CA2006047A1 - Printhead performance tuning via ink viscosity adjustment - Google Patents

Abstract

PRINTHEAD PERFORMANCE TUNING VIA
INK VISCOSITY ADJUSTMENT

ABSTRACT OF THE DISCLOSURE
The drop stability of a pen employed in an ink-jet printer is increased by increasing the viscosity of the ink. For inks comprising water/glycols, the increase in viscosity is easily accomplished by increasing the ratio of glycol to water. In one ink, a 20% increase in glycol concentration resulted in a 50% increase in viscosity and a decrease in the variation of angular misdirection by 43%.

Description

200604~
.. 1 PATENT

.
PRINTHEAD PERFORM~CE TUNING VIA
INK VISCOSI~Y ADJUS~ENT

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This invention relates ~o ink-jet printers, specifi-cally ~hermal 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 im-provement in st~bility and consistency of operation ex-tends the firing frequency range of the nozzle and reduces cross-talk, as well as desensitizes the exterior surfaces of the jetting nozzles to th~ir w~ttability state.
The design principles des~ribed herein are not limit-ed solely ~o th~rmal ink jet applications bu~ in fact are . o~ value in the design of athermally excited inX-jet prir.theads as well.
15This invention is of particular value in those cir-cumstances of design in which other means of attaining the above-mentioned operating benefits are not available through geometry changes due to manu~acturing process constraints.
B~ÇKGROUND ~RT

When designing printheads containing a plurality o~
inX-ejecting nozzles in a densely packed array, it is necessary ~o p~ovide some means of isolating the dynamics of any given nozzle from i~s neighbors, or else cross-talk will occur between the noz21es as they ~ire droplets of ink from elem~nts associated with the nozzles. This cross-talk seriously degrades print quiality and hence any providently designed ink-jet printhead must include some Case 188459 2~)6~7 1 ~atures to accomplish decoupl:ing between the nozzles and the common ink supply plenum so that the plenum does not supply a cross-talk path between neighboring nozzles.
Furthex, when an ink-jet printhQad is called upon to discharge ink drople~s a~ a very high rate, the motion o~
the meniscus present in each nozzle must be care~ully controlled so as to prevent any oscillation or "ringing"
o~ the meniscus caused by re~ill dynamics from inter~ering with the ejection of subsequently ~ired droplets. Ordi-narily, the "settling time" required between firings setsa limit on the maximum repe~i~ion rate at which the nozzle can operate. If an inX droplet is fired from a Aozzle 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 i~ the firing is superimposed on an undershoot condition: the ejected droplet is too small and extremely ~ast (this is known as a spear drop). There-~ore, in order to enhance the maximu~ printing rate of an ink-jet printhsad, it is necessary to include in its design some ~eans for reducing meniscus oscillation so as to ~inimize the settling time between sequential firings o~ any one nozzle.
In addition ~o cross-talk minimization, an important ob~ectiv~ of prin~head design optimization is the control o~ meniscus dynamics during refill. During the overshoot ph~se of rsfill, the ~o~entum o~ the fluid which has ~lowed into the firing cha~ber carries the meniscus beyond i~s equilibriu~ position. At that point where the compli~
ance o~ the meniscus has halted the fluid flow, the menis-cus has bulged out of khe bore and appears briefly as a spherical section or "igloo" o~ ink projecting out o~ the nozzle. Within microseconds, it has retrac~ed itsel~ back ;~ into the nozzle bore under the in~luence of surface ten~
sion force~ which s~rive to ~inimize the surface area o~
the meniscus. Viscous losses which are caused by the Case 188459 2~)01~147 ; ..

