Note: Descriptions are shown in the official language in which they were submitted.
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DROP-ON-DEMAND METHOD AND ~PP~RATUS
USING CONVERGING NOZZLES AN~ HIGH VISCOSITY FI.UIDS
Description
Field of Invention
This invention relates to an ink jet print head and,
more particularly, to an ink jet print head and method
for generating ink drops on demand under control of a
suitable electrical signal.
Description of Prior Art
Ink jet printing has been known in the prior art,
including systems which use a pressure genarated con-
tinuous stream of ink, which is broken into individual
drops by a continuously energized transducer. The
individual drops are selectively charged and deflected
either to the print medium for printing or to a sump
where the drops are collected and recirculated.
Examples o~ these pressurized systems include U. S.
Patents 3,596,275 to Sweet, and 3,373,437 to Sweet et
al. There have also been known in the prior art ink
jet printing systems in which a transducer is used to
generate ink drops on demand. One example of such a
system is commonly assigned U. S. Patent 3,787,884 to
Demer. In this system, the ink is supplied to a cavity
by gravity flow and a transducer mounted in the back of
the cavity produces motion when energized by an appro-
priate voltage pulse, which results in the generation
of an ink drop so that only those ink drops required
for printing are generated. A different embodiment
of a drop-on-demand printing system in which the
transducer is radially arranged is shown in U. S.
Patent 3,683,212 to Zoltan.
SA980051
The prior art drop~on-demand printlng systems have been
limited by low drop procluction rates, low resolution,
and low efficiency. Typical prior art drop-on-demand
printing systems have utilized a constant cross-section
nozzle and ink having a viscosity during operation
lower than 10 centipoise. Attempts to increase the
drop production rates have led to stream instability as
a result oE the low viscosity ink used. Attempts to
increase the ink viscosity to improve stream stability
have led to choking of the nozzles and termination of
ink flow due to the increased internal friction in the
nozzle. A decrease in the length of the nozzle in an
effort to decrease the friction resulted in unreliable
nozzle operation due to air intake caused by meniscus
dyn2mics.
SU~IARY OF THE INVENTION
Briefly, accordinq to one a.s~ect of the
invention, there is provided a drop-on-demand
ink jet printing method and a~paratus
comprisiny a print head having a fluid chamber supplied
with a suitable high viscosity marking fluid. An
orifice comprising a strongly converging nozzle is in
fluid communication with the fluid chamber, and an
electromechanical transducer is mounted in mechanical
communication with the fluid chamber. The transducer
is selectively energized with a series of signals so
that one drop of the marking fluid is ejected from the
orifice for each of the signals having at least a
predetermined amplitude.
Brief Description of ~he Drawings
FIG. 1 is a schematic view shcwins a conversing nozzle;
FIG. 2 is a droo-on~demand ink jet printer embodying a
converging nozzle;
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FIG. 3 is a section view taken along line 3-3 of FIG. 2
of the drop-on-demand ink jet print head.
FIG. 4 is a vlew, partially in section, of an alternate
embodiment of a drop-on-demand ink jet print head;
FIG. 5 is a right side view of an array of drop-on-
demand ink jet print heads;
FIG. 6 is a section view taken along lines 6-6 in FIG.
5.
Description of the Preferred Embodiments
Referring to FIG. 2, the printer apparatus comprises a
print head 10 to which is supplied high viscosity
li~uid ink from ink supply means 12. The viscosity
requirement is a function of nozzle size and maximum
drop-on-demand drop production rate. The viscosity
for inks for high resolution printing extends up to
100 centipoise, and the viscosity can be substantially
higher for applications in which lower resolution is
suitable. Control means 14 provides the voltage con-
trol pulses to selectively energize print head 10 to
produce one ink drop for each voltage pulse supplied to
print head 10. Print head 10 comprises head body 20
having a chamber or cavity 22 formed therein. Cavity
22 is maintained filled with ink through supply line 24
from ink supply means 12. Ink from supply means 12 is
not pressurized so the ink in cavity 22 is maintained
at or near atmospheric pressure under static condi-
tions. An exit from cavity 22 is provided by nozzle
portion 26 which is designed so that the ink does not
flow out of nozzle portion 26 under static conditions.
An intermediate ink reservoir 28 is formed in head body
20 and is separated from cavity 22 by internal wall
portion 30. The top of cavity 22, as shown in FIG. 2,
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is closed by a suitable transducer means which is fixed
to the head body. Internal wall portion 30 is designed
so that a narrow passageway 32 is provided for the
transfer of liquid ink from intermediate ink reservoir
28 to ink cavity 22. The transducer means comprises a
membrane member 34 which is fastened to an electro-
mechanical transducer 36. Transducer 36 displaces
radially when energized with a suitable voltage pulse
and bends membrane 34 inwardly (as shown dotted in FIG.
3), and produces a pressure wave in cavity 22 s~ that
liquid ink is e~pelled out through nozzle portion 26 to
form a single drop. Control means 14 provides the
voltage control pulses to selectively energize trans-
ducer 36 to produce one ink drop for each voltage pulse
applied to transducer 36.
According to the invention, nozzle portion 26 of the
drop-on-demand ink jet printing apparatus comprises a
converging nozzle. As shown in FIG. l, the nozzle has
an entrance dimension dl, which is larger than the
exit dimension d2. The nozzle shown in the drawing has
a substantially linear taper in the dimension of the
noæzle along its physical length e, however, other tapers
s~lch as a horn configuration would also be suitable.
