The photoconductive member accepts a charge of neyative polarity. In order to make full use of the exceptional features of this photoconductive member, it is necessary that the charge applied be substantially uniform over its entire surface.

053~;) One type o e corona generatiny element that is commonly used in electrophotography is in the form of a strand of fine metaI wlre of circular cross-section having a diameter o~ usually on the order of about 15 microns.
rrhe wire is stretched tauk be-tween anchoring end pieces and connected to a source of high voltage of sufficient :~ .
~ magnikude to ionize the air about the strand. The ions . ~ : :
so produced are then~attracted to;the photoconductive member by any suitable means. ~This type of corona generating l0~ element is~simple in construction, easy and inexpensive to fabricate, and will produce a substantially uniform , ~ .
~ corona throughout its length when used in the positive :
~mode.

A disadvantage o~ this ~ype of corone yene~ating , element is its inability to produce a uni~orm corona when used in the negative mode. More specifically, it has been found that when a strand of metal wire is used to generate a negative corona, the corona so produced will have a ~ tendency to vary in density from point to point along the ; 20 length of wire. It is believed that this nonuniformity , ~ is caused by randomly located and shifting hot spots which .: ~
develop for one reason or~another along the wire and become sources of intense radiation.
,;
; ~ In order to compensate for these hot spots and to produce a corona along the wire that is substantially uniform, various approaches have been suygested and/or actually put into use. One solution is to provide a 1~0~3~

plurality of metal wire corona genera-ting elements positioned parallel to and ro-tated abou-t a central axis.
Another proposed solution is to reciprocate a single metal wire corona generating element about its longitudinal axis. Other approaches to compensate Eor hot spots in a metal wire coFona generating element can be found in U.9. Patents 2,856,533 and 3,~33,156.
Another type of corona generating element frequently used in electrophotography comprises an array of needle or pin-shaped electrodes conductively , connected to each other. when a high voltage source is connected to the electrodes, a corona is generated around the tips o each electrode. This type of element does not have the problem of randomly located hot spots but other problems are encoun~ered. For example, the electrode~ must be spaced a relatively larcJe distance apart to produce a relatively uniform corona at voltages around 6 KV and if one electroda should ~ail to operate, there is a sizeable area over which no , corona is yenerated. Examples of this type of corona generating element may be found in U.S. Patents 3,581,149, ; 3,624,392, 3,649,380, 3,691,373, 3,765,154 and 3,959,690.
Examples of other corona generating elernents used ~tside the field of electrophotography are described in U.S. patents 2,852,093, 3,294,971 and 3,766,382.

