Note: Descriptions are shown in the official language in which they were submitted.
TITLE OF THE INVENTION
OFFSET ELECTROSTATIC PRINTER AND IMAGING PROCESS
UTILI71~G DEHUMIDIFIED AIR
BACKGROUND OF THE INVENTION
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Field of the Invention
The present invention relates generally to an offset
electrostatic printer which utilizes dehumidified air to extend the
lifetimes of the print head and of the dielectric imaging member
and to an offset electrostatic imaging process involving the
utilization of dehumidified air.
Description of the Prior Art
In a typical electrostatic imaging process, a latent
slectrostatic image is formed on a dielectric charge retentive
; surface using a non-optical means, such as an electrostatic print
head which generates ions by the corona discharge from a small
diameter wire or point source. The dielectric surface can be
; ~ ~ either on the final image recording or receiving medium or on an
intermediate transfer element, such as a cylindrical drum.
The latent electrostatic image is then developed by depositing
2 0 a developer material containing oppositely charged toner particles .
The toner particles are attracted to the oppositely charged latent
electrostatic image on the dielectric s~face. If the dielectric
surface is on the final recording medium,~hen the developed image
can be fixed by applying heat and/or pressure. If t~e dielectric
surface is on an intermediate transfer element, however, then the
developed image must first be transferred to the final recording
medium, for example plain paper, and then fixed by the application
of heat and/or pressure. Alternatively, the developed image may
be fixed to the final recording medium by means of the high
pressure applied between the dielectric-coated transfer element and
a pressure roller, between which the final recording medium
passes.
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The intermediate transfer element in an offset electrostatic
imaging process is typically a cylindrical drum made from an
electrically conductive, non-magnetic mateFial, such as aluminum or
stainless steel, which is costed with a dielectric material. Suitable
dielectric materials include polymers, such as polyesters,
polyamides, and other insulating polymers, glass enamel, and
aluminum oxide, particularly anodized aluminum oxide. Dielectric
materials such as aluminum oxide are preferred to layers of
polymers because they are much harder, and therefore, are not as
readily abraded by the developer materials and the high pressure
being applied. Metal oxide layers prepared by a plasma spraying
or detonation gun deposition process have been particularly
preferred as dielectric layers because they are harder and exhibit
longer lifetimes than layers prepared using other processes.
Vne major problem encountered with currently available
electrostatic printers of the ion deposition screen type has been
the limited lifetime of the electrostatic aperture board. These
types of electrostatic printers are disclosed in U . S . Patent Nos .
3,689,935, 4,338,614 and 4,160,257. Such electrostatic printers
have a row of apertures which selectively allow ionized air to be
deposited onto a dielectric sùrface in an imagewise dot matrix
pattern. It has been observed that a chemical debris tends to
build up around t~e apertures and on the corona wire as a
function of time and the humidity of the air. This chemical debris
was found to be a crystalline form of arnmonium nitrate. This
particular chemical is created when air containing water molecules,
such as is generally encountered, is ionized.
lt has also been observed that ~ when an electrostatic printer
of the type disclosed in U . S . Patent 4, 365, 549 is operated in a
moderately high relative humidity, the surface conductivity of the
dielectric drum increases where the ionized water molecules are
deposited. The ionized water molecules are complexes containing
hydronium ions. Water molecules in the air can become ionized by
the corona wire in the ion deposition print head or by the A . C .
scorotrons which are used to discharge residual charge on the
~ ~ drum. These conductive areas are observed on the final recording
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medium as weakly developed areas. This is believed to be caused
by the more conductive surfaces leaking off their latent
electrostatic images to the toner which has been made conductive
during the development operation.
A number of methods have been, suggested for alleviation of
this problem of contaminant buildup. It has been suggested that
the air being supplied to the corona discharge device first be
filtered through a filter for ammonia in order to prevent the
formation of ammonium nitrate. This method has not been found to
be effective because it does not remoYe the water molecules in the
air which under the influence of a corona discharge and in
combination with other components of air form precursors to
ammonium nitrate. Another method suggested for inhibiting
formation of ammonium nitrate in an ion generator which includes a
glow discharge device is to heat the glow discharge device above
its intrinsic operating temperature at or near the ion generation
sites .
SUMMARY OF THE INVENTION:
In accordance with the present invention, the operational
2 o lifetime of an offset electrostatic printer can be prolonged by an
order of magnitude by passing unheated dehumidified ~ir at, near
or through the ion modulated print head of the printer and at or
near the surface of the die]ectric imaging member.
An electrostatic printer in accordance with the present
2 5 invention comprises an ion modulated electrostatic print head for
forming latent electrostatic images, a dielectric imaging member
comprising a layer of dielectric material, means for developing a
latent electrostatic image on the dielectric imaging member, means
for transferring a developed electrostatic image from the dielectric
imaging member to an image, means for supplying unheated
dehumidified air at or near ambient temperature having a relative
humidity of less than about 20 percent, and means for directing
the dehumidified air at, near or through the print head and at or
near the dielectric imaging member. In a preferred embodiment,
the print head comprises a modulated aperture board having a
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plurality of selectively controlled apertures therein and sn ion
generator for projecting ions through the apertures. In this
embodiment, the dehumidified air is directed at or near the ion
generator and at, near or through the apertures. The offset
electrostatic printer may further comprise an ion generator for
erasing latent electrostatic images, and a means for directing
dehumidified air at or near such ion generator.
The process of the present invention comprises the steps of
forming a latent electrostatic image on a dielectric imaging member
using an electrostatic print head, developing the latent
electrostatic image, transferring the developed electrostatic image
from the dielectric imaging member to an image receiving surface,
providing unheated dehumidified air, and directing it at, near or
through the print head and at or near the dielectric imaging
member. The process may further comprise the steps of erasing
the latent electrostatic images by means of an ion generator and
directing the dehumidified air at or near such ion generator.
When unheated dehumidified air having a relative humidity of
less than about 20 percent, and preferably, less than about 5
percent, is used, the lifetime of the offset electrostatic printer can
be extended significantly. It has been found that the use of such
dehumidified air substantially inhibits the formation of ammonium
nitrate around the ion generatorsJ the apertures and the dielectric
imaging member, by removing the water molecules in the air which
in combination with other components of air and under the
influence of a corona discharge form precursors to ammonium
nitrate, such as nitric acid and ammonia. The use of unheated
dehumidified air also reduces oxidation of the electrodes used to
control the apertures, and provides for more uniform deposition of
ions across the print head. In addition~ the use of unheated
dehumidified air improves the retention of the latent electrostatic
images on the dielectric imaging member.