The flow through the nozzle is in the direction from
the larger opening to the smaller opening, as shown by
the arrow~
From a fluid mechanics viewpoint, the effective viscous
length Rd of a converging nozzle can be calculated as
R =l/3[(d /d )3 l]d R/(d d )
where dl, d2 are the dlmensions at the entrance and
exi-t or the converging section, respectively, and e is
the physical length of the nozzle (see FIG. 1). Thus,
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it can be seen tha-t the conver~ing nozzle is ph~sically
"long" but hydraulically "short"~ Since the converging
nozzles are "short", the converging nozzles do not
provide reliable drop-on-demand operation when using
prior art ink formu]ations having moderate viscosities
up to about 16 centipoise due to drop formation instabi-
lity. ~owever, it was found that highly reliable drop-
on-demand operation can be produced with converging
nozzles when using marking fluids having a substan-
tially higher viscosity than typical prior art systems.Although the prior art systems using constant cross-
section nozzles would not even work in the drop-on-
demand mode when utilizing marking ~luids of the sub-
stantially higher viscosity (up to 100 centipoise for
high resolution printing, for example), the combination
of the converging nozzle and the high viscosity marking
fluids produced not only highly reliable drop-on-demand
operation, but also much higher drop-on-demand drop
production rates than those obtainable by prior art
drop-on-demand ink jet printers.
The operation was superior in other ways as well. For
example, air ingestion into the nozzle is completely
inhibited and the stream stability is improved so that
a stream of drops of equal size and spacing can be
produced. The stream directionality is improved, and
the jet velocity is easily increased which is essential
for high speed printing. The nozzle can be operated at
any frequency in the frequency spectrum up to 120 kHz
without jet failure, and the nozzle can be operated
up to 80 k~z drop-on-demand drop production rate in
high resolution printing operation.
The converging nozzle can be produced by any suitable
technique. The preferred technique for producing a
converging nozzle is by anisotropically etching the
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nozzle in a silicon substrate. This technique will be
described with reference to the embodiment of the drop-
on-demand print head shown in FIG. 4. The print head
comprises cylindrical transducer member 60 closed at
one end ~y a nozzle plate 62, having formed therein
nozzle portion 64. The other end of the transducer is
fixed to body member 66. ~hen transducer 60 is actu-
ated by a suitable voltage drive pulse, transducer 60
is deflected to the position shown dotted in FIG. 4 to
cause a single drop of ink 78 to be expelled out
through nozzle portion 64.
Nozzle plate 62 comprises a silicon substrate formed of
single crystal material oriented with the (100) planes
parallel to the front surface. The front surface 68
and the rear surface 70 of the nozzle plate are coated
with etchant masking material. An aperture is made in
the masking material on the rear surface of the no~zle
plate. The nozzle plate is then sub]ected to a suit-
able anisotropic etching solution such as a water,
amine, pyrocatechol etchant, for example. It has been
known for some time that the (111) plane is a slow etch
plane in single crys~ai silicon. The nozzle is etched
in the form of a truncated pyramid type opening with a
square entrance aperture, tapered sides, and a smaller
s~uare exit aperture. The tapered sides form an angle
of 54.7 to the front surface since the etching is
along the crystal planes of the silicon substrate. The
etching is continued until an exit aperture of the
desired size is formed.
In a particular embodiment, the silicon nozzle plate
was five mils thick and the nozzle plate was etched to
produce a two mil square exit aperture. In an embodi-
ment similar to that shown in FIG. 4, the print head,
including the above-described nozzle plate, produced
reliable drop-on-demand operation up to a drop produc-
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tion xate of 60 kHz at a resolution of 240 pels/inch.This resolution is considered high resolution printing
since it produces print resolution approachiny that of
engraved type. However, the print quality began to
decline at drop production rates over 40 kHz. In this
apparatus, inks havinc3 a viscosity with a range from
about 15 centipoise up to 100 centipoise worked to
produce inX drops in a drop-on-demand mode, and the
preferred range of viscosity was from 20 to 40 centi-
poise.
In a second embodiment similar to that shown in FIG. 4,a 1.2 mil square nozzle was used and this apparatus
produced printing at a drop-on-demand production rate
of 80 kHz at a resolution of 450 pels/inch. This
apparatus worked to produce ink drops in the drop-on-
demand mode with inks having a viscosity from about
10 centipoise up to about 70 centipoise. The preferred
range of viscosity was from about 20 to 40 centipoise.
FIGS. 5 and 6 show a print head array 40 comprising
forty print heads 42 arranged in four rows 44 with
corresponding orifices 46 offset so that a line of
printing can be produced at a resolution approaching
engraved type as the print head moves across a print
sheet. Each of the print heads 42 comprises a hollow
cylindrical piezoelectric transducer 48 which forms an
ink chamber 50 to which ink is supplied from common
reservoir 52. A housing 54 is provided which includes
a tapered channel 56 for each print head which trans~
mits ink from ink chamber 50 to the corresponding
orifice 46 in nozzle plate 58. The orifices are
strongly convergent nozzles, as shown in FIG. 6. In
the preferred embodiment nozzle plate 58 comprises a
single crystal silicon substrate and orifices are
formed by anisotropic etching as described above to
form square orifices in nozzle plate 58, as shown in
FIG. 5-
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In a particular embodiment, a forty nozzle arraysimilar to that shown in FIGS. 5 and 6 was constructed
with 2 mil square nozzles. This array can be operated
to produce printing at a resolution of 240 pels/inch
at a drop-on-demand drop production rate of up to 40
kHz. The array operated successfully with ink having
a viscosity down to lS centipoise and up to 100 centi-
poise. However, the optimum range for the viscosity
was 20 to 40 centipoise.
While the invention has been particularly shown and
described with reference to a preferred embodiment
thereof, it will be understood by those skilled in the
art that various other changes in the form and details
may be made therein without departing from the ~pirit
and scope of the invention.
