Note: Descriptions are shown in the official language in which they were submitted.
Z~4~
MULTITONE INK JET PRINTER AND METHOD OF OPERATION
Technical Fle~d
This invention relates generally to thermal ink
j~t printers and more particularly to a multitone ink ~et
printer and method having an improvecl grey scale operation.
~k~
Thermal ink j et printing has baen described in
many recent technical articles, such a~ an article by Kuhn &
Myers in Scie~ific Ame~ica~, 1985, at pages 162 through
15 178, and also in an article by J. B. Angell et al. also in
Sc ~ e~ April 1983 a~ page~ 44 through 55.
In the art of multitone ink jet printing, one
approach to printing a dot with one of eight grey scale
20 level~ i~ to e~ploy a ingle inX jet drop gen~rator and fire
it from one (1) to ~ven ~7) ti~es at a gi:~n pixel in order
to provide the pixel from on~ to seven levels of ink drop
volume. ~owever, this approach suffers at least two dis-
tinct disadvantage~ when used ln a thermal ink-jet printer.
The ~irst of th~ disadvantages is that the substantial
repeated use o~ a 3ingle drop generator and its a~sociated
heater resistor increa~e~ the wear and failure rate
(decreacad lif~time) o~ the ther~al ink jet print head. As
30 used harain, th~ ter~ "wear" is defined as the accu~ulation
of drop e;~ction cycle~ in a drop generator with ~inite
liPeti~es. Such li~etimes are typically measured in tens of
millions o~ cycles. Secondly, when Lnk i8 ejected in a drop
sequence from a single drop generator, thare is a certain
35 recovery time relatad to the bubble collapse associated
with each in~ drop e~ection from thQ drop generator~ This
recovery time obviou31y imposes a limitation on the maximum
achievable rate at which pixel9 are printed using this
method of thermal inX jet printing.
Another approach to multitone ink jet printing
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involvas the use of mul~iple inX jet drop generators and
firing these drop generators simultaneously in different
5 numbers to achieva different corresponding ink jet drop
volumes. To some ex~ent, the use of these multiple drop
generators as contrasted to a single drop generator can
increase the lifetime of the thermal ink jet printer. One
such approach is disclosed fsr example by T. Kawanabe in U.
10 S. Patent ~,4~53,079 issued October 5, 1982. However, the
thermal ink jet recorder apparatus of ~he Kawanabe patent
identified above i8 possessed with certain other disadvan-
tages related to the requirement for simultaneous firing of
the multiple drop generators therein.
In particular, since these drop generators of the
prior art are simultaneously fired at a single location, th~
nozzles must be critically aligned with respect to each
other so that the ink drops will properly register within
the pixel on the recording medium (paper). Furthermore,
20 this alignment is predicated upon a particular spacing
between the nozzles and paper, and maintaining this distance
is critical to achieving a simultaneous combination of these
drop volumes on the pixel. In addition, since each drop
generator in the Kawanabe recorder of U. S. Patent 4,353,07g
produces only one uni volume of ink, then anywhere up to
seven drop genera~ors must be fired simultanesusly to
achieve the variation o~ one to seven levels on the grey
scale. This requirement significantly increases the
30 complexity,cost and unreliability of printhead design, and
it also increases the total drop generator~use time ~or the
print head and, aqain, imposes a limitation on the useful
print head lifetime.
Disc~Qsure of Invent~ion
The general purpose of this invention is to pro-
vide a new and improved thermal ink jet printer and method
of operation which overcomes the a~oredescribed disadvan-
tages of the prior art and consequentlv provides a print
head of decreased drop generator design complexity and
characterized by an extended lifetime.
To accomplish this purpose, I have discovered and
developed an improved multitone ink jet printer and
method wherein a plurality of ink drop volumes are
provided in a drop generator structure and are weighted
in a predetermined binary sequence. The drop generators
which are each assigned a binary number corresponding to
a specific ink drop volume are sequentially fired at a
chosen pixel as they come into alignment with the pixel
as the printhead moves with respect to the paper or vice
versa. Thus, firing one to three binary-weighted drop
generators produces 1 to 7 volume units of ink within
the pixel. This process produces 1-8 levels of grey
scale. The total number of drop generators required in
the print head and the total drop generator use time is
minimized to thereby maximize print head lifetime with a
minimum of associated drop generator design complexity.
.:
In an alternative embodiment of the invention,
there is disclosed and claimed a method for reducing the
optical density of ink ejected into a given pixel area
during a halftoning printing operakion. This method
includes ejecting a drop of untoned li~uid, termed "ink
vehicle" in the art, into a given area of a pixel and
also ejecting one or more drops of ink with a fixed dye
loading into khe same area of the pixel. In this
manner, the dye is dispersed over a laryer area, and the
objectionable optical characteristics of single, small,
high-contrast dots standing alone are eliminated in
favor of a more uniform gray tone.