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BRIEF DESCRIPTION OE THE DRAWI~GS:
The various objects, advantages and novel features of the
invention will be fully appreciated from the following detailed
description when read in conjunction with the appended drawings,
in which:
FIG. 1 illustrates an offset electrostatic printing system in
which the present invention may be employed;
FIG. 2 is a perspective view of the electrostatic print head,
with portions cut away to illustrate certain internal details;
FIG. 3 is an enlarged sectional ~Tiew of the corona wire and
aperture mask assembly of the print head;
FIG. g is a still further enlarged view of the aperture
electrodes carried by the aperture mask;
FIG. S is an enlarged view of the area around the dielectric
drum of the offset electrostatic printing system illustrated in FIG.
l;
FIG. 6 is a perspective view of a corona neutralizer, with
portions cut away to illustrate certain internal details;
FI G . 7 is an enlarged sectional view of the corona
2 0 neutralizer;
FIG. 8 illustrates the system which is used to supply
dehumidified air to the electrostatic print head and to the corona
neutralizer;
FIG. 9 is a schematic diagram of a test apparatus used to
determine the effect of dehumidified air on the lifetime of
electrostatic print heads;
FIG. 10 is a plot of corona kilovolts versus elapsed hours
based on the data presented in Example 1 below; and
~:: FIG. 11 is a plot of corona kilovolts versus elapsed hours based on the data presented in Example 2 below.
Throughout the drawings, like reference numerals will be
used to identlfy like parts.
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DETAILED DESCPcIPTlON OF THE PREFERRED EMBODIMENTS:
FIG. 1 illustrates an offset electrostatic label printing system
20 which may advantageously be used to practice the process of
the present invention. A web 22 of plain paper is fed from a
supply reel 24 and is carried by a number of guide wheels 26
through a brake roll nip formed by rolls 30 and 32 and then
between dielectric drum 39 and backup roll 36. A latent
electrostatic image is formed on dielectric drum 34 which has been
prepared by coating a conductive substrate with a metal oxide
layer using a plasma spraying or detonation gun deposition
process. The latent electrostatic image is formed by means of an
ion modulated electrostatic print head 28 as the drum 34 rotates.
The latent image is developed on the drum 34 by the developer
unit 38, and the developed image is then transferred to the paper
web 22 and simultaneously pressure-fixed thereon at the nip
between the drum 34 and the backup roll 36. A doctor blade 40 is
provided to scrape off the developer material residue followed by
cleaning of the dielectric layer with web cleaner 42. Any latent
electrostatic images remaining on the drum are then erased by
2 o corona neutralizer uni$ 180 in preparation for subsequent printing
cycles. An enlarged ~iew of the area around dielectric drum 34 is
shown in FIG. 5.
A web 46 of overlaminate material is fed from supply reel 48
through a nip formed by rolls 50 and 52 where it is applied over
the printed image on web 22. The overlsminated printed web is
then cut into finished labels by rotary die cutting station 54 and
passed through a drive roll nip formed by rolls 56 and 58. The
finished labels are wound onto rewind reel 60 and the cutout
;-i overlaminate web 46 is wound onto waste rewind reel 62.
FIG. 2 is a perspective view of the electrostatic print head 28
with portions cut away to illustrate certain internal details. FIG.
3 is an enlarged sectional view of the corona wire and aperture
mask assembly of the print head, and FIG. 4 is a still further
enlarged view of the aperture electrodes carried by the aperture
masX. The print head 28 is of the type disclosed and claimed in
U . S . Patent 3, 689, 935, issued to Gerald L . Pressman et al . on
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September 5, 1972 ~nd tl.S. Pstent 4,016,813, issued to Gerald L.
Pressman et al. on April 12, 1977. The print head 28 also
embodles certa,~n ~,lnprov~ents disc1Osed and claimed in
U . S . Patent 4, 338, 614, issued to Gerald L~ Pressman et
al r on Ju1y 6, 1982.
The plqnt head 28 of Fla. 2 generally comprises a pair of
electrical circuit boards 72, 74 mounted on either side of a
centrally-located coron~ wire and aperture mask assembly. The
coron~ wire 76 is enclosed within an elongated conductive corona
~hield 78 which has A U-shaped cross-section. The corona shield
78 is supported at each of its two ends by ~ manifold block 30 that
is formed with an oblong central cavity 82. The m~nifold block 80
is nested within a m&sk ~upport block 84 which is generally
C-shaped in cross-section. The mask support block 84 is formed
with ~n oblong centr~l opening 86 which registers with the cavity
82 in the m~nifold block 80 ~nd receives the corona shield 78.
The mask ~upport block 84 is secured at its edges to a print head
slider 88, the latter being the primary supporting structure of the
print head 28 and carrying the two circuit boards 72, 74. The
print head slider B8 is formed with a large central cut-out 90 and
is secured to driver board 92.
The cor~rla shield 78 is positioned in facing relationship with
an aperture mask formed by a flexible circuit board ~4. P~eferring
particularly to FIGS. 3 and 4, the circuit board 94 is formed with
two ~taggered rows of apertures 96 i 98 extending parallel to the
corona wire 76 and transverse to the direction of mo~rement of the
web 22 in ~IG. 1. Positive ions produced by ~he corona wire 76,
which is maintained ~t a positiYe DC potential of about 2 . 7
kilcnaats, ~re induced to pass through the apertures 9ô, g8 under
the ir~fluence of an accelerating potenti~l which is m~intained
between the corona wire 76 and the corlducffve core of the drum 39
of FIG. 1. ~he flexible circuit board 94 includes s central
insulating layer 100 ~nd carries a continuous conductive layer 102
on the side facing the corona wire 76. The opposite ~ide of the
insulating l~yer 100 c~rries ~ number of conductive segments 104,
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ln6 associated with the individual apertures 96, 98 as shown in
FIG. ~. Circuit board 94 is ~ecured to mask support block 84 by
a thin layer of adhesive 99 and to slotted focus plane 108 by an
insulsting sdhesive layer 109. Circuit board 99 is overl~T inated
with a thin insulating layer 107. In operation, individual
potentials ~re applied between the conductive ~egments 109, 106
and the continuous conductlve layer 102 3n ~rder to establish local
fringing fields with~ the apertures 96, 98. As described in the
aforementioned U.S. Patents 3,689,935 snd 4,016,813, these
fringing fields can be used to block or permit the flow of ions
from the corona wire 76 to the drum 34 of FIG. 1 through selected
ones of the apertures 96, 98. The apertures are controlled by
appropriate electronics carried by the circuit boards 72, 74. As
explained in the aforementioned ~.S. Patent 4,338,614, the
performance of the print head may be enhanced by interposing ~
slotted focus plane made of a conductive material between the
modulated apertures 96, 98 and the dielectric-coated drum 34.