1 ~tio~s o~ the fluid behind thl~ meniscus cause the seesaw oscillation o~ the meniscus to decay with time and eventu;
ally halt.
As such the time response oS a nozzle during refill S can be approximated by a damped second-order harmonic oscillator in which the ~ass of fluid entrained wîthin the nozzle, firing chamber and refill port "bounces" on the compliance of the meniscus while viscous dissipation gradually damps out the oscilla~ion. ( I~ will be noted 10 that none o~ the parameters involved - mass, compliance or resistance - are constants in this system; this is a linear approxi~ation . ) ' During ~he 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 liXely if the angle defined by the tangent ~o the meniscus bulge at the lip of the ori~ice ~quals or exceeds the wettin~ angla criterion for the material rrOm which the nozzle plate has been manufac-tured. If this happens, the meniscus will break free from ~he ~ip of the nozzle and the fluid bul~e 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 ~luid ba~k down the bore with it. A small and ~ery shal-low puddle o~ ink is typically "stranded" in the immediate vicinity o~ the nozzle a~ter each fsrinq and refill cycle.
At low frequency operating conditions - typically less than about 1,500 Hz - ample time exists between ~irings ~or essentially all o~ this stranded ink to be wicXed back up by the nozzles. However, at high ~requency operation -typically greater than about 1,500 H~ and above a new accumulation of puddled ink occurs at the nozzle lip.
This accumulation can al o occur at low frequencies if (1) ; the surface tension of ~he inX is sufficiently low, (2) the exterior surface of ~he orifice plate is sufficiently wettable, or t3) the bac~ pressure (de~ined as the abso-Case ~88459 ~! ~.i . ' ' ' ' ' 21)13~ 7 1 lute value of the s~atic negative gauge pressure in the inX plenum) is su~ficiently high (at least about -6 inch~s ~2) This accumulated ink has a deleterious e~ect upon print quality by capturing and deflecting the ejected ink droplet during the phase o~ droplet ejection when the tail is abo~lt to detach itsel~ rom the meniscus and follow the head of the dxoplet away from the nozzle. This causes breakoff to occur not ~rom ~le xetracted meniscus but instead from a random point around the wetted periphery of the nozzle; the drop is pulled of ~-axis in the direction of t~e puddle. This direction error is integrated over ~he flight time of the droplet to result in a dot place~
ment 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 so~e cases, the ink accumulation is severe enough to completely block droplet ejection ~rom the nozzle .
HQnce, any providsntly designed ink jet printhead ~ust include some features to minimize meniscus overshoot and ~inimize the time required for the meniscus oscilla~
tions to dacay away, so that the precedin~ 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 o~f the`-tail of the drop and rains back down on the nozzle plate. This is worst for low ~iscosi-ty, high velocity drops.
Traditionally, wet-out is prevented by ~aintaining a sta~ic negative pressure, also known as back pressure, throughout the i~k supply syste~ so as ~o de~ine an equi-librium position for the meniscus which lies inside the noz~le bore. Another method involves the use o anti-wetting coatings applied to the area surrounding the nozzle lip, which prevent ~eniscus breakoff during over-shoot. Yet another me~hod is to increase the amount of viscous damping present in the ink supply system, thereby holding overshoot below the value required to initiate Case 188459 ,' ~.' ! ~' ' ~' . `, ' . . .... ..

