Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF OPERATING AN ELECTROSTATIC PRTNTER
BACKGROUND AND SUMMARY OF THE INVENTION
This is a divisional of co-pending Canadian application
Serial No. 2,039,094 filed March 26, 1991.
IDAX and MIDAX printing techniques are commercial
electrographic imaging techniques that utilize what is
referred to as silent electric discharge. In such systems,
an ion cartridge is mounted adjacent an imaging drum. The
drum then moves into contact with a transfer sheet (e. g.
paper). The conventional cartridges utilized in these printing
systems include first and second electrodes, typically called
the driver and control electrodes, separated by a solid
dielectric member, such as a sheet of mica. The control
electrode, typically in the form of control fingers, defines
an edge surface disposed opposite the driver electrode to
define a discharge region at the junction of the edge surface
and the solid dielectric member. An alternating potential is
applied between the driver and control electrodes of
sufficient magnitude to induce charged particle producing
electrical discharges in the discharge region, and means are
provided for applying a charged particle extraction potential
between the control electrode and a further electrode, so
that imaging occurs on the imaging drum, or paper or like
dielectric moving past the ion cartridge. In most commercial
installations a screen electrode is also provided, between
the imaging drum and the control electrode, and separated by
an insulating spacer from the control electrode. A commercial
ion cartridge is
Trade-mark
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typically constructed of a plurality of driver,
control, and screen electrode units, in a matrix
form.
In commercial installations of MIDAX printers,
there typically are three major manners in which the
ion cartridges fail. The spot size produced by the
ion cartridge grows as the cartridge ages, and once
it gets to a particular level so that the print
quality is unacceptably poor, the cartridge must be
cleaned or retired; or under some circumstances
there is catastrophic failure of the cartridge.
One conventional way in which ion cartridges
fail is euphemistically referred to as "red death".
By-products formed in the ionization process, such
as oxides, build up on the cartridge control fingers
which can cause an uneven rush of electrons and
negative ions upon application of the extraction
voltage. Another mode of failure is euphemistically
referred to as "white death". In the white death
scenario, white crystals, which typically are
nitrates, build up on the screen electrode thereby
creating a dielectric layer and causing an
electrical defocussing of the electron and ion
stream as it exits the cartridge. A third typical
mode of failure, euphemistically referred to as
"black death", is caused by premature catastrophic
failure of the cartridge when conductive toner is
sucked up into the cartridge and creates unwanted
electrically conductive paths and also localized
heating.
According to the invention it has been found
that the mechanisms by which at least red and white
death occur arc dependent upon the characteristics
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of the atmosphere from which the ions are produced
by the ion cartridge. The atmosphere is typically
normal air, although it may be contaminated with
ammonia, benzene, or other gases depending upon the
particular plant in which the system is utilized.
Nitrogen, oxygen, and water vapor are the major
components of the atmosphere, and during operation
of the MIDAX printers after one stream of electrons
and ions is created and extracted from the cartridge
new air replaces that which was lost from the
cartridge. Most of the problems of ion cartridge
aging are caused by compounds made of or initiated
by oxygen and/or water vapor, and therefore the
process can be slowed or even eliminated by the
replacement of the air around the ion cartridge with
appropriate other gases. Even in situations where
ion cartridge life is not extended, however, there
may be significant advantages to providing a
particular atmosphere in the ion cartridges. For
example the quality of the print -- its uniformity
-- may be significantly enhanced. Uniformity
enhancements on the order of 40% are not unusual
when the atmosphere from which the electrons and
ions are created by the ion cartridge is properly
controlled.
According to the present invention, it has been
found that if a substantial portion of the air at
the discharge region of the ion cartridge is
replaced with nitrogen, elemental noble gases,
mixtures of noble gases, or mixtures of nitrogen
with one or more noble gases, uniformity and/or
cartridge life can be significantly enhanced. If
the gas is supplied in a particular manner even
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black death catastrophic failure can be eliminated
or minimized.
Gases that are particularly effective in the
practice of the invention are nitrogen, mixtures of
nitrogen and helium, and mixtures of nitrogen with
argon, xenon, neon, and/or krypton. It has been
found that completely dry pur,.e nitrogen is not
particularly effective since nitrogen is not easily
ionized, and therefore there must be some "catalyst"
present to enhance the nitrogen ionization. However
the catalyst must be present in small enough amounts
so that arcing does not occur, since arcing can be
destructive and reduce cartridge life. While water
vapor that naturally occurs can provide this
catalyst effect, it is desirable for other reasons
to keep the amount of water vapor to a minimum.
Therefore it is most desirable to add another gas,
such as a noble gas, to the nitrogen.
