TECHNICAL FIELD
[0001] This invention relates to ink-jet printers, specifically thermal ink-jet printers,
and more particularly, to a structure for substantially improving the performance
of the nozzle(s) in a thermal ink-jet printhead. This improvement in stability and
consistency of operation extends the firing frequency range of the nozzle and reduces
cross-talk, as well as desensitizes the exterior surfaces of the jetting nozzles to
their wettability state.
[0002] The design principles described herein are not limited solely to thermal ink jet
applications but in fact are of value in the design of athermally excited ink-jet
printheads as well.
[0003] This invention is of particular value in those circumstances of design in which
other means of attaining the above-mentioned operating benefits are not available
through geometry changes due to manufacturing process constraints.
BACKGROUND ART
[0004] When designing printheads containing a plurality of ink-ejecting nozzles in a densely
packed array, it is necessary to provide some means of isolating the dynamics of any
given nozzle from its neighbors, or else cross-talk will occur between the nozzles
as they fire droplets of ink from elements associated with the nozzles. This cross-talk
seriously degrades print quality and hence any providently designed ink-jet printhead
must include some features to accomplish decoupling between the nozzles and the common
ink supply plenum so that the plenum does not supply a cross-talk path between neighboring
nozzles.
[0005] Further, when an ink-jet printhead is called upon to discharge ink droplets at a
very high rate, the motion of the meniscus present in each nozzle must be carefully
controlled so as to prevent any oscillation or "ringing" of the meniscus caused by
refill dynamics from interfering with the ejection of subsequently fired droplets.
Ordinarily, the "settling time" required between firings sets a limit on the maximum
repetition rate at which the nozzle can operate. If an ink droplet is fired from a
nozzle too soon after the previous firing, the ringing of the meniscus modulates the
quantity of ink in the second droplet out. In the case where the meniscus has "over-shot"
its equilibrium position, a firing superimposed on overshoot yields an unacceptably
large ejected droplet. The opposite is true if the firing is superimposed on an undershoot
condition: the ejected droplet is too small and extremely fast (this is known as a
spear drop). Therefore, in order to enhance the maximum printing rate of an ink-jet
printhead, it is necessary to include in its design some means for reducing meniscus
oscillation so as to minimize the settling time between sequential firings of any
one nozzle.
[0006] In addition to cross-talk minimization, an important objective of printhead design
optimization is the control of meniscus dynamics during refill. During the overshoot
phase of refill, the momentum of the fluid which has flowed into the firing chamber
carries the meniscus beyond its equilibrium position. At that point where the compliance
of the meniscus has halted the fluid flow, the meniscus has bulged out of the bore
and appears briefly as a spherical section or "igloo" of ink projecting out of the
nozzle. Within microseconds, it has retracted itself back into the nozzle bore under
the influence of surface tension forces which strive to minimize the surface area
of the meniscus. Viscous losses which are caused by the motions of the fluid behind
the meniscus cause the seesaw oscillation of the meniscus to decay with time and eventually
halt.
[0007] As such the time response of a nozzle during refill can be approximated by a damped
second-order harmonic oscillator in which the mass of fluid entrained within the nozzle,
firing chamber and refill port "bounces" on the compliance of the meniscus while viscous
dissipation gradually damps out the oscillation. (It will be noted that none of the
parameters involved - mass, compliance or resistance - are constants in this system;
this is a linear approximation.)
[0008] During the brief time that the meniscus has overshot its equilibrium position and
is bulging out of the nozzle, it is possible for the fluid in the bulge to spill out
onto the material surrounding the lip of the nozzle. This spillage becomes very likely
if the angle defined by the tangent to the meniscus bulge at the lip of the orifice
equals or exceeds the wetting angle criterion for the material from which the nozzle
plate has been manufactured. If this happens, the meniscus will break free from the
Dip of the nozzle and the fluid bulge will then spread out across the nozzle plate.