~201)~0~17 . s 1 wet-out. Still another me~hod is to provide a contact-line barrier that preven~s the puddle fro~ advancing out past a certain radius.
~nti-wetting coatings are of limited utility in S preventing wet-out during overshoot since their lifetimes are typically shorter than that of the printheads to which ~hey have been applied, causing wet-out to reappear prior to the completion of the printhead's service life. Fur-ther~ore, it is difficult to sufficiently immobilize these coatings so that wiping detritus from nozzles does not ~orce the coating down into the bores, wreaking ha~oc irreversibly upon the prin~head.
It is often impossible in practice to draw down overshoot via static negative bac~pressure, 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.
But a third op~ion, increasing the amoun~ of viscous da~pin~ has been ~ound ~o be the most practical solution to the wet-out problem. ~his is because it (1) lasts the li~e of t~e pen, (2~ does not slow refill so much that the firi.~g frequency limit is compromised and (3) damps rip-ples and waves on the meniscus surface and hence makes the process more stable.
Increasing the hydraulic resistance (via ink-channel d~mensional changes) is not the most practical method of increasing damping, at least in some situations, as print-head geo~etries push the li~it of the smallest dimensions attainable with a particular material and process. (There ! 30 is an ongoing need to scale down printhead geometries to allow the firin~ of s~aller droplets, as is desirable when printing very high quality tex~, high resolution graphics or images containing gray levels or ~half~oning". At drop volumes below 50 picoliters, nozzle wet-out due to insu~-~icient damping becomes ~he 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 ele~ents) in the feed channel Case 188459 i ,., ~ .. . . . ... .
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1 struct~re would exceed the aspect ratio li~its of the resist ~ rom which the barrier structure containing the feed channels is ~ormed. For DuPont ~Vacrel" film, this aspect ratio is approximately 1:1. Henca, 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, 0O002 inch wide for 0.002 inch thick film and so on. ~ence, to be manu~acturable, some o~her means of obtaining sufficient meniscus damping to prevent wet-out 10 must be included in the printhead structure.
Previous approaches to the problem of cross-talk, or minimizing in~er-noz21e coupling, can be separated into three classes: resistive, capaci~ive, and inertial. The following is a brief discussion of each me~hod and a 15 critique of the typical embodiments of these methods.
Resistive decoupling (to hydraulically "decouple" the nozzlss fro~ one another) uses the fluid friction present in the ink feed channels as a means of dissipating the ~nergy content of the cross-talX surges, thereby prevent-20 $ng the dyna~ics of any singl~ meniscl~s fro~ being strong-ly fel~ by its nearest neighbors. In the prior art, this is typically implemented by making the ink feed channels lon~er or smaller in cross-section than the main supply plenum. While these axe simpie solutions, ~hey have 25 s~veral drawbacks. First, such solutions rely upon fluid motion to generate the pressure drops associated with the anergy dlssipation; as such, they can only attenuate the cross-talk surges, not completely block the~. Thus, some cross-talk "leakagesn will always be presentO Second,,any 30 attempt to shut of~ cross-talk complétely by these methods will necessarily restrict the refill rate of t~e nozzles, thereby compro~ising the maximum rate at which the print-head can print:. Third, the resistive decoupling tech-niques as pract:iced in the prior art add to the inertia of 35 the fluid refill channel, which has serious implica~ions ~or the printhead performance (as will be explained at the end of the inertial decoupling exposition which follows shortly).
Case 188459 ZOOG04t7 ,.;; 7 1 In capacitive decoupling, an extra hole is put in the nozzle plate above that point where the inX feed channel meets the ink supply plenum. Any pressure surges in the ink feed channel are transformad into displacements of the meniscus present i.n the extra hole (or "dummy nozzle~). In this way, the ho:le acts as an isolator ~or brief pressure pulses but does not interfere with re~ill ~low. The location, size and shape o~ the isolator hole ~ust b~ carePully chosen to derive the required degree of 10 decoupling without allowing the hole to e ject droplets o~
ink as i~ it were a nozzle. This ~ethod is extremePy e~fective in preventing cross-talk (but can introduce problems with nozzle meniscus dynamics, as will be dis-cussed below).
In inertial decoupling, the feed channels are made as long and sl~nder as possible, thereby maxi~izing the inertial aspect of the fluid entrained within them. The inertia of the fluid "clamps" its ability to respond to crosstalk surges in proportion to ~he suddenness o~ the surge and thereby inhibi~s the trans~ission of cross-talk pulses into or out of the ink feed channel. While this deco~pling scheme is used in the prior art, it requires considerable area within the print head to implement, making a compact structure imposslbla. 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 s~aller of the two cross-section dimensions, the flow resistance can grow to an unacceptable level, conpromising refill speed. ~ore 30 importantly, however, are the dynam~'c effects caused by: :
the coupling o~ this inertance to the compliance o~ the nozzle meniscus, as will be discussed below. -With regard to the pro~lem o~ 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 accommo-date higher and higher repe~ition rates. Clearly, any method used to decouple the dynamics of neighboring noz-Case 188459 . :.,. - . , -,., ~ . . . .
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l 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 decouplin~
means as the oscillatory damping mean~. These problems can b~ traced to the synergistic effects between the nozzle meniscus and the fluid entrained within the ink feed channel, as outlined below.
If resistive decoupling is attempted by reducing the width of ~he entire ink feed channel, the inertia of the fluid entrained within ~he feed channel increases. When this inertia is coupled to the co~pliance of the meniscus in the nozzle, it results in a lower re onant frequency of oscilla~ion 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.
Capacitive ~ecoupling has been proven effective at droplet ejection frequencies below that corresponding to the resona~t frequency of the nozzle meniscus coupled to the feed channel inertia. Howev~r, its implementation at *requencies near meniscus resonance is also complicated by int~ractiv~ effects. Specifically, the isolator orifice acts as a low impedance shunt path for high frequency surges. ~ence, the high frequency impedance of an ink feed chann~l 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, wh~ch results in smaller droplet size and lower droplet velocities and thus lower ejection effic:iency. During the bubble collapse ph~se, the isolator orifice meniscus pumps fluid ~low back into the refill chal~er, which excites a resonant mode in which the two ~enisci trade fluid between themselves via the ink ; feed channel. Since these two menisci are for most prac-~ical designs similar in si2e, and since they are effec-tively "in series", the equivalent co~pliance of the Case 188459 ;' i~;.~ ' '.'. '." ' ' ';! ' ' . ' . . . .

