CA1300972C

CA1300972C - Barrier structure for thermal ink-jet printheads

Info

Publication number: CA1300972C
Application number: CA000568284A
Authority: CA
Inventors: Howard Hyman Taub; Gordon D. Denler
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1987-06-02
Filing date: 1988-06-01
Publication date: 1992-05-19
Anticipated expiration: 2009-05-19
Also published as: EP0294032A3; EP0294032A2; EP0294032B1; JPS63307957A; HK91694A; US4794410A; DE3886266D1; DE3886266T2; JP2752374B2

Abstract

ABSTRACT OF THE DISCLOSURE

A three-sided barrier structure (22), comprising three walls (24a-c), is provided in conjunction with a resistor (10) used in a thermal ink-jet printhead. Place-ment of the structure less than about 25 µm from the re-sistor results in longer resistor life and an improvement in the static bubble purging ability of the printhead.
Two-slded barrier structures (14), comprising two walls (16a-b), associated with the resistor at spacings less than about 25 µm also improve the resistor life.

Description

i3~0g~2 ~AR~I~R STRUCTURE FOR
T~ERMAL IN~-JET PRINTHEADS

1 T~CHNICAL FIELD

The present lnventlon relates to lnk-~et prlnters, and, more partlcularly, to lmproved thermal ink-~et print-heads employed ln such prlnters.

BACKGROUND ART

In thermal lnk-~et prlntheads, thin film resistors are employed as heaters to form a bubble of lnk over the resistor surface. ~he growth and collapse of the bubble causes an lnk droplet to be e~ected from an orlfice asso-clated with the re~lstor. The eJected droplet of lnk 18 dlrected toward a medlum, such as paper.
At a predetermined tlme, as determlned by a slgnal sent to the printer from, say a computer, the resistor is heated (by I2R heating) to a temperature sufficient to vaporize a thin layer of lnk dlrectly over the resistor, which rapldly expands into a bubble. This expansion, in turn, causes part of the lnk remalnlng between the re~ls-tor and the orlflce to be expelled through the oriflce toward the ~edlum. In present use, the reqlstor is heated to provlde a surface te~perature of a few hundred degrees, at repetltlon frequencieQ up to 50 ~Hz and above. How-ever, heating of the reslstor ltself lasts less than about10 psec.

13~t(:~972 1 The presence of wall-like structures, commonly called "barrlers", ln ~he immedlate viclnlty of a thermal lnk-Jet resistor has slgnificant effects on the performance of the device.
When a vapo~ bubble collapses over a resl~tor whlch has no barrler ~tructuro ln its lm~edlate vicinlty ~bar-rlers whlch are several mlls away have little effect), the event approximately has axlal symmetry wlth the flnal collapse point at the center of the reslstor. In thls case, fluid can flow freely from all directions as the bubble collapses.
When a wall or barrler 18 placed near the reslstor, reflll cannot occur from thls dlrectlon, thus the bubble appears to be pushed towards the wall by fluid filllng from all other dlrectlons. A slngle-~lded barrler struc-ture for an array of resistors 18 lmpractical to lmple-ment, slnce lt would not actually lsolate ad~acent resls-tors, whlch 18 the orlglnal function of the barrier.
A two-slded barrler conflguratlon causes reflll to occur from two dlrectlons: the flnal stages of bubble collapse occurs ln an approxlmate llne across the center of the resistor. Thus, the slngle collapse point (whlch ln prac-tlce may be a small area) 18 spread lnto a llne whlch reduces the rate or magnitude of impacting at any one polnt on the llne. Nowever, the bubbie collapse attalned does permlt bubble collapse on the reslstor and does per-mlt reflll to occur from more than one directlon.
Three-slded barrlers have been shown, but due to thelr configuratlon, have not resulted ln improvlng resls-tor life or expulsion of statlc bubbles. See, for exam-ple, U.S. Patents ~,502,060; 4,503,~4~; ~,5~2,389; and 4,550,326.

~3~9~:~
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DISCLOSURE OF INVENTION

In accordance with the invention, a three-sided barrier structure adjacent a resistor in a thermal ink-jet print-head can provide a number of advantages if placed within certain critical distances. Placement of such barrier less than about 25 ~m from such resistors can provide (1) an increase in the life of a resistor by helping to sweep away the collapsing bubble from the center of the resistor and ~2) an improvement in the self-purging by the printhead of static bubbles.
A two-sided barrier structure, if placed less than about 25 ~m from the resistor, also privides an increase in the life of the resistor. However, the self-purging of static bubbles is not as readily attained as for the three-sided barrier structure.
Various aspects of the invention are as follows:
A thermal ink-jet printhead including at least one resistor for firing droplets of ink normal to the plane of said resistor toward a medium, characterized by a three-sided barrier structure having three walls and encompass-ing said resistor to provide an open side for replenishing of ink from a reservoir, each said wall of said barrier structure spaced from said barrier, said spacing being less than about 25 ~m from an edge of said resistor.
A method for extending resistor life of a resistor employed in a thermal ink-jet printhead, said resistor adapted to eject droplets of ink normal to the plane of said resistor, said method comprising providing a barrier structure having three walls and placing each wall less ,1 ~