While helium can be effective as a catalyst for
nitrogen ionization, if helium is used in a
commercial environment it can be dangerous to a
human operator since the helium and nitrogen
ionization may generate gases that would make an
operator dizzy. Argon, xenon, neon, and krypton do
not have that effect, however, yet they provide an
effective catalyst for nitrogen ionization. The
amounts of argon, neon, krypton, or xenon must be
controlled, however, to make sure that they are low
enough so that arcing does not occur.
In the preferred form of the present invention,
nitrogen is mixed with argon, xenon, neon, or
krypton so that there is a volume ratio of about 5 -
1 to about 20 - 1 of nitrogen to other gas. The
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invention is most effective in some actual operating environ-
ments when nitrogen and argon are mixed at a ratio of about
to 1. Typically the gas mixture is supplied to the
discharge region at a rate of about 4.75-6.25 cubic feet per
hour, typically about .5 cubic feet per hour of argon, xenon,
neon, or krypton, and about 5 cubic feet per hour nitrogen.
A number of particularly advantageous mechanisms for
introducing the gas to the discharge region are provided
according to the invention. Black death can be significantly
reduced if the gas is introduced through the insulating
spacer between the control electrode and the screen electrode.
The gas is typically introduced at a pressure above atmospheric
pressure so that a positive pressure is provided in this area,
and conductive toner can therefore not be easily sucked into
the ion cartridge. Alternatively, the gas may be injected
through a plenum and holes spaced about one-half inch along a
pre-existing cartridge mounting rail, typically the first
rail in the direction of rotation of the imaging drum.
Alternatively, a pair of gas manifolds may be provided at
opposing ends of the imaging drum, and a pair of spray tubes
extending between the gas manifolds with a plurality of
openings provided along their length. The gas is then
supplied by regulators and conduits to the gas manifolds,
and thus introduced uniformly between the ion cartridge and
the imaging drum.
According to the invention, there is provided a method
of operating an electrostatic printer which prints, with an
acceptable determinable print quality, electrostatic images
on a dielectric utilizing electrostatic discharges at an
electrostatic region formed by application of a predetermined
voltage high enough for good print quality while not
significantly adversely affecting printer components, and
utilizing a depletable source of gas under pressure which is
supplied to the region of electrostatic discharge effective
to prolong the life of printer components, comprising the
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steps of sequentially: (a) upon a degradation in the print
quality, increasing the voltage effective to cause the
electrostatic discharge, so as to increase print quality to
approximately said acceptable determinable level; (b) removing
the source of gas under pressure once it is depleted, and
connecting up a new source of the gas so that it is supplied
to the region of electrostatic discharge; and (c) returning
the voltage effective to cause electrostatic discharge back
to approximately its value prior to the practice of step (a);
said steps (a)-(c) practiced without interrupting printer
operation.
It is the primary object of the present invention to
provide for enhanced uniformity and/or enhanced cartridge
life in silent electric discharge electrographic imaging.
This and other objects of the invention will become clear
from an inspection of the detailed description of the invention
and from the appended claims.
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BRIEF DESCRIPTION OF 1'~ DRAi~IINGS
FIGURE 1 is a side schematic view, partly in
cross-section and partly in elevation of an
apparatus for electrostatic imaging according to the
present invention, and for practicing the method of
the present invention;
FIGURE 2 is a side schematic primarily
cross-sectional view of the details of the ion
cartridge of the FIGURE 1 apparatus;
FIGURE 3 is a detail schematic cross-sectional
view of another embodiment of apparatus for feeding
gas to the ion discharge region of apparatus
according to the invention;
FIGURE 4 is an exploded perspective schematic
view of still another embodiment for the supply of
gas to the discharge region, according to the
invention; and
FIGURE 5 is a top schematic view of the
apparatus of FIGURE 4, also showing the gas sources
and regulating apparatus.
DETaILFSD DESCRIPTION OE 1'~ DRAWINGS
An exemplary apparatus according to the present
invention is shown generally by reference numeral 10
in FIGURE 1. The main components include the silent
electric discharge ion generating system 11, and an
imaging drum 12 or like device for moving a
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dielectric, such as dielectric belt or dielectric
paper web 13 or dielectric surface of drum 12, past
the SED apparatus 11. Most of the components of the
SED apparatus 11, and the imaging drum, are
conventional.
One of the major components of the SED
apparatus 11 comprises the ion cartridge 14 which is
mounted by a cartridge mounting block 15 within a
casing defined by driver printed circuit board 16
and cartridge connectors 17. The structures 16, 17
are supported by a pair of cartridge mounting rails
18, 19 that are elongated in the direction of
elongation of the drum 12 and the ion cartridge 14.