2qD06~7 ,,`, g l coupled system is roughly half of that with only one orifi~e in it. The two-ori~ice system will thus re~onate at a higher frequency, which is a benefit from a settling time point of view, but the energy stored in the resonat-ing syste~ still needs to b~ dissipated and th~reforeconstrictive damping will be necessary in such an imple-mentation. While the ef~ects of these resonances is poorly understood at this ti~e, the efficiency decrease ~ay be severe enough to prevent the printhead from work-ing.
It is clear tha~ wha~ is needed is a method of prin~-ing that accomplishes bo~h (l) 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 ef~ects.

~I~CLQSU~_QF TR~ I~VE~TION
In accordance with the present invention, the viscos-ity ~f the jetted fluid is adjusted to control the quanti-ty of damping presen~ in the fluid supply channels or refill ports of ~he ink jet printhead. Since any ~iscosi-ty increase acts to increase viscous damping presentthroughout the ink supply circuit, the feed channel dimensions in the supply circuit may be increased in order to pr~vent excessive pressure drops within the supply circuit. From a processing and manu~acturin~ standpoin~, enlargement or these features is simple, in contrast to the much more ~ icult problem of ma~ing the sa~e fea-tures s~aller, as would be required to enhance damping via the traditional techniques discussed above in the 8ack-ground Art.
There are ~wo examples of how this principle may be . used to enhance the operation of ink-jet printheads. In the ~irst example, the directionality proble~ arising from nozzle wet-ou~, referred to as "streaking", is eliminated Case 188459 .:.: . " . - , . . : . . .

04~

1 via an ink viscosity increase from an original value of 5 cp to an adjusted value o~ 7.5 cp.
In the second example, the issue o~ insufficient damping at the manufacturable limit o~ the barrier struc-ture is addressed. In this case, the channel architec-tures 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 t.hickest ink had a viscosity o~ p.
This invention involves adjustment of ink viscosity as a means of enhancing printhea~ performance in situa-tions where hydraulic tuning is impossible or impractica~, i'.e., head architectures which are already at the limits of manu~acturability and/or which are no longer available for changes due to other design constraints. Adjustment of inX viscosity allows an otherwise impossible range of tradeoffs between nozzle refill and meniscus settling time to be made i~ such printheadsO The operating speed im-provements which this ~ec.hnique permit are quite large: a three- to five-fold increase in operatin~ speed over current state o~ the art.
~ While the drop stability in the pen is improved with an increase in the viscosity o~ the ink, such increase in viscosity does not generate crusting, clo~ging an~ prob-l~ms in print quality over the entire environ~ental oper-ating range ~typically given as 30-C/70% RH to 15-C/20 RH, where RH is relative humidi~y).
This invention allows s~all drop-volume printers to operate witho~t the ordinarily-encoun~ered stability problems at droplet ejection ra~ed a~ove 10 kHz. I~ also allows the dyna~ics of ink droplet formation to be decou-pled from the wettability of the orifice plate, which prevents uneven frequency response, trajectory errors ~nd air ingestion. It allows these printheads to avoid such characteristics even in those situations where similar tuning effor~s (via inclusion of lumped resistive ele ments, ~or instance) are prohibited.