than about 25 ~m from said resistor, heating said resistor to form a vapor bubble for ejecting a droplet of ink, and collapsing said vapor bubble and sweeping said collapsing vapor bubble away from the center of said resistor thereby extending the life of said resistor.
A method for purging static bubbles from a resistor employed in a thermal ink-jet printhead, said resistor adapted to eject droplets of ink normal to the plane of said resistor, said method comprising providing a barrier structure having three walls and placing each wall less than about 25 ~um from said resistor, heating said resistor to form a vapor bubble for ejecting a droplet of ink, collapsing said vapor bubble and sweeping away said collapsing vapor bubble, and self-purging static bubbles by confining said static bubbles to the immediate vicinity of said resistor.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-3 illustrate the collapse of a vapor bubble at the center of a resistor for (1) a resistor with no neighboring barrier structure: (2) a resistor with a two-sided barrier structure in accordance with the invention;
and (3) a resistor with a three-sided barrier structure in accordance with the invention.
BEST MODES FOR CARRYING OUT THE INVENTION
Referring now to the drawings wherein like numerals of reference designate like elements throughout, a resistor 10 is depicted. In the following description, in each case, the ink droplet is ejected normal to the plane of the resistor. This is in contrast to configurations, in which the ink droplet is ejected parallel to the plane of the resistor.

. ~

~3~97;~

1FIG. la illustrates a top plan vlew of a resistQr 10 with no nelghborlng barrler structure. FIGS. lb-d are line drawings of a portion of a photographic sequence showing how a vapor bubble 12 collapses near the center of 5the resistor 10. The lifetlme of the resistor 10 18 typl-cally less than about 20 x 1o6 flrlngs.
FIG. 2a illustrates a top plan view of a resistor 10 with a two-sided barrler structure 14 comprlslng two walls 16a, 16b. FIGS. 2b-d are llne drawings of a portlon of a photographlc sequence showing a bubble 18 elongatlng across the width of the reslstor 10 as lt collapses, fi-nally breaking up into several bubble fragments before vanishlng completely.
It i8 seen that for the two-slded barrler conflgura-lS tion depicted, the bubble collapses in a band across thecenter of the resistor 10. Such bubble collapse ls at-talned 80 long as the dlstance from the edge of the resls-tor 10 to the wall 16 18 less than about 25 ~m, as dls-cussed below ln connectlon wlth the three-slded barrler structure.
In conflguratlons wlth dlstances greater than about ~m, the bubble collapse 18 slmllar to that attained with no barrler structure. Thus, the bubble collap~e band ls an lmprovement over an essentlally bubble collapse point, and accordingly, llfetime of the resistor 18 ln-creased. For example, the llfetime of the reslstor 10 where the walls 16 are greater than about 25 ~m from the reslstor ls typlcally less than about 20 x 106 flrlny~, whlle the llfetlme of the reslstor where the walls are less than about 25 ~m from the reslstor may range up to about 100 x 106 flrlngs.
However, the bubble does not move off the reslstor 10 unless the barrlers are offset, that 18, closer on one slde than on the other. An offset two-slded barrler may, therefore, be acceptable.

~3VV~

1 While a parallel conflguratlon 18 depicted, lt wlll be appreciated that non-parallel conflguratlons, as well as varlations of parallel conflguratlons, e.y., a "brack-et" shape, may also be employed ln the practlce of the s lnvention.
Flnally, ~t~tlc bubbl~ ellmlnatlon, achleved wlth the three-slded barrler structure, as descrlbed below, is not attalned wlth the two-sided barrler structure 14, even wlthln the lnd~cated dlstance separatlon. Nonetheless, slnce resistor llfetlme lmprovement 18 attained, this configuration 1Y considered to fall wlthin the scope of the lnventlon.
FIG. 3a illustrates a top plan vlew of a reslstor 10 wlth a three-slded barrier structure 22 ln accordance wlth the lnvention. The barrler structure comprises three walls 24a, 24b, 24c. FIGS. 3b-d are line drawings of a portion of a photographic sequence showing a collapsing bubble 26 whlch i9 shlfted toward the third side 24c of the barrier structure 22 by the refllllng llguld (not shown) whlch enters from the open slde of the barrler structure, as indlcated by arrow 23. The final stages of bubble collapse take place off the reslstor 10, forming bubble fragments 30 along the rear wall 2~c.
The three-sided barrler structure 22 of the lnventlon may comprlse, for example, a block U-shaped conflguration, with the re~lstor 10 placed in the blght of the U, as deplcted ln FIG. 3a, or variants thereof, 80 long as one side remains open for entry of ~nk, indicated by arrow 23, from an lnk reservolr ~not shown).
It should be noted that the photographs upon which the line drawlngs of FIGS. lb-d, 2b-d and 3b-d are based were for a pond test and that the detalls of the collaps-ing bubbles ln a completely as~embled prlnthead ~with an oriflce plate - not shown) may be somewhat dlfferent.
However, the basic prlnclples would remaln the same.