As the meniscus retracts back into the bore, it reattaches itself to the edge of the
nozzle and in so doing pulls most but not all of the fluid back down the bore with
it. A small and very shallow puddle of ink is typically "stranded" in the immediate
vicinity of the nozzle after each firing and refill cycle. At low frequency operating
conditions - typically less than about 1,500 Hz - ample time exists between firings
for essentially all of this stranded ink to be wicked back up by the nozzles. However,
at high frequency operation - typically greater than about 1,500 Hz and above - a
new accumulation of puddled ink occurs at the nozzle lip. This accumulation can also
occur at low frequencies if (1) the surface tension of the ink is sufficiently low,
(2) the exterior surface of the orifice plate is sufficiently wettable, or (3) the
back pressure (defined as the abso lute value of the static negative gauge pressure
in the ink plenum) is sufficiently high (at least about -6 inches H₂O).
[0009] This accumulated ink has a deleterious effect upon print quality by capturing and
deflecting the ejected ink droplet during the phase of droplet ejection when the tail
is about to detach itself from the meniscus and follow the head of the droplet away
from the nozzle. This causes breakoff to occur not from the retracted meniscus but
instead from a random point around the wetted periphery of the nozzle; the drop is
pulled off-axis in the direction of the puddle. This direction error is integrated
over the flight time of the droplet to result in a dot placement position error on
the print medium. Since these errors are random in magnitude and direction, the result
is an unpredictable and serious degradation of print quality. In some cases, the ink
accumulation is severe enough to completely block droplet ejection from the nozzle.
[0010] Hence, any providently designed ink jet printhead must include some features to minimize
meniscus overshoot and minimize the time required for the meniscus oscillations to
decay away, so that the preceding scenario (referred to as "nozzle wet-out") is avoided.
It should be noted that wet-out can be caused or exacerbated by spray that breaks
off the tail of the drop and rains back down on the nozzle plate. This is worst for
low viscosity, high velocity drops.
[0011] Traditionally, wet-out is prevented by maintaining a static negative pressure, also
known as back pressure, throughout the ink supply system so as to define an equilibrium
position for the meniscus which lies inside the nozzle bore. Another method involves
the use of anti-wetting coatings applied to the area surrounding the nozzle lip,
which prevent meniscus breakoff during over-shoot. Yet another method is to increase
the amount of viscous damping present in the ink supply system, thereby holding overshoot
below the value required to initiate wet-out. Still another method is to provide a
contact-line barrier that prevents the puddle from advancing out past a certain radius.
[0012] Anti-wetting coatings are of limited utility in preventing wet-out during overshoot
since their lifetimes are typically shorter than that of the printheads to which they
have been applied, causing wet-out to reappear prior to the completion of the printhead's
service life. Furthermore, it is difficult to sufficiently immobilize these coatings
so that wiping detritus from nozzles does not force the coating down into the bores,
wreaking havoc irreversibly upon the printhead.
[0013] It is often impossible in practice to draw down overshoot via static negative backpressure,
since this backpressure acts to retard the refill of ink in the nozzles between firings.
Hence, sufficient backpressure to prevent wet-out also compromises operating speed.
[0014] But a third option, increasing the amount of viscous damping has been found to be
the most practical solution to the wet-out problem. This is because it (1) lasts the
life of the pen, (2) does not slow refill so much that the firing frequency limit
is compromised and (3) damps ripples and waves on the meniscus surface and hence
makes the process more stable.
[0015] Increasing the hydraulic resistance (via ink-channel dimensional changes) is not
the most practical method of increasing damping, at least in some situations, as printhead
geometries push the limit of the smallest dimensions attainable with a particular
material and process. (There is an ongoing need to scale down printhead geometries
to allow the firing of smaller droplets, as is desirable when printing very high quality
text, high resolution graphics or images containing gray levels or "halftoning". At
drop volumes below 50 picoliters, nozzle wet-out due to insufficient damping becomes
the dominant factor in degrading print quality.) The feed channel dimensions of such
structures are already so minute that to include pinch points (as lumped resistive
elements) in the feed channel structure would exceed the aspect ratio limits of the
resist film from which the barrier structure containing the feed channels is formed.