Case 188459 ~I 2q~0160~'~

FIG. 1 is a perspective view of a nozzle plate and no2zles therein, depicting an emerging droplet of ink from 5 a nozzle and a puddle of ink associated therewith;
FIG. 2 is a cross-section taken aiong th~ line 2-2 of FIG. 1, showing the s~ruct~re of one par~icular drop generator: and FIG. 3 is a view similar 1:o that of FIG. 2, showing a 10 portion of an ink-filled drop generator with a bulging meniscus and a drop of ink wetting the nozzle plate at characteristic wetting angle ew.

~E~OD~S ~OR CAR~UT T}~NVENT~QN
Referring now to the drawinqs wherein liXe numerals of reference designate like elements ~hroughou~, a portion o~ a printhead is depicted in FIG. l. In particular is seen a noæzle plate lO in which are recessed a plurality of nozzles 12 in individual recesses 13. Ink 14 is fired ~rom resistors through the nozzles in a particular ar-rang~ment toward a print medium (e.g., paper) to form alphanumeric characters and graphics.
FIG. 2 d~picts a portion of a feed cha~ber 16 in which is located a resistor 18; there is one resistor associated with each nozzle 12. Ink is ~ed into the feed chambers from a plenum tnot shown). Upon receiving a puls~ o~ energy from an external source, ~he resistor 18 is heated to a level sufficient to expel a droplet of ink 14 toward the print medium. Pollowing ejection of the ink droplet 14, additional ink fills the chamber 16 in prepa-ration for another ~iring.
The nozzle ~2 has a nozzle diameter d; ~ach resistor covers a square area with side dimension s; the channel ~5 width is given by w. The thicXness of the nozzle plate lO
is tp, while the thickness of barrier layer 20 is tb. In a preferred example, the printhead employs a barrier layer 20 comprising ~lacrel 55 ~ thick and a nozzle plate lO
Case l88459 ~ 7 1 comprising gold-plated nickel 63 ~m thick. The nozzles 12 are 47 ~3 ~m diameter, with resistors 64 ~m x 64 ~m, and channel width 84 ~m wide.
As indicated in the Background Art section, during the overshoot phase, a puddle 22 of ink may forD adjacent th~ nozzle 12~ rf not wicXed back into ~he chamber, such a puddle may have a deleteriou~; ef~ect upon print quality by interfering with the droplet 14 of ink as it is ejected from the noz21e 12.
During the refill process, the meniscus overshoots its equilibrium position, is slowed, stopped, and eventu-ally reversed by the surface tension of the meniscus. The ~aximum overshoot occurs when the meniscus is stopped. Xn PIG. 3, e corresponds to the maximum overshoot of the meniscus. The angla e is defined by a tangent to the meniscus sur~ace at the nozzle perimeter and a line drawn parallel to ~he top plate surface. To avoid spillage onto t~e top pla~e, e should ~e less than ew, the characteris-tic wetting angle for the ink and top plate materials.
As used herein, a s~able drop gen~rator is one ~hat makes drops with consistent trajectories, volumes, speeds, and ~reak-up patterns. In accordance with the invention, this stability becomes more likely as the viscosity is increased. This is becaus~ it is the damping effect of 2~ viscosity that will balance and control the inertial and surface forces that drive the refill and ejection process-~s. Unstable drop qenerators with low viscosity are characterized by chaotic meniscus movement, large meniscus overshoots, erra~ic spray patterns, and puddles 22.
This stability can be ~easured by looking at the accuracy and consistency of dot placemen and size.
Stability was measured by looking a~ line spacing on paper. Tha od~-numbered nozzles in the pen were fired across th~ page, forming a set of parallel lines. Then, an identical pattern was made with the even-numbered nozzles on a di~ferent part of the page. A ~ision system then exami~ed the patterns, measuring line spacing uni-formity and line width uniformity.
Case 1~8459 $