The slotted focus plane is illustrated at 108 in FIG. 3, with the
slot 110 01igned with the aperture rows 96, 98.
2 0 In an alternative embodiment, the corona wire 76 may consist
of a dielectric-coAted conductor u~ing a high-frequency AC voltage
source. Ion generstors of this type generate ~oth positive and
neg~tive ~ons~ ~lthough only one type of ion (in this case positive)
is drawn through the apertures 96, 98 by the DC accelerating
2 5 potential existing between the corona wire and the drum 34 .
Dielectric-coated AC corona devices are described in 1~ . S . Patent
4,057,723, issued to Dror S~rid e~ ri). on November 8, lg77; V.S.
Pstent 4,110,614, issued to Dror S~rid et ~1. on August 29, 1978;
U.S. P~ent ~,409,604, issued to Richa:rd A. Fotland on
October 11" 1~83; and U.S. Patent 9,446,371, ~ssued to Harold W.
Cobb on May 1, 1984O
In practice, it: has been found that deposits of ammonium
nitrate form in and around the fipertures 96, 98, principally on the
side facing the corona wire 7O. Some dep~sits also form on the
corona wire itself, thereby reducing its output and producing a
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nonuniform corons. After the print head has been in operation
with an air flow which has not been, dehumidified for about 50-75
hours, the deposits of ammonium nitrate in and around the
apertures 96, 98 begin to restrict the flow of ions through the
apertures. The effect on output can be counteracted somewhst ~y
increasing the potential on the corona wire 76, but eventually a
point is reached at which the apertures become substantially
completely blocked. When this occurs, the print head 28 must be
removed from the printing apparatus and the flexible circuit board
99 carrying the apertures 96, 98 must be replaced or cleaned.
The flexible circuit board 94 is rather difficult and expensive to
manufacture, since it must be etched with a pattern of fine,
closely-spaced conductors for controlling the individual apertures.
Therefore, frequent replacement of this component is undesirable.
Frequent cleaning is also undesirable because there is the
possibility of damaging the delicate circuit and because it is time
consuming .
FIG. 6 is a perspective view of a corona neutralizer, with
portions Cllt away to illustrate certain internal details. FIG. 7 is
2 0 an enlarged sectional view of a corona neutralizer . The corona
wire 400 is enclosed within an elongated conductive corona shield
402 which has a U-shaped cross-section and a series of holes 404
therethrough. The corona shield 402 is supported by a manifold
block 406 which is fGrmed with an oblong central cavity 408. A
filter screen 41û is disposed between corona shield 402 and
manifold block 406 over the entire length of the cavity 408. An
air inlet tube 412 for suppl~ng a flow of air to the corona
neutralizer is connected with casnty 408. ~ solid diffuser disk 414
is nested within block 406 adjacent to filter screens 410, 411
opposite air inlet tube 412. An electrically grounded screen 416 is
wrapped over the outside surfaces of the corona shield 402 and the
manifold block 406. The two ends of screen 416 are secured
between plates 418 and 420 in order to tighten the screen against
the outside surfaces of the corona shield and manifold block. An
identical corona neutralizer 45 is shown in phantom in FIG. 7
adjacent to corona neutralizer 44.
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ln operation, an AC potential is applied to the corona wire
400 so that both positive and negative ions are generated. Some
of the negative ions are drawn through the screen 416 by the
residual positive charges on the dielectric drum 34, and in this
manner the drum surface is neutralized. The screen 416 is
maintained at or near ground potential; as a result, the electric
field existing between the screen and the drum surface will drop
to ~ero when the drum surface has been completely neutralized,
and the flow of negative ions toward the drum will cease. In
10 general, the flow of ions between the corona wire 400 and the
drum surface will cease when the potential of the drum surface
becomes equal to the screen potential. When two corona
neutralizers 44, 45 are used, as in the preferred embodiment, the
screen potential of the first neutralizer may be made slightly
15 negative in order to accelerate the rate of charge neutralization.
In accordance with the present invention, a flow of
dehumidified air st or near ambient temperature is provided
through the electrostatic print head 28 in order to inhibit the
formation of ammonium nitrate in and around the apertures 96, 98
20 and on the corona wire 76, and through corona neutralizer unit
180 in order to inhibit the formation of arnmonium nitrate on the
corona wires and screen. An exemplary system for supplying
dehumidified air to the print head 28 and corona neutralizer unit
180 is illustrated in FIG. 8. Compressed air at a minimum of 80
25 psi and generally about 80-100 psi enters the system through a
section of tubing 120 and is conducted to the input side of a
CoAlescing oil filter 122. The coalescing oil filter operates to
remove any oil or water droplets which may be present in the
source of compressed air. The output side of the filter 122 is
30 connected by means of a further length of tubing 124 to a
timer-operated solenoid valve 126. ~The solenoid valve is part of a
commercially available air dryer system which also includes a pair
- ~ of desiccant towers 128, 130. A suitable system of this type is
the Model 311B air dryer manufactured by O'Xeefe Controls
35 Company of Monroe, Connecticut. The solenoid valve 126 operates
on a 30-second cycle and directs the compressed ~dr through the
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lengths of tubing 132, 134 and desiccant towers 12B, 130 in an
alternating manner. During each 30-second cycle, one of the
desiccant towers is supplying dehumidified air to the output tubing
136 and the other desiccant tower is receiving a backflow of
dehumidified n r from the first tower in order to regener~te the
desiccant rneterial within the inoperative touer. Humid air- from
the tower being regenerated i6 discharged from the system through
an exhaust muffler.