For Dupont "Vacrel" film, this aspect ratio is approximately 1:1. Hence, in Vacrel,
a feature desired to be free of Vacrel must be at least 0.001 inch wide if the basis
thickness of the film is 0.001 inch, 0.002 inch wide for 0.002 inch thick film and
so on. Hence, to be manufacturable, some other means of obtaining sufficient meniscus
damping to prevent wet-out must be included in the printhead structure.
[0016] Previous approaches to the problem of cross-talk, or minimizing inter-nozzle coupling,
can be separated into three classes: resistive, capacitive, and inertial. The following
is a brief discussion of each method and a critique of the typical embodiments of
these methods.
[0017] Resistive decoupling (to hydraulically "decouple" the nozzles from one another) uses
the fluid friction present in the ink feed channels as a means of dissipating the
energy content of the cross-talk surges, thereby preventing the dynamics of any single
meniscus from being strongly felt by its nearest neighbors. In the prior art, this
is typically implemented by making the ink feed channels longer or smaller in cross-section
than the main supply plenum. While these are simple solutions, they have several drawbacks.
First, such solutions rely upon fluid motion to generate the pressure drops associated
with the energy dissipation; as such, they can only attenuate the cross-talk surges,
not completely block them. Thus, some cross-talk "leakages" will always be present.
Second, any attempt to shut off cross-talk completely by these methods will necessarily
restrict the refill rate of the nozzles, thereby compromising the maximum rate at
which the printhead can print. Third, the resistive decoupling techniques as practiced
in the prior art add to the inertia of the fluid refill channel, which has serious
implications for the printhead performance (as will be explained at the end of the
inertial decoupling exposition which follows shortly).
[0018] In capacitive decoupling, an extra hole is put in the nozzle plate above that point
where the ink feed channel meets the ink supply plenum. Any pressure surges in the
ink feed channel are transformed into displacements of the meniscus present in the
extra hole (or "dummy nozzle"). In this way, the hole acts as an isolator for brief
pressure pulses but does not interfere with refill flow. The location, size and shape
of the isolator hole must be carefully chosen to derive the required degree of decoupling
without allowing the hole to eject droplets of ink as if it were a nozzle. This method
is extremely effective in preventing cross-talk (but can introduce problems with nozzle
meniscus dynamics, as will be discussed below).
[0019] In inertial decoupling, the feed channels are made as long and slender as possible,
thereby maximizing the inertial aspect of the fluid entrained within them. The inertia
of the fluid "clamps" its ability to respond to cross-talk surges in proportion to
the suddenness of the surge and thereby inhibits the transmission of cross-talk pulses
into or out of the ink feed channel. While this decoupling scheme is used in the prior
art, it requires considerable area within the print head to implement, making a compact
structure impossible. Furthermore, since the resistive component of a pipe having
a rectangular cross-section scales directly with length and inversely with the third
power of the smaller of the two cross-section dimensions, the flow resistance can
grow to an unacceptable level, compromising refill speed. More importantly, however,
are the dynamic effects caused by the coupling of this inertance to the compliance
of the nozzle meniscus, as will be discussed below.
[0020] With regard to the problem of meniscus dynamics, there are apparently no solutions
offered in the prior art. Apparently, this is a problem that has only recently surfaced
as printhead designs have been pushed to accommodate higher and higher repetition
rates. Clearly, any method used to decouple the dynamics of neighboring noz zles
will also aid in damping out meniscus oscillations, at least from a superficial consideration.
In practice, problems are experienced when trying to use the decoupling means as the
oscillatory damping means. These problems can be traced to the synergistic effects
between the nozzle meniscus and the fluid entrained within the ink feed channel, as
outlined below.
[0021] If resistive decoupling is attempted by reducing the width of the entire ink feed
channel, the inertia of the fluid entrained within the feed channel increases. When
this inertia is coupled to the compliance of the meniscus in the nozzle, it results
in a lower resonant frequency of oscillation of the meniscus, which requires a longer
settling time between firings of the nozzle. The inertial effect and the resistive
effect are tied together, with the net effect being that settling time cannot be reduced.