4~

1 ~hese measure~ents of line spacing and width were then combined into an overall print quality number", with 4 being a perfect grade. Test:ing at 30-C (the worst-case operating temperature for print quali~y) revealed that 40%
H20/60% DEG ink (DEG is diethylene glycol) had a print quality number of 3.2, which was a ~ull two points better than the l~ss viscous 50% H20/50% DEG ~print quality number of 1.2). ~lso, consi~ering dot placement only, measuremen~s of cross-scan directionality using the same plot showed that going from an ink viscosity of S to 7.5 cp (50/50 water/DEG to 40/60 water/iEG) decrea~ed the variation in anqular misdirection by 43~:

_==========_=====================______=__======__========
TABLE I.
Printing Results (30 C/70%XH) InX ambient viscosity cross-scan 3-sigma ( %H20/DEG ) ( Cp ) ( degreQs ) 50/50 5O0 1.20 40J60 7.5 0.69 Co~putex ~odeling of the ink flow in the printhead confirmed the improved stability obtained with the higher ~iscosity ink modeling o~ a pen e~ploying 59 ~ ~ac.el and nozzles 12 having a diameter of 43 ~m and showed ~hat a change in vehicle composition from 50/50 water/DEG to 40/60 water/DEG should result in the following changes in . 30 refill time, overshoot~ and damping:Y

Case 138459 20~604~
14 :~

TA~LE II.
% Chan~e Relative to 50/50 at 30 C
.... ..
Ink Temp. Refill Overshoot Da~ping ¦ 40/60 H20/DEG 60-C ~2.3% -25.1% +36.9%
. 40/60 H20/DEG 30 C +18.2 49.3 +67.4 In another experimental comparison, 50/50 in~ evi~
dence~ a spear drop onset at 3,500 Hz. (Spear drops are headless, very ~ast, and us~lally 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. :.
In yet another experimental comparison, various compositions of ink were ~ired ~rom pens with the follow~
ing results:

TABLE III.
Properties for Various Viscosi~ies of H20/DEG ¦:

% H2Vmin/Vss Directionalityviscosity . cp (25 C) 0.729 1.5 3.ag 0.833 4 5.48 0.860 7.5 8.61 >0.95 9 Y 13.~1 :, 20% N~P 0.901 7.5 7.94 :

Notes: ( 1 ) Vmin/Vss is the ratio between the min-imum velocity in a frequency resonance plot and ; the st:eady state velocity. A value close to 1.0 :.
indicates stability over the frequency range.

Case 1884S9 ~0(~6~4~7 ~:: 15 ( 2 ) Direc~ionality i5 on a scale o~ 0 to 9, where 0 i~; bad and 9 is good.
( 3 ) 20% NMP = 409~ H20, 40% DEG, 209~ N-methyl pyrrolidone.

Print quality was d~ermined for a variety o~ compo-sitions, using the pre~err~d printhead con~iguration given above. The r~sults are listed below, with average print 10 quali~y given ~or ~he indicated vehicle composition. The higher the value, the better the print quali~y. Each pen ha~ three groups of ten nozzles each; each such group i~s ~alled a primitive. In the test, a visual deter~ination W2S made for each half-primi~ive (the odd or even nozzles), an~ su~med for all six half-primitives to arrive a~ the average PQ rating. The rating is based on 0 = very poor, 1 = poor, 2 = fair, and 3 = good; values of 2 and abo~e there, 12.0 and above) are deemed acceptable.