The drum 12 is mounted for rotation in the direction
A by a shaft, bearings, and like conventional
components, and so that it is spaced only a small
distance from each of the rails 18, 19, defining
gaps 20, 21 therewith. Typically the gaps 20, 21
have a width of less than about .002 inches. An
interior volume 22 is provided between the ion
cartridge 14 and the imaging drum 12.
The ion cartridge 14 is of conventional
construction, such as shown in U.S. patents
4,155,093, 4,160,257, 4,267,556, and/or 4,381,327.
FIGURE 2 very schematically illustrates one
component of the ion cartridge 14, there being many
such components arranged throughout the length of
the ion cartridge 14 (typically in matrix form) to
provide electrostatic charges to the dielectric web
or belt 13. The major components of the cartridge
14 schematically illustrated in FIGURE 2 comprise a
first or driver electrode 24, a second or control
,electrode 25 typically formed by a plurality of
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control fingers, and a solid dielectric member 26
disposed therebetween. Typically the member 26 is
mica in commercial installations, however according
to the invention improved performance and longevity
are possible so that other solid dielectric members
besides mica may be practical.
A high voltage alternating potential 28 is
applied between the driver and control electrodes
24, 25 to cause the formation of a pool or plasma of
positive and negative charged particles in the
region adjacent the dielectric 26 at an edge surface
of the control electrode 25, which charged particles
may be extracted to form a latent electrostatic
image on the dielectric belt or web 13 or drum 12
periphery. Charged particles of a given polarity
may be extracted from the plasma by applying a bias
potential 29 of appropriate polarity between the
second electrode 25 and a further electrode, which
typically would comprise the image drum 12 itself.
Also in most commercial installations, a~screen
electrode 31 defining a screen aperture 32 is
provided by an electrical insulator 30 from ~..he
second electrode 25. The screen voltage should be
in a relatively narrow range, e.g. -400 to -900 volts.
The screen voltage is determined in part by the
distance of the screen 31 from the drum 12. At a
distance of 0.0010 inches, the optimum screen
voltage is about -700, and could be increased to
about -800 before arcing occurs.
As seen in FIGURE 2, constant power supply 33
(typically a voltage of about -700) and variable
power supply 34, and a switch 27, are provided in
addition to power supply 29 (typically a voltage of
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about -275). The power supply 34 typically has~a
range of about +200 to about +300 (e. g. about
+250). When switch 27 is in the right (no-print)
position in FIGURE 2, the power supply 29 is
bypassed, and there is a voltage of about -450 to
the control electrode 25 (e. g. -700 + +250 = -450).
When the switch 27 is in the left position in FIGURE
2, that is the print position, there is about a -725
voltage to control electrode 25 (e.g. -700 + +250 +
-275 = -715). The screen electrode 31 provides an
electrostatic tensing action preventing accidental
image erasure and focussing of the electrostatic
discharge onto the drum 12 periphery. In most
commercial installations, a dielectric belt or web
13 need not pass past the ion cartridge 14, but
rather the peripheral surface of the imaging drum 12
is dielectric, and that surface moves into operative
association with a receptor sheet, such as a paper
sheet, which cooperates with a transfer roll.
What has been heretofore described is
conventional. According to the invention, at least
some of, and preferably the vast majority of. the
gas in the volume 22 (i.e. at the discharge region
of the control electrode 25) is replaced with gas
having particular qualities so as to enhance the
uniformity of the print quality, and/or to extend
the life of the ion cartridge 14.
The apparatus according to the present
invention comprises means for supplying a control
gas to the discharge region during the generation of
charged particles. The control gas -- which in the
FIGURE 1 embodiment is supplied directly to the
volume 22 -- comprises a gas selected from the group
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consisting essentially of nitrogen, elemental noble
gases, mixtures of elemental noble gases, and
mixtures of nitrogen with one or more elemental
noble gases. It is not essential that all
contaminants be removed from the gases, and in fact
where pure nitrogen is utilized it is necessary that
water vapor, or some other catalyst to facilitate
nitrogen ionization, be present in order for the
system to work properly. However it has been found
that almost 100% pure nitrogen supplied as
illustrated in FIGURE 1, or in a like manner,
combined with the natural water vapor from the paper
or other components introduced into the system,
works satisfactory to at least enhance print
uniformity. Nitrogen mixed with helium is also
effective, however in commercial installations where
an operator will be located adjacent to the printing
apparatus 10 helium is not desirable since
by-product gases are produced which can have
undesirable side effects when inhaled, and thereby
pose a safety hazard. It has been found that it is
particularly desirable, however, to provide a
particular mixture of nitrogen with one or more of
argon, krypton, xenon, or neon, most preferably
argon.