~n aspect of the invenkion is as follows:
A scanning printhead for firing ink drops in a
predetermined binary sequence including, in combination:
(a) a thin film resistor substrate having a
plurality of thin-film heater resistors thereon for
generating vapor bubbles in the ink to effect the
generation of ink-droplets,
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3a
(b) a barrier layer disposed on said thin film
resistor substrate and having a plurality of channels
defining respectively a plurality of drop generators,
(c) a drop generator orifice plate disposed on
: 5 said barrier member and having respectively a plurality
of different drop generator diameters representative of
said predetermined binary sequence of drop volumes and
disposed respectively over said plurality of heater
resistors and barrier channels forming a drop
generator, whereby said drop generators may be
seguentially first by the application of electrical
pulses to said heater resistors as said print head is
provided with relative motion with respect to paper
receiving ink droplets from said printhead.
A scanning printhead for firing ink drops in a
predetermined binary sequence including, in combination:
: (a) a thin film resistor substrate having a
; plurality of thin film heater resistors thereon for
generating vapor bubbles in the ink to effect the
generation of ink-droplets,
(b) a barrier layer disposed on said thin film
resistor substrate and having a plurality of channels
defining respectively a plurality of drop generators,
(c) a drop generator orifice plate disposed on
said barrier member and having respectively a plurality
of different drop generator diameters representative of
said predetermined binary sequence of drop volumes and
disposed respectively over said plurality of heater
resistor.s and barrier channels forming a drop generator,
whereby said drop generators may be seguentially fired
by the application of electrical pulses to said heater
resistors as said printhead is provided with relative
motion with respect to paper receiving ink droplets from
said printhead.
B
7~
3b
Brief Description of the_Drawin~s
Figure lA - lD are sequential isometric views
looking up through the paper at the bottom of a scanning
printhead having the "1", 'l29' and 9-4l- binary volumes for
ejecting ink onto a given pixel.
Figure 2 is a graph of optical dot density versus
print volume for the binary drop generator depicted in
Figures lA - lD.
Figures 3A and 3B illustrate isometrically, and in
B
~L%7~
cross-section respectively, a print head structure consti-
tuting a preferred embodiment of the invention.
Figures 4A-4E illustrate schematically an alterna-
tive embodiment of tha inv~ntion wherein untoned ink vehicle
is combined with an ink ~rop of fixed dye loading. This
process is carried out in order to obtain a mixture of ink
vehicle and ink on paper to produce a reduction in optical
lOdensity o~ the fix d dye loading to thereby eliminate
grairyness of small dots.
Figures 5A and 5B illustrate isometrically and in
plan view, respectively, a three-color, eight-level halftone
printer in accordance with the present invention.
Bes~Mode fQr~Ca~rying___t the Invention
Re~erring now to Figures lA-lD, there is shown in
sequence the firing of the binary-weighted "4", "2" and "l"
volume ink drops at the pixel p. The direction of scan for
20 a printhead lO is from left to right, and the "4" volume ink
drop "a" has printed into the pixel at displacement xl As
indicated in Figure lB, the "2" volume drop "b" is on its
way toward the pixel ~here it ubsequently combines with the
1-4-1 volume drop to provide a "4" + "2" combined volume spot
on the pixel at displacement x2 as indicated in Figure lC.
Also shown in Figure lC is the firing of the "l" volume ink
"c" which combines with the already deposited ~'4" + "2"
volumes of ink to give a seven volume ink spot or level
30 eight on the grey scale and completely covering the pixel as
indicated in Figuxe lD at displacement X3.
There i8 much less critical alignment with the
drop generator scheme of ~he present in~ention than w.ith the
approach of firing seven drop generators simultaneously.
35 Furthermore, since there are only three drop generators for
the eight grey levsls rather than the seven drop generator
scheme of the prior art, this feature results in a simpler
electrical interconnect requirement for the printhead heater
resistors, less complex driver electronics, a less complex
plumbing scheme for feeding ink to the three drop genera-
tors, and a more compact and reliable printhead.
Referring to Figure 2, the scheme wher~y combined
5 ink drops with total volume "1" through "7" produce eight
grey levels is de~cribed. Thi~ figure shows how optical
density of a pixel is increased fro~ the re~lectivity of the
untoned paper ("white") to the reflectivi~y of ink covering
the paper ("black") a~ succe~ively larger volume~ of ink
10 are applied to the region of a single pixel. The curve in
the figure obeys a typical rela~ion between spot area and
drop volume on a coatsd paper, and the total optical density
of the pixel-is computed ~rom th2 reflectivities and areas
of the toned and untoned re~ion~ within a pixel boundary.
15 Next to each print volume derived ~rom the ~ ', "2'l, and "4"
binary-weighted dot printars is shown schematically the
co~bination of drops producing this volume.
Referring now to Figures 3A and 3~, the partially
cut-away isometric vie~ of a printhead employing the three
20 drop generator sche~Q o~ the pre~ent invention includes a
9ilicon thin fil~ resiRtor sub~trate 30 which is fabricated
u ing silicon planar processing and thin film deposition
techniques which are w~ll known to thos~ skilled in the art.