Dehumidified air from the output of the air dryer ~ystem
pas~es through an output regul~tor 138 which controls the air
pressure to the print head 28. A gage 140 allows the air pressure
at the output of the regulator 138 to be monitored. From the
output of the re~ator 138, the dehumidified air passes via tubing
142 to the input side of ~ hydrocarbon filter 152. The output side
of the hydrocarbon filter 152 is connected via ~ short length o
tubing to a tee 148, one output of which is ronnected to the input
~ide of an adjustable flow meter 14~ of the floating ball type. In
the preferred embodiment, the flow meter 1~1 is set to pro~ride an
air flow of ~bout 41 cubic feet per hour to the electrostatic print
head 28. A knob 146 on the f~ow meter ~llows the flow rste of the
dehumidified air to be adjusted if necessaryO The output 6ide OI
the ~low meter 144 is connected via ~ length of tubing 149 to a
pressure ~ensor 150. The function of the pressure ~ensor 150 is
to insure th~t adequate air pressure is being provided to the print
head 28, and to ~nterrupt the oper~tion of ~e machine when this
condition is not satisfied. The output side of the pressure sensor
150 is connected Vi8 a length of flexible tubing 156, which ~ill not
introduce any hydrocargons, e . g. Bev-A-Line * I~/ av~-lable from
Cole Parmer, Chicago, or Teflon* to disconnect coupling 154 which
is conne~ed to a rigid tube iS8 carried by the print head 28.
The~ tu~e 158 passes through a ~upport member 160 ~nd is
conn~cted to the input side of ~ particulate filter 162. Referring
to FIG. 3, the' output ~ide of the filter 162 is connected to an
aperture 164 located at one end of the oblong central opening 82 in
~; 35 the frame 80. The aperture 164 delivers dehumidified air into the
enclosed chamber formed by the openings 82, 86 and the cut-out
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90 in the rear frame member 88. The dehumidified air flows
around the sides of the corona shield 78 and passes through the
gap between the corona shield and the aperture mask 94 to the
interior of the corona shield, where it surrounds the corona wire
76 in the course of passing out of the print head through the
apertures 96, 98 and the slotted mask 108.
The second output of the tee 198 is connected via tubing 185
to tee 166. C)ne output of tee 166 is connected via tubing 168 to
the input side of an adjustable flow meter 172 of the floating ball
type. The other output of tee 166 is connected via tubing 170 to
the input side of an identical adjustable flow meter 174. Flow
meters 172, 174 are connected via tubing 176, 178 to corona
neutralizer unit 180. Corona neutralizer unit 180 comprises two
identical side-by-side corona neutralizers 44 and 45. Referring to
FIG. 7, tubing 178 is connected to tubin~ 412 which delivers
dehumidified air into the enclosed cavity 408. The dehumidified
air flows around diffuser disk 414, through fglter screens 410, 411
and through the series of holes 404 through corona shield 902,
where it surrounds corona wire 400. The dehumidified air then
passes through screen 416 against the dielectric coating of drum
34.
The flow of dehumidified air through the electrostatic print
head 28 has been ~ound to retard the buildup of ammonium nitrate
on the corona wire 76, and in and around the electrically
2 5 controlled apertures 96, 98, to a point where the useful life of the
print head can be extended by an order of magnitude. This
represents an enormous increase over the average lifetime of a
print head not supplied with dehumidiffed air, which is typically
about 75 hours. The flow of dehumidified air through the corona
neutralizers, such as corona neutralizer 44, has been found to
retard the buildup of ammonium nitrate on the CQrOna wire ~00 and
screen 416. The use of dehumidified air has also been found to
improve the retention of latent electrostatic images on the dielectric
drum 34. The following examples, provided merely by way of
illustration and not being intended as limitations on the scope of
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the invention, will assist in an understandin~ of the in~rention and
the manner in which these advantageous results are obtained.
EXAMPLE 1
A test was conducted to determine the effect of dried air on
the lifetime of electrostatic print heads. An ~pparatus was
constructed which was capflble of testing four print heads in
parallel. Print performsnce was assessed quantitatively by
measuring print qu~lity as a function of time.
A schematic diagram of the test apparatus used is shown in
FlG. 9. Referring to ~ 3. 9, compressed air at about 100 psi
entered the apparatus through tubing 300. All tubing used to
connect the components of the apparatus was Bev-A-Line IV
tubing. Tubing 300 was connected to coalescing oil filter 302
(t~7ilkerson* ~20-02-F00) and coalescing oil ~ filter 304 (l~'ilkerson*
M20-02-F00) which were used to remove oil and water droplets
present in the ~ource of compressed air. A pressure switch 306
: stopped power to the print heads from power source 308 in the
event of air ~upply failure. The coalescing oil filters were
eonnected to a charcoal ~Iter 310 (Balston*C1-150-19) which was
- 2 o used to remove oil or water droplets in the ~ir . The charcoal
filter was connected by a Tee joint 312 to the "wet" side of the
apparstus 314 and to the '7dry" ~ide of the apparatus 316.
On the wet ide 314, the Tee joint was connected first to a
regulat~r 318 (0-60 psi~ which permitted the air flow on the wet
25: ~ide to ~e balanced with that on the dry side. Regulator 318 was
connected to humidifier 320, which consisted of a steel tank, about
12 inches in dia~Teter ar~d about 24 inches long and having rounded
ends, through a t~lree-way vslve 319. Air entered and e~ted the
tank coaxi~lly at the ends. lqater was added to the humidifier 320
~ by means of funnel 322 and valve 324, through three-way valve
319, i:ntering air became humidified by picking up water
contained in the tank. The humidifier 320 was connected to a
coalescing filter 326 (Balston Type BX) which was used to remoYe
liquid water droplets ~rom the humidifier and ~llow water vapor to
pass throu~h. Filter 326 was connected to a hygrometer in a
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pressurized box 328, which permitted quick messurement of the
humidity in the humid air stream. Bec~use it was preRsurized, the
humidity at atmospheric pressure W8S calculated from the pressure
(P) and the relative humidity ~RH) measured at pressure according
to the following relationship:
P measurement _ % RH measurement
P ~tmospheric % RH atmospheric
Pressure gage 330 facilitated the above ralculation. Hygrometer
328 was connected to wet Rir distribution manifold 332.
On the dry 6ide 316, the Tee joint 312 was connected to air
dryer 334 (O'Keefe * Model OKC-079-2) . Air dryer 334 was
connected to a regul~tor 336 of the type used for regulator 318 on
the wet side of t~e apparatus. Regulator 318 was connected to
dry air distMbution manifold 338. ~'et ~ir distribution menifold 332
and dry air distribution mani~old 338 were connected through six
1~ identical f~ow meters 340~ (Dwyer Rate Master*Type RMA-8-SSV,
0-100 scfh flow). Flow meters 340 controlled the air flow to print
head 342, print head 344, print head 346, and print head 348. All
four print heads ere of the type 6hown in FIGS. 3 and 4. The
percent relative humidity (% RH) to p~nt heads- 344 ~nd 346 was
2 0 controlled by controlling the relative amounts of wet and dry ~ir
from manifolds 332 and 338, respectively. Arrow 350 points in the
direction of increasing humidity of the air supplied to the print
heads .