According to the invention it has been
determined that the amount of noble gas to be mixed
with nitrogen (when a nitrogen noble gas mixture is
utilized) 'should be enough to provide a catalyst for
nitrogen ionization. However since elemental noble
gas present in too large a quantity will cause
arcing to occur, the amount of noble gas must be
limited by that criteria. In actual experiments it
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has been found that a mixture of nitrogen and one or
more of argon, krypton, xenon, or neon gases --
particularly argon -- is most suitable, the volume
ratio of nitrogen to other gas being in the range of
5 to 1 to 20 to 1, most desirably about 10 to 1.
The flows of the gases making up the mixture are
controlled so that the total gas mixture flow to the
discharge region is at a rate of about 4.75-6.25
cubic feet per hour, most typically by supplying
nitrogen at about 5 cubic feet per hour and the
other gas, e.g. argon, at about .5 cubic feet per
hour.
Supply of gas to the volume 22 in the FIGURE 1
embodiment is provided by utilizing the pre-existing
cartridge mounting rail 18 at the "first" portion of
the imaging drum 12 as it rotates in direction A
into the volume 22, so that gas passes with the
rotating drum toward the gap 21. This is preferably
provided by forming a plenum 35 in the rail 18, with
a plurality of through-extending openings or jets 36
from the plenum 35 to the volume 22, preferably the
openings or jets 36 being spaced from each other
about one half inch along the length of the rail
18. A conduit 37 leading from a source 38 of
pressurized nitrogen, or other gas pursuant to the
invention, supplies the controlled gas to the plenum
35. The source 38 can be either compressed nitrogen
or like gas, or a liquid nitrogen dewer, or a Prima
Alpha Separated nitrogen filter attached to a
compressed air source.
With the proper control of gas to the volume
22, the apparatus 10 of FIGURE 1 can greatly assist
in extending the life of the ion cartridge. That is
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the avoidance of red death and white death may b~~
provided. However a small amount of air, and other
materials, may still pass into the volume 22, and
therefore it is possible that conductive toner
particles may accidentally be drawn into the volume
22, which conductive toner would burn and result in
premature catastrophic failure of the cartridge 14.
In order to prevent this "black death", the
apparatus illustrated in FIGURE 3 may be utilized.
In the FIGURE 3 drawing, elements that are
comparable to those in the FIGURES 1 and 2
embodiment are illustrated by the same reference
numeral only preceded by a "1".
In FIGURE 3, the first or driver electrode 124
is shown mounted on a conventional backing insulator
40, which in turn is connected to an aluminum
backbone 41. The mica dielectric member 126 is
disposed between the driver electrode 124 and the
control electrode fingers 125, with an insulating
spacer 130 separating the screen electrode 131 from
the control fingers 125. In this embodiment, the
nitrogen or like gas under pressure (that is greater
than ambient pressure) is introduced into the
discharge region through the insulating spacer 130,
having a vector generally parallel to the control
fingers 125, by the openings or jets 136 connected
to the plenum 135. The gas for ionization at the
discharge region flows outwardly through the opening
132 in the'screen electrode 131, along with the
ions, and since a positive pressure is maintained at
the discharge region it is extremely unlikely that
conductive toner particles could enter that area and
thereby cause "black death".
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The embodiment of FIGURES 4 and 5 is still
another embodiment of the apparatus for supplying
the desired gases to the discharge region, according
to the invention. In the FIGURES 4 and 5
embodiment, structures comparable to those in the
FIGURES 1 and 2 embodiment are illustrated by the
. same reference numeral only preceded by a "2". In
this embodiment, the ion cartridge 214 is shown in
operative association with a support 45, which
provides a positive electrical connection, adjacent
the image drum 212. Gas is supplied via the gas
manifolds 47, 48 which are mounted on opposite
ends of the cartridge 214 and drum 212. A pair of
spray tubes 49, 50 having a plurality of openings
51, 52 respectively therein extend between the
manifolds 47, 48 and supply gas directly to the
"top" of the drum 212 (as oriented in FIGURE 4), and
just below the ion cartridge 214, to provide the
vast majority of the gas at the discharge region.
Gas is supplied to the manifolds 47; 48 by
conduits 54, 55 which are connected to a tee fitting
56, which in turn is connected by conduit 58 to a
second tee fitting 59 (see FIGURE 5). In the
preferred embodiment illustrated herein, a source of
nitrogen under pressure, 61, and a source of argon
under pressure, 62, are provided to supply the
ionizing gas. The sources 61, 62 are connected by
conventional regulators and metering devices 63, 64
to the tee fitting 59. The regulator/metering
devices 63, 64 control the flow rates of nitrogen
and argon (or xenon, krypton, or neon) so that they
are in the appropriate range.