1~0S3~

Thus a need arises or the.p:rovision o~ a rnetal wire type of corona generatiny element that will produce a substantially uniform corona over its length when it is used in the negative mode.
S ~Accordingly, the invention provldes a corona element for use in applying a u~iform electrostatic charge ~ over a surface when the element is connscted to a sourcs of high voltage, said element comprising a strand of metal wire having a'plurality of generally uniform thin portions ~10 ~ at spaced intervals along i.ts length, each thin portion terminating ln at least one sdge constituting a corona emission surface, whereby, when a source of high voltage is connected to the corona generatiny element a corona i will be generated along the edge o~ said thin portions along the length o:E the metal wire.
T'he preferr~d embodi.ments of t'his invention now will 'be descrlbed, 'by way o:E example, with reference to the drawings accompanying this specification in which:
Figure 1 is a perspective view of a corona ge'nerating elemént constructed in accordance with the ~ invention including a block diagram and -~ : Figure 2 is a fragmentary perspective view of a modifled form of the invention.
Referring to the dra~ing, in Figure 1 there is illustrated a corona generating element identified generally by reference numeral 11. A corona power supply 13 whose output is a neyative D.C. voltage of 6 KV is connected ~o l~Z~)530 corona generating element l:L. A photoconductive mer~er 15 of the type that is charged with a neg~tive volt~ye is positioned to accept a charge from corona yenerating element 11 on its upper surface 17.
corona generating element 11 comprises a strand of wire 19 which can be made of tungsten or any other metal suitable for corona emission. Wire 19 is of circular cross-section and has a series of identical, B generally rectangular configures ~ thin, flat portions 21, qenerally also having a rectangular cross-section. Flat portions 21 are located alony the length of wire 19 at closely spaced in-tervals and are formed such that they lie in a common plane which includes '~ the longitudinal axi~ of wire 19 and have a thickness less than the diameter of wire 19. Flat portions 21 contain sides which extencl :Laterally outward ~yond the body of wire 19 and terminate at side edges 23. These side edges 23 function as knife-edge type corona emission surfaces. Typical dimensions for corona generating element 11 are as follows:
Cross-sectional diameter of wire ~ 75 microns hickness of flat portions - - - - - - - - 25 mlcrons I,ongitudinal dimension of flat portions - 0.5 millimeter (i.e., dimension along longitudinal axis of wire) Lateral di~nension of flat portions - - - - 1 millimeter Distance between centers of successive flat portions ~ - - - 3 millimeters ~ IL;Z~530 Since the thickness of khe flat sections in the exemplary dimensions listed above is one-third -the diameter oE the wire, khe intensity of t'ne co~ona produced at each flat section is twen-ty-seven times greater than the corona that would otherwise be produced at that location.
When wire 19 is energized by corona power supply 13, a corona of high intensity is produced along the side edges 23 of each flat portion 21~ The net result of the corona generated at each of these locations is a continuous series of high intensity coronas substantially uniformly distributed over the entire length of wire 19.
; In Figure 2, there is illustrated another ernbodiment o the corona generating element 11. Irl this ernbodiment, the corona generating element 11 cornprises the strand of wire 19, with a series o~ thin axehead portions 25 located along the length of wire 19 at closely spaced intervals. ~xehead portlons 25 are generally similar to flat portions 21, except that axehead portions 25 have a ....
cross-section normal to the longitudinal axis of the wire 19 ~` that is substantially diamond-shaped and they have razor edges 27, which extend laterally beyond the body of wire 19.
These razor edges 27 function as the knife-edge type corona ~ emission surfaces.
- 25 Axehead portions 25 are forrned in such a way that razor edges 27 lie in a cor~mon place which includes the ~20~3~

longitudina:L axis of wire 19. The slopiny part 29 of axehead portions 25 which are between razar edyes 27 have a thickness less than the diameter of wire l9. RazOr edges 27 are thinner than side edges 23 of flat poxtions 21.
When wire 19 is energized by corona power supply 13, :a corona of high intensity is produced along razor edges 27 of each axehead portion 25. The net result of the corona generated at each o:E these locations is a continuous series :; of hlgh intensity coronas substantially uniformly distributed over the length of wire 19.
The thin flat portions 21 and thin axehead portions 25 are located along the length of the wire l9 at closely spaced intervals to produce a continuous series o~ high intensity coronas which are substantially uniformly distributed over the entire lenyth of wire 19. The intervals between portions 21 or 25 ~an be reyular or irregular as l.ong as a continuous series of coronas substantially uniformly distributed over the entire :: length of wire l9 are produced.
Preferably portiOn 25 has two razor edges 27 extending beyond the body of wire l9. One razor edge 27 on axehead portion 25 would operate to form the desired corona, but two edges are preferred. Also, if one of the razor edges 27 should break off, the newly formed edge will function as a corona generating surface~ Furthermore, since the corona is generated at ths razor edges 27 and since C)53~

the razor edges 27 are thinner than the o-ther portions of the w~re, and the side edges 23 of flat portions 21, the intensity of the corona at the razor edyes 27 is much greater than the corona that would othe~ise be produced.
Finally, and most importantly, since the corona .
is generated along the closely spaced razor edges 27 and not along the other portions of the wire, the likelihood of randomly located and shifting hot spots being generated when the wire i5 used in the negative mode is greatly reduced if not totally eIiminated.
corona generating element 11 can be fabricated by taking a length of metal wire of circular cross-section ~; and then forming the thin portions by hammering, stamping, crimping, ~r any other suitable means ~nown in the art.
Whil0 the invent:ion has been described her0:in in respect o~ the appl:ication o~ an electrostatic charge o~ negative polarity, it is to be understood that the invention also is useful in applying an electrostatic charge of pos_tive pol~rity.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A corona generating element for use in applying a uniform electrostatic charge over a surface when the element is connected to a source of high voltage, said element comprising a strand of metal wire having a plurality of similar discontinuous unitary thin portions at uniformly spaced intervals alongs its length, each thin portion terminating in at least one knife edge constituting a corona emission surface, whereby, when a source of high voltage is connected to the corona generating element, a corona will be generated along the knife edge of each of said thin portions along the length of the metal wire.