In order to assess the changes in print quality over a period
2 5 of time due to the effect of the air humidity, prints were made
periodically using the print heads and the decrease in image
density was ob~erved. Image density in an area is a function of
charge density depcsited by the print head in that area.
Deposited ch~rge density decreases as a function of aperture
occlusion by the ammoJ~ium nitr~te crystals which form as a result
of the water in the xir ~upplied to the print head. Therefore,
measurement ~f image density uniformity will eharacterize the
degree to which water in the ~ir ~upply is degrading the print
quality. Another indication of the buildup of ammonium nitrate
crystals is the gradual incre~e in voltsge needed to maintsin a
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constant current from the corona wire to the mask and corona
shield. This current was periodically measured.
Test prints ;vere made periodically to permit measurement of
image density. A portion of the test print was solid black which
was printed by allowing all of the apertures to print. Such a test
print allowed the assessment of the degree of occlusion of the
apertures across the width of the print head by measurement of
the relative image density across the print. Since print head to
print head variations are possible, each test print was compared to
a test print made with that particular print head at the start of
the test.
The corona voltage of all four print heads was adjusted to
give a total current of 200 IIA to both mask and shield and was
maintained at that value. Voltage readings at the start of the test
are set forth in Table 1 below:
TABLE 1
Print Head Gorona KV
2.50
2 2.50
3 2.42
4 2.49
SeYeral test prints were made from each print head and saved.
The test apparatus was placed in a room having a controlled
temperature of 70F (21.1C). The compressed air in tubing 300
had a dew point of 20F (-6.6C). The humidity of air coming out
of the humidifier 320 at equilibrium is a function of the
temperature of the room and the flow rate which is held constant.
The humidifier 320 was allowed to equilibrate to the room
temperature and flow conditions used. The equilibrium point was
about 55~ RH at 6 psig and 72F (22.2C~. This corresponded to
39~ RH at atmospheric pressure for air from the humidifier. The
four print heads were to be tested under the following conditions:
:
.
~æ~
--16--
Print Head 342 - very dry air from the air dryer;
essentially 0% RH
Print Head 344 - 5~ RH
Print Head 346 - 109~ RH; This was selected to represent
the absolute best conditions for year
round operation without a dryer.
Print Head 348 - very wet air; 100% humidified air of about
39% RH
In order to obtain those various humidities, the six flow meters
340 were set as fol]ows:
Print Head 342 - dry air (60 scfh)
Print Head 344 - dry ~r (52 scfh)
wet air (8 scfh)
Print Head 346 - dry air (45 scfh~
wet air (15 scfh)
Print Head 348 - wet air (60 scfh)
Test prints were made periodically by removing the print heads
from the test apparatus and inserting them in a Markem Model 7000
electrostatic printer. Attempts were made to maintain the same roll
2 0 of dielec~ric paper and toner lot . All four print heads were
turned on at 16: 20 hours on day 1 of the test . The pressure
reading on the hygrometer was increased to 15 psig.
At 07: 25 hours on day 2, the test was stopped because the
humidity of the air coming out of the humidifier had equilibrated
overnight at 59~ RH a t 15 psig for an atmospheric relative
humidity of about 30%. This was considered to be too low as the
maximum relative humidity for the test ~ In order to increase the
humidity of air from the humidifier, the Elow rate through $he
humidifier was decreased in order to increase the residence time of
the air in the humidifier. The flow through the humidifier was
decreased by decreasing the flow through the masks. The flow
meters to print heads 344 and 346 having a range of 0-100 scfh
were not calibrated finely enough to accurately meter the
humidified air to these print heads~ A flow meter having a range
of 0-5 scfh was used for print head 344 and a flow meter having a
range of 0-10 scfh was used for print head 346.
:
,
.
'
-17-
At 15:41 hours on day 2, the print heads were restarted.
Equilibrium was reached at 60% RH at 5 psig, which corresponds to
about 45~ at standard presxure. The flow rates were set as
follows:
Print Head 342 (dry) - dry air (30 scfh)
Print Head 344 (5% RH) - dry ~ir (27 scfll)
wet air (3.3 scfh)
Print Head 346 (10% RH) - dry air (23 sc:~)
wet air (6.6 scfh)
Print Head 348 (45% RH) - wet air (30 scfh)
The data for the four print heads tested are set forth in
Table~ 2-5 below:
. ~ :
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- ~ ' '
J~
-18-
TABLE 2