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The board 45 may have spring loaded pins 67~for
engaging 214, and electrical connectors 68 for the
drive electrode of the ion cartridge.
In the preferred embodiment, the ratio of
nitrogen to argon (or xenon, krypton, or neon) is
about 5 to 1 to 20 to 1, most preferably about 10 to
1. The flow rate is regulated so that the gas
mixture supplied to the region by the tubes 49, 50
is (for the FIGURES 4 and 5 embodiment of apparatus)
at a rate of about 4.75-6.25 cubic feet per hour.
This rate may change depending upon the particulars
of the geometry for applying the gas to the
discharge region, but would be at an equivalent
range taking into account the differences in the
supply apparatus. Most desirably, the nitrogen
would be supplied at about 5 cubic feet per hour
while the argon (or xenon, neon, or krypton) at a
rate of about .5 cubic feet per hour. The nitrogen
flow rate could vary about plus or minus 10%, and
the argon flow rate could vary about plus or minus
50%. It is necessary, however, that the amount of
argon, or like gas, be supplied to the nitrogen
stream so as to be effective to provide a catalyst
for nitrogen ionization; however the amount must be
low enough to prevent arcing since arcing more
readily occurs the higher the percentage of argon or
the like.
Utilizing the ratios heretofore described~it is
possible in an actual commercial installation of a
MIDAX printer to increase the cartridge life between
two and five times (i.e. with respect to red and
white death). At ratios significantly outside this
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range, for the supply apparatus illustrated in
FIGURES 4 and 5, the same results cannot be expected.
Reference is made to the following non-limiting
examples which show some of the results achievable
according to the invention:
Example 1
Utilizing an apparatus generally similar to
that in FIGURE 1, so that the volume surrounding a
conventional MIDAX ion cartridge associated with an
imaging drum is shrouded, approximately 100%
nitrogen gas was supplied to the volume 22. In
actual operation of the system 10, the uniformity of
the hole to hole ion cartridge output increased
approximately 40%. Sufficient water vapor or like
components were able to enter the system so as to
provide a catalyst for the nitrogen ionization.
Examr~le 2
Again utilizing the apparatus generally such as
illustrated in FIGURE 1, a mixture of about 4:1,
nitrogen to helium, by volume, was added to the
volume 22. Again the print quality uniformity was
significantly enhanced. While sufficient testing
was not done to know for positive whether or not the
ion cartridge life was extended in its real life
environment, extrapolation of the results indicated
that it clearly would be.
Example 3
Utilizing apparatus as illustrated generally in
FIGURES 4 and 5, about 5 cubic feet per hour of
nitrogen and about .5 cubic feet per hour of argon
were mixed in tee fitting 59 and in the subsequent
conduits, being supplied in controlled quantities by
regulators 63, 64, and were introduced through the
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spray tubes 49, 50. In an actual commercial plarit
environment, the life of the MIDAX ion cartridge 214
was extended to approximately four times its
expected longevity. If the argon concentration was
reduced below a volume ratio of about 20 to 1, there
was insufficient argon to provide a catalyst for
ionization and poor and/or intermittent ionization
will take place, resulting in poor print quality.
When the amount of argon is increased above about 5
to I, in its real life testing there was too high a
potential of arcing to expect the type of longevity
desired.
It is also noted that in the MiDAX control
system, control of internal operating voltages may
be effected from an operator control panel (not
shown). Thus in the practice of the invention, if
the operator notices that the print quality is
degrading, he can increase the voltage to ion
cartridge 14, and operate the regulators 63, 64 to
close of the cylinders 61, 62. While good quality
printing (due to the increased voltage) continues,
he can then replace the gas bottles 61, 62, and once
he reestablishes the gas supply utilizing regulators
63, 64, he can then reduce the voltage back to
normal. In this way the system can be continuously
run without a degradation in print quality while
changeover of gas supplies takes place.
It will thus be seen that according to the
present invention enhanced print uniformity and/or
ion cartridge longevity for a MIDAX printer can be
achieved by supplying the desired gas at the
discharge region. While the invention has been
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herein shown and described in what is presently '
conceived to be the most practical and preferred
embodiment thereof, it will be apparent to those of
ordinary skill in the art that many modifications
may be made thereof within the scope of the
invention, which scope is to be accorded the
broadest interpretation of the appended claims so as
to encompass all equivalent structures and methods.
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