13~97~

1 The three-sided barrier structure 22 of the inventlon should be placed such that none of the walls 24a-c are no further than about 25 ~m from the reslstor 10. Such placement provides an lncrease ln the life of the reslstor 10 by helplng to sweep away the collapsln~ bubble from the center oS the re~l~tor, ~e ehown ln F~S. 3b-d. For exaa-ple, the llfetlme of the reslstor 10 where the walls 24 are greater than about 25 ~m from the reslstor ~8 typlcal-ly less than about 20 x 106 firlngs, whll~ the lifetlme of the re~istor where the walls are leqs than about 25 ~a from the resistor may range up to about 200 x 106 flrlngs.
Where the walls 24 are less than about 10 pm from the reslstor 10, the llfetlme may exceed 200 x 106 flrlngs.
Sweep~ng the collapslng bubble from the center of the reslstor 10 lncreases the llfe of the reslstor, since cavltation, whlch 18 a problem wlth structures of lcss than three sldes, 18 ~reatly reduced. Such cavltatlon results ln a shock wave whlch strikes the saae area ~typl-cally the central area) on the resistor 10 each tlme the reslstor 1B pulsed to flre a bubble. The cavitatlon ef-fect lead~ to erosion of the bubble collapse area and concomltant early fallure of the reslstor. Thl8 problem 18 further exacerbated by the fact that the center o~ the resistor 10 18 also the hottest reglon, and the colncl-dence of the bubble collapse area wlth the center of thereslstor results ln addltlonal eroslon.
U8e of the three-slded barrler structure 22 of the lnventlon and placement thereof less than about 25 ~m froa / the reslstor 10 also¦placeaent~provldes an lmproveaent ln the self-purglng by the prlnthead of statlc bubbles.
Statlc bubbles (not shown) contaln ~ases rather than va-porlzed lnk vehlcle and enter the head by a varlety of mechanlsms. Their "collapse", by dlssolvlng back lnto the lnk, can take from about 10 to lO9~1on~er than vapor bub-bles, dependln~ on thelr slze.

~ f ~ ._ i3~)V97Z

1 Preferably, the barrler 22 should be wlthln about 10 ~m of the re~lstor 10, and most preferably wlthln about 5 pm, ln order to fully reallze the beneflts of the sweeplng effect. Al~o, accumulatlon of ~lcrobubbles and ~rowth thereof on the walls 24a-c of the barrler 22 lo mlnlmlzed a8 the walls are moved clo-or to th~ reslstor, especlally ln the range of less than about lo ~m.
Asymmetrical placement of the barrler structure 22 about the reslstor 10 18 not crltlcal, ~o long as the max~mu~ dlstance llsted above 18 not oxceeded on any of the three sldes ad~acent a barrler wall 24. It appears that the smallest dlQtance between the reslstor 10 and the wall 24 controls where the bubble wlll move to. However;
it will be remembered that static bubbl~s tend to be stored ln large spaces, 80 that whlle ~ome misalignment between the reslstor 10 and the barrler stxucture 22 1 acceptable, such mlsallgnment should be mlnlmlzed.
The barrier structure 22 may comprlse sultable poly-meric or metallic ~aterlals. ~xamples of such materlals include dry Sllm reslsts, such as VACREL and RISTON, avallable from E. I. duPont de Nemours ~Wllmlngton, DE), polyimlde composltlons, plated nlckel, and the llke.
The three-slded barrler structure 22 of t~e lnven-tlon, wlth walls 24 wlthln the crltlcal dlstance of the reslstor 10, afford several advantages over one- and two-barrl~r configuratlons. Flrst, because reflll 18 ~rom one dlrectlon, the collapslng bubble 26 lo sw~pt off th~ re-slstor toward the "bac~" barrler wall 24c. There lo also a tendency for the bubble 26 to dlvlde lnto several co~po-nent~ 30, whlch weakens the collapse energy at any glvenpolnt.
Further, the barrler structure 22 asslstQ the purglng of ~tatlc bubbles whlch may have several orl~lns: (lJ alr trapped ln the prlnthead when lt 18 flrst fllled wlth lnk;
~2) gases dlssolved ln the ln~ whlch co~e out of oolutlon;