2. A corona generating element as defined in claim 1 in which said thin portions each have at least one knife edge defined by converging opposite surface planes.

3. A corona generating element as defined in claim 1 in which said thin portions have opposed planar surfaces lying in generally parallel planes.

4. A corona generating element as defined in any one of claims 1, 2 or 3, in which each thin portion includes a pair of knife edges.

5. A corona generating element as defined in claims 1, 2 or 3 in which the knife edge of the thin portions extends outward beyond the body of the wire.

6. A corona generating element as defined in any one of claims 1, 2 or 3 in which the wire is circular in cross-section.

7. A corona generating element as defined in any one of claims 1, 2 or 3 in which the wire has a circular cross-section and the knife edge of the thin portions is disposed offset outwardly of the wire.

8. A corona generating element as defined in any one of claims l, 2, or 3 in which the thin portions are disposed in a common plane along the length of the wire.

9. A corona generating element as defined in any one of claims 1, 2, or 3 in which said thin portions are disposed in a common plane with the longitudinal axis of the wire lying in the plane occupied by the thin portions.
,

10. A corona generating element as defined in any one of claims 1, 2, or 3 in which said thin portions are disposed in a common plane with the longitudinal axis of the wire lying in the plane occupied by the thin portions, said thin portions being substantially identical over the length of the elements.

11. A corona generating element defined in claim 1 or 2 in which the thin portions have a cross-section normal to the longitudinal axis of the wire and a substantially diamond-shaped configuration, the outer edges of which defining a knife edge.

12. A corona generating element defined in claim 1 or 3 in which the thin portions are of substantially rectangularly shaped configuration extending in a direction along a line normal to the longitudinal axis of the wire.

13. A corona generating element defined in any one of claims 1, 2 or 3 in which those edges of said thin portions not knife edges have rounded corners.

14. A corona generating element as defined in any one of claim 1, 2, or 3 in which the maximum thickness of the thin portions is about one-third the diameter of the wire, whereby the intensity of the corona produced at each thin section is about twenty-seven times the intensity of the corona that would otherwise be produced at that location,

15. A corona generating element as defined in any one of claims 1, 2 or 3 in which the thickness of the thin portions is about 25 microns and the diameter of the wire is about 75 microns.

16. A corona generating element as defined in any one of claims 1, 2 or 3 in which the wire is made of tungsten.

17. A corona generating element as defined in any one of claims 1, 2, or 3 in which the distance between centers of successive thin portions is about 3 millimeters.

18. A corona generating element as defined in any one of claims 1 or 3 wherein the thin portions are identical flat formations having opposite parallel planar surfaces arranged extending outwardly along a line normal to the longitudinal axis of the wire.

19. A corona generating element as defined in any of claims 1 or 3, wherein the thin portions are identical along the length of the wire and have planar surfaces with opposite terminal ends disposed outwardly beyond the body of the wire.

20. A corona generating element as defined in any one of claims 1 or 3, wherein the wire is circular in cross-section and each thin portion has opposite planar surfaces and define a pair of knife edges disposed on opposite sides of the wire.

21. A corona generating element as defined in any one of claims 1 or 3, wherein the wire is circular in cross-section and each thin portion has opposite parallel plane surfaces all said thin portions occupying a common plane and the knife edges are located in diametrically opposite sides of the wire.

22. A corona generating element as defined in any one of claims 1, 2, or 3 and, in combination therewith, a power source having a negative D.C. output voltage and the said element connected to said output.