[0022] Capacitive decoupling has been proven effective at droplet ejection frequencies below
that corresponding to the resonant frequency of the nozzle meniscus coupled to the
feed channel inertia. However, its implementation at frequencies near meniscus resonance
is also complicated by interactive effects. Specifically, the isolator orifice acts
as a low impedance shunt path for high frequency surges. Hence, the high frequency
impedance of an ink feed channel terminated at its plenum end with an isolator orifice
will be lower than an equivalent channel without an isolator. This means that during
the bubble growth phase, blow-back flow away from the nozzle is increased by the isolator
orifice. This robs kinetic energy from the droplet emerging from the nozzle, which
results in smaller droplet size and lower droplet velocities and thus lower ejection
efficiency. During the bubble collapse phase, the isolator orifice meniscus pumps
fluid flow back into the refill chamber, which excites a resonant mode in which the
two menisci trade fluid between themselves via the ink feed channel. Since these two
menisci are for most practical designs similar in size, and since they are effectively
"in series", the equivalent compliance of the coupled system is roughly half of that
with only one orifice in it. The two-orifice system will thus resonate at a higher
frequency, which is a benefit from a settling time point of view, but the energy stored
in the resonating system still needs to be dissipated and therefore constrictive
damping will be necessary in such an implementation. While the effects of these resonances
is poorly understood at this time, the efficiency decrease may be severe enough to
prevent the printhead from working.
[0023] It is clear that what is needed is a method of printing that accomplishes both (1)
isolation of any given nozzle from its neighbors and (2) reduced oscillation of the
meniscus during refill (to minimize interference with the ejection of subsequently
fired droplets. This method must do the above while not introducing any adverse side
effects.
DISCLOSURE OF THE INVENTION
[0024] In accordance with the present invention, the viscosity of the jetted fluid is adjusted
to control the quantity of damping present in the fluid supply channels or refill
ports of the ink jet printhead. Since any viscosity increase acts to increase viscous
damping present throughout the ink supply circuit, the feed channel dimensions in
the supply circuit may be increased in order to prevent excessive pressure drops within
the supply circuit. From a processing and manufacturing standpoint, enlargement of
these features is simple, in contrast to the much more difficult problem of making
the same features smaller, as would be required to enhance damping via the traditional
techniques discussed above in the Background Art.
[0025] There are two examples of how this principle may be used to enhance the operation
of ink-jet printheads. In the first example, the directionality problem arising from
nozzle wet-out, referred to as "streaking", is eliminated via an ink viscosity increase
from an original value of 5 cp to an adjusted value of 7.5 cp.
[0026] In the second example, the issue of insufficient damping at the manufacturable limit
of the barrier structure is addressed. In this case, the channel architectures are
enlarged to accommodate the thicker ink. The original ink had a viscosity of 1.2 cp.
The intermediate ink viscosity was 5 cp. The thickest ink had a viscosity of 11 cp.
[0027] This invention involves adjustment of ink viscosity as a means of enhancing printhead
performance in situations where hydraulic tuning is impossible or impractical, i.e.,
head architectures which are already at the limits of manufacturability and/or which
are no longer available for changes due to other design constraints. Adjustment of
ink viscosity allows an otherwise impossible range of tradeoffs between nozzle refill
and meniscus settling time to be made in such printheads. The operating speed improvements
which this technique permit are quite large: a three- to five-fold increase in operating
speed over current state of the art.
[0028] While the drop stability in the pen is improved with an increase in the viscosity
of the ink, such increase in viscosity does not generate crusting, clogging and problems
in print quality over the entire environmental operating range (typically given as
3°C/70% RH to 15°C/20% RH, where RH is relative humidity).
[0029] This invention allows small drop-volume printers to operate without the ordinarily-encountered
stability problems at droplet ejection rated above 10 kHz. It also allows the dynamics
of ink droplet formation to be decoupled from the wettability of the orifice plate,
which prevents uneven frequency response, trajectory errors and air ingestion. It
allows these printheads to avoid such characteristics even in those situations where
similar tuning efforts (via inclusion of lumped resistive elements, for instance)
are prohibited.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030]
FIG. 1 is a perspective view of a nozzle plate and nozzles therein, depicting an emerging
droplet of ink from a nozzle and a puddle of ink associated therewith;
FIG. 2 is a cross-section taken along the line 2-2 of FIG. 1, showing the structure
of one particular drop generator; and
FIG. 3 is a view similar to that of FIG. 2, showing a portion of an ink-filled drop
generator with a bulging meniscus and a drop of ink wetting the nozzle plate at characteristic
wetting angle ϑw.