Print Head 342
Elapsed Corona
Hours KV Comments
0 2.50
33,2 2.46
63.4 2.49 60.0% RH @ 5.00 psig, 70F
87.5 2.49 60.D~ RH @ 5.00 psig, 74F
109.7 2.50 60.0~ RH @ 5.00 psig, 75F
128.2 2.51 60.0% RH @ 5.00 psig, 71F
153.4 2.50 59.0~ RH @ 5.00 psig, 72F
194.2 2.51 56.5% RH ~ 5.00 psig, 74F
214.4 2.52 57.0% RH @ 4.00 psig, 74F
254.1 2.50 60.0% RH @ 4.00 psig, 72F
281.9 2.50 56.5% RH @ 4.00 psig, 71~F
346.2 2.4g 52.0% RH @ 4.00 psig, 75F
384.2 2.50 58.0% RH @ 4.00 psig, 71F
406.0 2.50 62.0% RH @ 5.00 psig, 74F
434.8 2.51 59.0% RH ~ 5.00 psig, 73F
463.1 2.50 54.0% RH @ 5.00 psig, 75F
486.4 2.50 55.0% RH @ 5.00 psig, 73~F
500.8 2.51 56.0% RH Q 5.00 psig, 73F
508.3 2.50 53.0% RH @ 4.75 psig, 74F
532.8 2.49 53.0% RH @ 4.60 psig, 73F
556.0 2.47 52.0% RH @ 4.50 psig, 73F
578.6 2.49 54.0% RH @ 5.00 psig, 73F
594,8 2.49 56.0% RH @ 5.00 psig, 73F
649.9 2.50 51.0% RH @ 4.75 psig, 74F
688.8 2.53 52.0% RH @ 5.00 psig, 73F
~ 30 695.3 2.52 52.0% RH @ 5.00 psig, 73F
-~ ~ 716.5 2.50 50.0% RH @ 5.00 pSig3 76~F
~ 772.9 2.50 49.0% RH @ 4.75 psig, 75F
~:
,
'~
~: :
:
'
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--19--
TAB LE 3
P~nt Head 344
Elapsed Corona
Hours KV Comments
o 2.50
33.1 2.49
63.0 2.5260.0% RH @ 5.00 psig, 70F
87.3 2.5360.0% RH @ 5.00 psig, 74~F
109.5 2.5460.0% RH ~ 5.00 psig, 75F
127.8 2.5660.0% RH @ 5.00 p9ig, 71F
152.8 2.5459.0% RH @ 5.00 psig, 72F
193.3 2.5556.5% RH @ 5.00 p9ig, 74F
213.4 2.5557.0% RH Q 4.00 psig, 74F
252.g 2.5460.0% RH @ 4.00 psig, 72F
280.4 2.5356.5% RH @ 4.00 psig, 71F
344.1 2.S352.0% RH @ 4.00 psig, 75F
381.7 2.5358~0% RH @ 4.00 psig, 71F
403.3 2.5362.0% RH @ 5.00 psig, 74DF
431.9 2.5359.0% RH @ 5.00 psig, 73F
459.9 2.5354.0% RH @ 5.00 psig, 75F
483.1 2.5455.0% RH @ 5.00 psig, 73F
497.4 2.5556.0% RH Q 5.00 psig, 73F
504.9 2.5353.0~ RH @ 4.75 psig, 74F
529.2 2.5353.0% RH @ 4.50 psig, 73F
` 25 552.2 2.5152.0% RH @ 4.50 psig, 73F
574.7 2.5354.0% RH @ 5.00 psig, 73F
590.7 2.5356.0% RH @ 5.00 psig, 73F
645.4 2.55~51.0~ RH~ @ 4.75 psig, 74F
-~ ~ 684.1 2.5552.0% RH @ 5.00 psig~ 73F
690.6 2.5552.0% RH @ 5.00 psig, 73F
711.6 2.5550.0% RH @ 5.00 psig, 76F
767.5 2.5349.0% RH @ ~.75 psig, 75F
: :
.,
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;~: :
.
.
-
: .
- .
' '
. .
:: ' . . .
- 20-
TAB LE 4
Print Head 346
. _
Elapsed Corons
Hours KV _ Comments
0 2.42
32.9 2.48
62.8 2.50 60.0% RH @ 5.00 psig, 70~
87.3 2.51 60.0~ RH ~ 5.00 psig, 74~F
109.4 2.52 60.0% RH @ 5 00 psig, 75F
127.7 2.54 60.0% RH @ 5 00 psig, 71F
152.6 2.52 59.0% RH @ 5 00 psig 72F
192.8 2.53 56.5% RH @ 5 00 psig 74F
212.9 2.54 57.0~ RH @ 4 00 psig 74F
252.2 2.52 80.0% RH @ 4 00 psig 72F
279.7 2.51 56.5% RH @ 4.00 psig, 71F
343.3 2.52 52.0% RH @ 4.00 psig, 75F
380.8 2.51 58.0% RH @ 4.00 psig, 71F
402.5 2.52 62.0~ RH @ 5.00 psig, 74F
431.0 2.55 59.0% RH @ 5 00 psig, 73F
458.9 2.52 54.0% RH ~ 5 00 psig, 75F
482.0 2.54 55.0~ RH @ 5.00 psig, 73F
496.3 2.54 56.0% RH @ 5.00 psig, 73F
503.7 2.52 53.0% RH @ 4.75 psig, 74F
527.9 2.52 53.0~ RH @ 4.50 psig, 73F
550.8 2.51 52.0% RH @ 4 sa psig 73F
573.2 2.50 54.0% RH @ 5 00 psig 73F
589.3 2.50 56.0~ RH @ 5 00 psig 73F
; ~ 643.7 2.51 51.0~ RH @ 4 75 psig 7~F
682.3 2.51 52.0% RH @ 5 00 psig 73F
688.8 2.51 52.0% RH @ 5 00 psig 73~F
710.0 2.51 50.0% RH @ 5.0D psig, 76F
765.2 2.49 49.0% RH ~ 4.75 psig, 75F
~ ~ ,
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.
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.
--21-
TABLE S
Print Head 348
Elapsed Corona
Hours KV Comments
- 5 0 2.49
33.0 2.57
63.0 ~.66 60.0% RH @ 5.00 psig, 70F
87.4 2.69 60.0% RH @ 5.00 psig, 74F
109.5 2.69 60.0% RH @ 5 . 00 psig, 75F
127.8 2.69 60.0~ RH @ 5.00 psig, 71~F
152.7 2.68 59.0% RH @ 5.00 psig, 72F
193.0 2.70 56.5% RH @ 5.00 psig, 74~
213.1 2.70 57.0% RH @ 4.00 psig, 74F
253.4 2.68 60.0% RH @ 4.00 psig, 72F
279.9 2.66 56.5% RH @ 4.00 psig, 71F
343.4 2.69 52.0~6 RH @ 4.00 psig, 75F
380.9 2.68 58.0% RH @ 4.00 psig, 71F
402.6 2.68 62.0% RH @ 5.00 psig, 74F
~31.1 2.70 59.0% RH @ 5.00 psig, 73F
459.1 2.68 54.0% RH @ 5.00 psig, 75~F
482.2 2.70 55.0% RH @ 5.00 psig, 73~F
496.5 2.70 56.0% RH @ S.00 psig, 73F
504.0 2.68 53.0% RH @ 4.75 psig, 7~F
528.2 2.71 53.0% RH ~ 4.50 psig, 73F
551.1 2.69 52.0% RH @ 4.50 psig, 73F
573.5 2.71 54.0~6 RH @ 5.00 psig, 73F
589.6 2.70 56.0% RH @ 5.00 psig, 73F
644.1 2. 72 51.0% P~H @ 4.75 psig, 74F
682.8 2.72 52.0~ RE~ @ 5.00 psig, 73F
689.1 2.73 52.0% RH @ 5.00 psig, 73F
-~ 710.0 2~73 50.0~ RE~ ~ 5.00 psig, 76F
765.8 2.70 49.0% RH~@ 4.75 psig, 75F
Although most of the print quality ~rom, print head 344 was
uniform, a band of apertures about 2 cm wide did not print. The
print head was removed from the test apparatus and examined.
Ammonium nitrate had built up on both the inside and the outside
of the apertures in that band. The remainder of the mask was
clear of obstructions and printed well.