g~2 1 (3) air gulped in from outslde durin~ operation due to a menlscus folding back on itself; (4) ga eous products of chemical corrosion; and (5) agglomeratlon of microbubbles.
With other prior art approaches, when a statlc bubble resldes in the lm~ediate neighborhood of the resistor 10, it rec~lves a otron~ lmpuloe ~orc~ ev~ry tlme a vapor bubble exposlon occurs; this moves the statlc bubble to another locatlon. Wlth the three-slded barrler structure of the inventlon, the bubble 18 conflned to remaln ln the lmmedlate vlclnlty of the reslstor by three physlcal wall-~ 24a-c and one vlrtual wall, whlch 18 the refill flow from the fourth dlrectlon, shown by arrow 28 in FIG. 3a.
It 18 also possible for the statlc bubble to be moved lnto the fluld region dlrectly above the reslstor, in which case it may be eJected fro~ the printhead wlth the next drop. In fact, this may be expected to happen even-tually after some number of impulses.
For one- or two-slded barrlers, the static bubble may move away from the resistor to a re~ion where the vapor explosion force cannot lnfluence lt (although the static bubble may have a large effect on devlce operation). It should be noted that this problem i8 likely to occur with placement of the three-slded barrler 22 at a distance much ~reater than about 25 ~m from the reslstor 10, slnce the bubble can be trapped between the reslstor and the bar-rler wall and not be influenced by vapor bubble explo-sions.

I~DUST~IAL APPLICABILITY
Two- and three-slded barrier wall confl~uratlons assoclated with reslstors used in thermal lnk-~et prlnt-ers, spaced less than about 25 ~m from such reslstors, are expected to flnd use ln prlnter~ to improve resistor life 13(~72 1 and, ln the caQe of three-sided barrler structures, otatlc bubble purglng ablllty of the prlnthead.

Thus, two- and three-slded barrier wall conflgura-tlons, to be used ln asqoclation wlth a resistor employed ln a thermal lnk-Jet prlnthead and sp~ced no more than about 25 ~m from the reslstor, have been dlsclosed.
Placement of such barrlers withln the crltlcal dl~tance from the re~lstor results ln longer reslstor life and, ln the case of three-slded conflguratlons, an lmprovement ~n the statlc bubble purglng ablllty of the prlnthead. Many modlflcatlons and changes of an obvlous nature wlll make themQelve apparent to those of ordlnary 8~111 ln the art, and all such modlflcatlon~ and change~ are deemed to fall wlthln the scope of the lnventlon, as deflned by the ap-pended clalmQ.

Claims

1. A thermal ink-jet printhead including at least one resistor for firing droplets of ink normal to the plane of said resistor toward a medium, characterized by a three-sided barrier structure having three walls and encompass-ing said resistor to provide an open side for replenishing of ink from a reservoir, each said wall of said barrier structure spaced from said barrier, said spacing being less than about 25 µm from an edge of said resistor.

2. The printhead of claim 1 wherein said walls are connected so as to form a substantially U-shaped structure, encompassing said resistor in the bight thereof.

3. The printhead of claim 1 wherein each said wall is less than about 10 um from said resistor.

4. The printhead of claim 3 wherein each said wall is less than about 5 µm from said resistor.

5. A method for extending resistor life of a resistor employed in a thermal ink-jet printhead, said resistor adapted to eject droplets of ink normal to the plane of said resistor, said method comprising providing a barrier structure having three walls and placing each wall less than about 25 µm from said resistor, heating said resistor to form a vapor bubble for ejecting a droplet of ink, and collapsing said vapor bubble and sweeping said collapsing vapor bubble away from the center of said resistor thereby extending the life of said resistor.

6. The method of claim 5 wherein said walls are connected so as to form a substantially U-shaped structure, encompassing said resistor in the bight thereof.

7. The method of claim 5 wherein each said wall is placed less than about 10 um from said resistor.

8. The method of claim 7 wherein each said wall is placed less than about 5 µm from said resistor.

9. A method for purging static bubbles from a resistor employed in a thermal ink-jet printhead, said resistor adapted to eject droplets of ink normal to the plane of said resistor, said method comprising providing a barrier structure having three walls and placing each wall less than about 25 µm from said resistor, heating said resistor to form a vapor bubble for eject-ing a droplet of ink, collapsing said vapor bubble and sweeping away said collapsing vapor bubble, and self-purging static bubbles by confining said static bubbles to the immediate vicinity of said resistor.

10. The method of claim 9 wherein said walls are connected so as to form a substantially U-shaped structure, encom-passing said resistor in the bight thereof.

11. The method of claim 9 wherein each said wall is placed less than about 10 µm from said resistor.

12. The method of claim 11 wherein each said wall is placed less than about 5 µm from said resistor.