CORONA GENERATING ELEMENT

ELEMENT GENERATEUR DE COURONNES

English 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.

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




11



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.


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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.


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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


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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

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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


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~ 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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