In order to quantitatively measure the print quality, the
4 0 optical densities of the printed images from the ~our print heads
were measured. The instrument used for this purpose was a Welch
Densichron Model 1 photometer with a Model 3832A reflection unit
measuring head. This instrument illuminated the printed image
`~: :
.,
' ~ ' ' - . . .
-22 -
with a light and measured the reflected light from a spot
approximately 1/8 inch in diameter.
The instrument was allowed to warm up and was adjusted to
read 100% reflected on a standard white glass tile and 0%
transmitted on a standard black glass tile. The clear filter was
used. Readings were taken of the printed images and the
variations of the reflectance across the image.
' ~ ' ' ' ' . '
.
3L~ iOl
-23 -
o
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OD ~ C~ ~r o o~ o ~ o c~ o co o
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h Ir~N 0 ~ 0 O~ 0 C
P~ ~ ~ t:d ON O~ --i V
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¢ ~ ,~ ~ OD O ~0 ,~~ E E
U~o U~ P~ .
C~ O tD C~ O O O O E~~ d~ O O
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p., ~ ~ ^ E
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_I O ~ O O Or~l ON O C-- o .UJ C C
: ~ ~ ; N : ~ Cq ~ ~ 3~ ~ Q
~ ~ 0~ u~ ~ O ~ ~
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I ~ O C`~ O U~ O ~- O C- O O~ O
.C~
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--24-
The four print heads were run for about 773 hours under the
four different humidity conditions. The dats was reviewed in an
effort to determine the level of dehumidification required to achieve
a print head life of 300 hours with good print quality. The values
for percent relative humidity were initially selected based on the
belief that they would bracket the 300-hour mark. Periodic print
tests as well as measurements of the corona voltage, shield current
and mask current were made. The following results for the four
print heads were obtained:
Print Head 342 - (very dry air) The print tests showed
that this print head had substantially
unchanged print quality throughout the
773-hour test.
Print Head 344 - (nominal 5% RH~ This print head showed
an anomolous area of light print which
was probably due to print head geometry
with a self-reinforcing cycle of ammonium
nitrate formation, which began to manifest
itself about 150 hours into the test. The
2 0 remainder of the printed image appeared
very uniform with no substantial
degradation of print quality after 773
hours ~
Print Head 346 - (nomin~l 10% RH) This print head showed
reasonable print quality beyond 300
- hours, although at over 700 hours the
print quality and uniformity were not as
good as the prints of print head 342 or of
the unaffected portion of print head 344.
Print Head 348 - tnominal 4096 RH) The performance of
this print head was unacceptable. The
; print quality was very non-uniform even
after only 63 hours of operation.
The change in corona voltage over time was found to be a
good indication of the buildup of ammonium nitrate, and therefore,
of the print qurlity from the mask. Thls }s due to the fsct thst
-: ,
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:
6~ ~
-25 -
the high voltage supply to the corona wire operates in a current
regulated mode. As contaminants build up on the corona wire, the
voltage required to maintain a constant current increases. The
data for corona voltage are set forth in Tables 2-5 above. A plot
of corona kilovolts versus elapsed hours appears in FIG. 10. The
corona voltages for print heads 3~2, 344 and 346 were
approximately the same, while the corona voltage for print head
348 quickly rose to the limit imposed by the current limited power
supply. The corona voltage would have gone higher without this
1 0 limit .
The optical test which was conducted in an effort to quantify
the print quality as a function of time indicated that the images
printed by print heads 342 and 344 (with the exception of the
anomolous region) and 346 were very similar. One reasonable
measure of pFint uniformity is the ratio of the reflectance of the
least reflective area on the print to the re~lectance of the most
reflective area. If the print were perfectly uniform, this ratio
would be equal to 1, since there would be no difference between
the most and the least reflective areas. At the conclusion of the
2 0 test, the values of this ratio for the four print heads were as
follows:
Print Head 342 - O .43
Print Head 344 - 0.17
Print Head 346 - 0.32
Print Head 348 - 0.18
If print head 344 had not performed so anomolously, its ratio
would probably be between those of print heads 342 and 346, so
that the drier the ~ir flowing through the print head, the more
uniform the prints produced by that print head.
This test demonstrated that satisfactory print quality and
uniformity can be obtained at 300 hours by passing air at 10% RH
or less through the print head and that drier air can extend the
lifetime of the print head for beyond this point, whereas air at 40%
RH leads to substantial non-uniformity of the print at only 63
hours .
.
:~
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~: , ' .' -
. ' ' , .~ ' ~ '
-26-
EXAMPLE ~
A second test was conducted to expand the range of relative
humidities of the air flowing through the print heads. One of the
four print heads in this test was run with very dry air and the
S others were run with air having relative humidities of 10%, 20g6 and
30~. The test apparatus of FIG. 9 was changed slightly to
accommodate the different range of flow rates by installing more
accurate flow meters. In this test, the air flow to the various
print heads was adjusted each time the humidity and the pressure
of the humidified air source was checked. This permitted more
accurate long term testing regardless of the drift in the humidity
of the air going through the system.
The print heads used in Example 1 were cleaned and the
aperture mask in print head 344 was replaced. New corona wires
were installed. Each print head was adjusted to have a combined
mask and shield current of 200 llA. The four print heads were
tested under the following conditions:
Print Head 342 - essentially 0% RH dry air; (30 scfh)
Print Head 344 - nominal 10% RH; dry air (24 scfh)
wet air (~ scfh)
Print Head 346 - nominal 20% RH; dry air (19 scfh)
wet air (11 scfh)
Print Head 348 - nominal 30% RH; dry air (13 scfh)
wet air (17 sc~h)
2 5 Test prints were made periodically as described in Example
above .
The data for the four print heads tested are set forth in
Tables 7-10 below:
~ :
$~ ;L
-27-
TABLE 7
Print Head 342
Elapsed Corona % RH @
~ours KV Atmos. P
0 2.51 53
29.0 2.50 5~
53.7 2.50 52
81.3 2.47 49
105.5 2,47 ~8.7
163.~ 2.48 52.2
191.5 2.49 49.9
230.3 2.48 46.8
295.8 2.49 43.6
319.7 2.51 48.6
360.1 2.50 51.5
407.0 2.50 50.7
TABLE 8
Print Head 344
Elapsed Corona % RH @
Hours KV Atmos. P
0 2.49 53
28.9 2.50 54
53.5 2.51 52
~1.0 2.49 49
104.9 2.49 48.7
163.0 2.49 52.2
~: ~ 190 4 2.49 49.9
229 1 2.49 46.~
294 1 2.51 43.6
317 8 2.52 ~8.6
358.0 2.51 51.5
404.5 2.50 50.7
: ~ :
: ~: :
~:.