7~-~--~=~7~D=== ~
TABLE IV.
Print Quality vs. % H20/DEG

H20/DEG Av~. PQ Rating 70/3~ 0 . 8 60/40 8 . 0 50/50 ~.1 . 9 40/60 16 . 5 30/70 18 . 0 3 0 I y 5% ~JMP 14 0 10~ IP 13.8 15~ IP 16.0 ;~

35 Note: ~ NMP plus equal portions of ~2 and DEG ~ :~
::, ~.

Case 188459 ~ ~

~ 21D(~60~
i-1 From the foregoing, it is evident that an increase in v~scosity of ink i~proves the print guali~y considerably.
~owever, there i5 an upper limit on the viscosity of ink that may be e~ployed, since higher ,viscosity inks taXe longer to dry and increase the refill time. Indeed, in conjunction with some print ~eclia (e.g., Mylar transparen- :
cies)~ the upper limit is severely constrained. For exa~ple, 30/70 H20/DEG is useful wi~h paper, but cannot be used with Mylar transparencies.
Although diethylene glycol was used to increase the viscosity of the in~s in the ~oregoing examples, it Wi7 1 ¦ be readily clear to those skille~ in this art that the ~eachin~s 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 ~he many I glycols that are used in ink-jet printing, and an increase in the glycol conten~ rela~ive ~o water will accomplish the same purpose, with the sa~e end result as indicated a~ove.

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Case 188459

Claims (11)

1. A method of increasing drop stability in a pen having a plurality of nozzles (12) in a nozzle plate (10) associated with a drop-firing means (18) for firing drops of ink (14) through said nozzles, said pen employed in an ink-jet printer, said method comprising increasing the viscosity of said ink.

2. The method of Claim 1 wherein said viscosity is increased by at least about 50%.
.

3. The method of Claim 2 wherein said ink comprises a vehicle consisting essentially of about 30 to 40% water and the balance diethylene glycol.

4. The method of Claim 3 wherein said ink comprises a vehicle consisting essentially of about 40% water and about 60% diethylene glycol.

5. The method of Claim 1 wherein said pen comprises a plurality of ink feed chambers (16) defined in a barrier layer (20) comprising Vacrel about 55 µm thick and a nozzle plate (10) comprising gold-plated electro-formed nickel about 63 µm thick, with a plurality of nozzles (12) formed in said nozzle plate about 47 µm diameter, and with a resistor (18) about 64 µm x 64 µm formed in the bottom of each of said chambers and operatively associated with one of said nozzles, said ink feed chambers fluidically communicating with a source of ink (14) by an ink feed channel having a width 90 µm wide at the top thereof and wherein said viscosity of ink is at least about 7.5 cp at 30°C.

6. A method of increasing drop stability in a pen having a plurality of nozzles (12) in a nozzle plate (10) associated with a drop-firing means (18) for firing drops of ink (14) through said nozzles, said pen employed in an ink-jet printer, said method comprising increasing the glycol content relative to the water content to thereby increase the viscosity of said ink.

7. The method of Claim 6 wherein said viscosity is increased by at least about 50%.

8. The method of Claim 7 wherein said ink comprises a vehicle consisting essentially of 30 to 40% water and the balance diethylene glycol.

9. The method of Claim 8 wherein said ink comprises a vehicle consisting essentially of about 40% water and about 60% diethylene glycol.

10. The method of Claim 6 wherein said pen comprises a plurality of ink feed chambers (16) defined in a barrier layer (20) comprising Vacrel about 55 µm thick and a nozzle plate (10) comprising gold-plated electro-formed nickel about 63 µm thick, with a plurality of nozzles (12) formed in said nozzle plate about 47 µm diameter, and with a resistor (18) about 64 µm x 64 µm formed in the bottom of each of said chambers and operatively associated with one of said nozzles, said ink feed chambers fluidically communicating with a source of ink (14) by an ink feed channel having a width 90 µm wide at the top thereof and wherein said viscosity of ink is at least about 7.5 cp at 30°C.

11. A method of increasing drop stability without increasing crusting or clogging or decreasing print quali-ty in a pen having a plurality of nozzles (12) in a nozzle plate (10) associated with a drop-firing means (18) for firing drops of ink (14) through said nozzles, said pen employed in an ink-jet printer, said method comprising employing an ink comprising a vehicle including water and diethylene glycol and a dye, said method comprising pro-viding an ink consisting essentially of about 40% water and about 60% diethylene glycol.