BEST MODES FOR CARRYING OUT THE INVENTION
[0031] Referring now to the drawings wherein like numerals of reference designate like elements
throughout, a portion of a printhead is depicted in FIG. 1. In particular is seen
a nozzle plate 10 in which are recessed a plurality of nozzles 12 in individual recesses
13. Ink 14 is fired from resistors through the nozzles in a particular arrangement
toward a print medium (e.g., paper) to form alphanumeric characters and graphics.
[0032] FIG. 2 depicts a portion of a feed chamber 16 in which is located a resistor 18;
there is one resistor associated with each nozzle 12. Ink is fed into the feed chambers
from a plenum (not shown). Upon receiving a pulse of energy from an external source,
the resistor 18 is heated to a level sufficient to expel a droplet of ink 14 toward
the print medium. Following ejection of the ink droplet 14, additional ink fills the
chamber 16 in preparation for another firing.
[0033] The nozzle 12 has a nozzle diameter d; each resistor covers a square area with side
dimension s; the channel width is given by w. The thickness of the nozzle plate 10
is t
p, while the thickness of barrier layer 20 is t
b. In a preferred example, the printhead employs a barrier layer 20 comprising Vacrel
55 µm thick and a nozzle plate 10 comprising gold-plated nickel 63 µm thick. The nozzles
12 are 47 ±3 µm diameter, with resistors 64 µm x 64 µm, and channel width 84 µm wide.
[0034] As indicated in the Background Art section, during the overshoot phase, a puddle
22 of ink may form adjacent the nozzle 12. If not wicked back into the chamber, such
a puddle may have a deleterious effect upon print quality by interfering with the
droplet 14 of ink as it is ejected from the nozzle 12.
[0035] During the refill process, the meniscus overshoots its equilibrium position, is slowed,
stopped, and eventually reversed by the surface tension of the meniscus. The maximum
overshoot occurs when the meniscus is stopped. In FIG. 3, ϑ corresponds to the maximum
overshoot of the meniscus. The angle ϑ is defined by a tangent to the meniscus surface
at the nozzle perimeter and a line drawn parallel to the top plate surface. To avoid
spillage onto the top plate, ϑ should be less than ϑ
w, the characteristic wetting angle for the ink and top plate materials.
[0036] As used herein, a stable drop generator is one that makes drops with consistent trajectories,
volumes, speeds, and break-up patterns. In accordance with the invention, this stability
becomes more likely as the viscosity is increased. This is because it is the damping
effect of viscosity that will balance and control the inertial and surface forces
that drive the refill and ejection processes. Unstable drop generators with low viscosity
are characterized by chaotic meniscus movement, large meniscus overshoots, erratic
spray patterns, and puddles 22.
[0037] This stability can bo measured by looking at the accuracy and consistency of dot
placement and size. Stability was measured by looking at line spacing on paper. The
odd-numbered nozzles in the pen were fired across the page, forming a set of parallel
lines. Then, an identical pattern was made with the even-numbered nozzles on a different
part of the page. A vision system then examined the patterns, measuring line spacing
uniformity and line width uniformity.