: ~ :
..
` . ' '` . '. '
:
-2~-
TABLE 9
Print Head 346
Elapsed Corona % RH @
Hours KV Atmos. P
0 ~.50 53
28.6 2.52 54
53.0 2.54 52
80.3 2.52 ~9
104.1 2.52 4~.7
162.0 2.53 52.2
189.1 2.53 49.9
227.4 2.5~ 46.8
292.2 2.55 43.6
315.7 2.56 48.6
355.6 , 2.55 51.5
401.~ ~.55 60.7
TABLE 10
Print Head 348
Elapsed Corona % RH @
2 o Hours KV Atmos . P
0 2.52 53
28.7 2.56 54
53.2 2.58 52
80.7 2.56 49
104.6 2.57 48.7
162.5 2.59 52.2
189,7 2.61 49.9
228.4 2.61 46.8
293.4 2.64 43.6
317.0 2.66 48.6
357.0 2.66 51.5
403.5 2.67 50.7
:
:
The results of the prlnt tests and a comparlson of the corona
voltages for the four print heads over time indicates 8 clear
3s difference in print head performance at different percent relative
humidities of the air flowing through the print heads. The
measurement of corona voltage versus time is especially significant.
~:
Corona voltage has historically been a measure of cleanliness of the
print head, ~ince the corona voltage needed to maintain the same
o current increases as contaminants buildup. A plot of corona
,': ~ ~ :
'~
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. ` ~
'
-29-
kilovolts versus elapsed hours based on the data set forth in
Tables 7-10 above appears in FIG. 11.
As in Exarnple 1 above, print head 344 showed some anomolous
results, even though the aperture mask was replaced. This is
probably due to a geometric feature of this particular print head.
It was observed that one side of the printed image became lighter
due to the buildup of arnmonium nitrate in part of the mask.
Disregarding the anomolous results from print head 344, print
head 348 (3ûg6 RH) was the first one to show a lightening of the
print on the edge of the image. This lightening was readily
apparent at 106 hours. Print head 346 (20~ RH) began to show a
lightening at the edge of the printed image at 164 hours, which
became very evident by 296 hours. By contrast, in the case of
print head 342 (very dry air - dew point < -50~F), there was no
perceptible difference in appearance of the printed image even
after 407 hours of operation. Therefore, the lifetime of a print
head is a function of the degree of dehumidification of the air
passing through the print head.
For the purpose of printing with an electrostatic print head
of the type used in the Examples, a lifetime of less than about 300
hours has been deemed to be unacceptable. This lifetime was
selected as desirable even though the use of this type of print
head without any dehumidification of the air, at a relative humidity
OI 50-60 percent, will gener~lly only maintain print quality and
2 5 uniformity for about 60 hours . As shown by these tests,
acceptable print quality for about 300 hours of operation can be
obtained if the air flowing through the print head has a relative
humidity of less~ than about 20 percent, and preferably less th~ l5
percent . There appear~ to be no lower limit for the humidity of
the air that wi~l result in acceptable print quality within the limits
of economically reasonable drying equipment.
If a print head were to be desi~ned which was less expensive
to manufacture or service than those employed in the Examples, a
relative humidity higher than 20 percent may be found to be
3 S acceptable . Although the lifetime of the print head would be
: ~ .
~ ' ' ' ' ": ' ` '
~ : ' . . ':
-30-
shorter at higher percent relative humidity, the print head could
be economically replaced at the end of its shorter lifetime.
EXA~IPLE 3
A test was conducted to determine the effect of dried air on
the lifetime of the A . C . corona scorotrons (neutralizers) used to
discharge the residual charge on the dielectric drum.
A grounded conductive aluminum drum was placed adjacent to
and above a double unit scorotron similar to that shown in FIGS. 6
and 8 to simulate an offset electrostatic printer of the type shown
in FIG. 1. A separate supply of air was connected to each side of
the scorotron unit. Compressed ambient air was pumped through
tubing into the "wet" side of the scorotron unit. This air was
also pumped through an air dryer (OIKeefe Model 311B) and then
into the "dry" side of the scorotron unit. Air was pumped into
each side of the scorotron unit at a rate of about 30 cfl~. 60 Hz
A . C . was applied to the corona units . The power supply was a
8000 Y rms A . C . The scorotron units were run for about 755
hours and then examined. A significant difference was noted
between the 7'dry" air side and the "wet" air side of the unit.
2 0 The screen on the wet air side had many spots covered with rust
and ammonium nitrate crystals. An extensive amount of ammonium
nitrate crystals were present at the ends of the wet air side. The
screen on the dry air side had less ammonium nitrate crystals and
less rust.
There were patterns on the aluminum drum which
corresponded to the rusted areas on the screen. Ammonium
nitrate was also deposited on the drum with more present on the
wet air side than on the dry air side.
EXAMPLE 4
The A . C . corona scorotron lifetime test of Example 3 above
was repeated with a number of important differences. The 303
stainless steel screen (200 mesh~ used in the scorotron unit in
~ ' .
Example 3 was replaced with a 316 stainless steel screen (200
mesh) which was more corrosion resistant. The scorotron holder
was redesigned so that air was distributed through a series of 11
small holes in the back~ An anodized aluminum drum was coated
with aluminum foil. The foil was grounded so that a record could
be preserved with the pattern of ammonium nitrate deposition.
Compressed air was pumped into the "wet" air side of the
scorotron unit and through the air dryer into the "dry" air side of
the unit as described in Example 3. The flow rate of the air into
each side was about 30 cfh.
The scorotron units were run for about 740 hours and then
examined. The screen on the wet air side of the scorotron unit
had 11 spots of ammonium nitrate crystals deposited on it
corresponding to the location of the air inlet holes. The wet air
side screen also exhibited a yellowish discoloration between the
spots. There was no pattern of spots corresponding to the air
entry holes on the screen on the dry air side of the scorotron
unit. The yellowish discoloration was somewhat present on the dry
air side screen, but much less prominent than on the wet air side.
2 0 On the inside surfaces of the two screens, the difference was
much more pronounced.
Inside the corona units, a large amount of ammonium nitrate
crystals formed at the nodes on the corona wire on the wet air
side and a small amount of buildup was present on the dry air
2 5 side .
A large amount of ammonium nitrate crystals were deposited
on the aluminum foil covering the drum on the wet air side and a
small amount were deposited on the dry air side.
~ .
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