[0038] These measurements of line spacing and width were then combined into an overall "print
quality number", with 4 being a perfect grade. Testing at 30°C (the worst-case operating
temperature for print quality) revealed that 40% H₂O/60% DEG ink (DEG is diethylene
glycol) had a print quality number of 3.2, which was a full two points better than
the less viscous 50% H₂O/50% DEG (print quality number of 1.2). Also, considering
dot placement only, measurements of cross-scan directionality using the same plot
showed that going from an ink viscosity of 5 to 7.5 cp (50/50 water/DEG to 40/60 water/DEG)
decreased the variation in angular misdirection by 43%:
TABLE I
Printing Results (30°C/70%RH) |
Ink |
ambient viscosity |
cross-scan 3-sigma |
(%H₂O/DEG) |
(cp) |
(degrees) |
50/50 |
5.0 |
1.20 |
40/60 |
7.5 |
0.69 |
[0039] Computer modeling of the ink flow in the printhead confirmed the improved stability
obtained with the higher viscosity ink modeling of a pen employing 59 µm Vacrel and
nozzles 12 having a diameter of 43 µm and showed that a change in vehicle composition
from 50/50 water/DEC to 40/60 water/DEC should result in the following changes in
refill time, overshoot, and damping:
TABLE II
% Change Relative to 50/50 at 30°C |
Ink |
Temp. |
Refill |
Overshoot |
Damping |
40/60 H₂O/DEG |
60°C |
+2.3% |
-26.1% |
+36.9% |
40/60 H₂O/DEG |
30°C |
+18.2 |
-49.3 |
+67.4 |
[0040] In another experimental comparison, 50/50 ink evidenced a spear drop onset at 3,500
Hz. (Spear drops are headless, very fast, and usually misdirected; they appear above
certain critical frequencies.) 40/60 ink evidenced a spear drop onset at frequencies
of about 4,800 Hz, while 30/70 ink evidenced a spear drop onset at frequencies greater
than 5,500 Hz.
[0041] In yet another experimental comparison, various compositions of ink were fired from
pens with the following results:
TABLE III
Properties for Various Viscosities of H₂O/DEG |
% H₂O |
Vmin/Vss |
Directionality |
Viscosity cp (25°C) |
60 |
0.729 |
1.5 |
3.89 |
50 |
0.833 |
4 |
5.48 |
40 |
0.860 |
7.5 |
8.61 |
30 |
>0.95 |
9 |
13.81 |
20% NMP |
0.901 |
7.5 |
7.94 |
Notes: (1) Vmin/Vss is the ratio between the minimum velocity in a frequency resonance plot and the steady
state velocity. A value close to 1.0 indicates stability over the frequency range. |
(2) Directionality is on a scale of 0 to 9, where 0 is bad and 9 is good. |
(3) 20% NMP = 40% H₂O, 40% DEG, 20% N-methyl pyrrolidone. |
[0042] Print quality was determined for a variety of compositions, using the preferred
printhead configuration given above. The results are listed below, with average print
quality given for the indicated vehicle composition. The higher the value, the better
the print quality. Each pen has three groups of ten nozzles each; each such group
is called a primitive. In the test, a visual determination was made for each half-primitive
(the odd or even nozzles), and summed for all six half-primitives to arrive at the
average PQ rating. The rating is based on 0 = very poor, 1 = poor, 2 = fair, and 3
= good; values of 2 and above (here, 12.0 and above) are deemed acceptable.
TABLE IV
Print Quality vs. % H₂O/DEG |
H₂O/DEG |
Ave. PQ Rating |
70/30 |
0.8 |
60/40 |
8.0 |
50/50 |
11.9 |
40/60 |
16.5 |
30/70 |
18.0 |
5% NMP |
14.0 |
10% NMP |
13.8 |
15% NMP |
16.0 |
Note: % NMP plus equal portions of H₂O and DEG |
[0043] From the foregoing, it is evident that an increase in viscosity of ink improves the
print quality considerably. However, there is an upper limit on the viscosity of ink
that may be employed, since higher viscosity inks take longer to dry and increase
the refill time. Indeed, in conjunction with some print media (e.g., Mylar transparencies),
the upper limit is severely constrained. For example, 30/70 H₂O/DEG is useful with
paper, but cannot be used with Mylar transparencies.
[0044] Although diethylene glycol was used to increase the viscosity of the inks in the
foregoing examples, it will be readily clear to those skilled in this art that the
teachings of this invention are applicable to any of the water-miscible glycols typically
used in ink-jet printing. Thus, in addition to diethylene glycol, ethylene glycol
and propylene glycol are but a few examples of the many glycols that are used in ink-jet
printing, and an increase in the glycol content relative to water will accomplish
the same purpose, with the same end result as indicated above.