The silicon ~bstrat~ 30 include~ a com~on ink feed-hole 32
25 in the form o~ a cylinder or ~lo~ throug~ substrate 30 and
configured u~ing diamond saw blade or laser drilling tech~
riques. As i~ also well known, thQ thin film resistor
silicon sub3trate 30 is constructed to have a plurality of
heater ra~i~tor~ 34, 36 and 3~ thereon, and these resistors
are configured to correspond to th~ different "1", '12" and
"4" volume ink channels 40, 42 and 4~ in the barrier layer
46. This ~arrier layer 46 i~olates individual drop yenera-
tor~ to reduc~ cros~talk and is i~portant to obtaining long
35 operating lifQ for the ~her~al lnk je~ re~i3tor~. Typically
the barrier layer 46 i~ ~a~rica~ed ~rom a well known commer-
cial polymer material with tha trade mark "VACREL", nickel,
glass, or some o~her mater~al impgrviou~ to attack by the
liquid ink contained therein.
The printhead in Figure 3A further includes a drop
genexator orifice plate 48 typically constructed of nickel
.2~
,
and includes the plurality of orifices 50, s2 and 54 for
ejecting tAe "1", "2" and "4" binary ink volumes, respec-
5 tively. These orifices 50, 52 and 54 will emit the "1", "2"and ~ l volumes of ink when their corresponding thin-film
resistors 34, 36 and 38 respectively are heated by the
application thereto of current pulses. T~e means by which
the ink bubble for~ on the heater resistor and provides the
10 energy to eject a droplet of ink is well known to those
skilled in the thermal inX j~ printing art, and is des-
cribed, ~or example, in the ~ewl et~ __rd Journal, Volume
36, Number 5, May 1985.
These current pulses are applied by way of
15 conventional surface metallization patterns (not shown), but
typically consisting of very thin conductive traces of
aluminum or gold on th~ upper ~urfac~ of the silicon sub-
strate 30 and d~po~ited u~i~g ~tandard evaporation processes
well known in the axt.
Although th~ substrat~ memher 30 is referred to
herein as a "silicon sub3trate", it will be unders~ood by
those s~ill2d in the art that the sub~trate 30 will ~ypical-
ly by a thin film compo~it~ or layered s~ructure wherein a
first layor of silicon dioxide Slo2, will be grown or
depo ited on a ~ilicon ~ub~trata sur~ace to provide surface
pasivation thereo~, and then a resistive layer such as
tantalum-aluminum will b~ ~po~ited on the Slo2 layer. Next
the conductivQ traca material will be evaporated on the
tantalum-aluminu~ lay~r and lithographically defined so that
opening~ in this trace mat~rial define ~he boundaries of the
heater re~istora. Finally, a inert outer passivation or
barrier layer ~at~rial su¢h as silicon carbide, SlCl, or
silicon nitrid~, Si3N4, or tantalu~ pentoxide, Ta205, or
combination thereo~ in succes~iv~ layex~ is for~ed on ~he
sur~ace of the al~minu~ trace material and the exposed
heater resistors in order to provide a good isolation bar-
rier layer betwee~ the heater resistorq and the inX in the
reservoirs above these heater resistors. This inert barrier
layer is necessary due to the highly corrosive nature of the
ink and al~o because of ther cavitation-produced wear during
., i
~I!l27~3~fi~l
ink ejection from the drop generators. Therefore, as used
herein, "silicon substrate'l is intendad to mean a thin film
5 re~is~or silicon sub~trate of the a~ove type of layered
structure.
Referring now to Figures ~ through 4E, there is
illustrated an alternative embodiment of my invention which
includes an additional drop generator which is indicated as
- 10 drop generator 60 in Figure 4A. This generator 60 produces
untoned droplets of fluid which is the ink without toning
dyes. This liquid is known in the art aa the ink vehicle.
The volu~ of the untoned drop will be betw2en the "1" and
"4" volume. When a ~ingle drop 64 is ejected from the
15 printhead onto the paper 65 a~ indicatad in Figure 4B, it
will typically exhibit an ~ptical density profile in the
for~ indicated in Figure 4C. In this Figure, re~erence
number 66 indicate~ th~ optical den ity profile acxoss the
diameter of printed dot 67. A~ a result o the sharp steep
20 profile 66 of optical density at the edge, the dot 67 in
FigurQ 4C, when se~n at a normal viewing distance, will
produce a grainylik~ charactQri~tic a~ perceived by the eye
and will consequently re ult in undesirable shading ~r
grainyne~s for c~rtain image printing and background appli-
25 cations - particulary in th~ reproduction of highlights in
continuous tona imag~s. Thus, it is frequently de~irable to
remove thi~ grainylike characteri~tic and thi~ may be
accomplish~d by th~ utilization og an untoned liquit vehicle
drop 68 as indicated in Ftguro 4D. This vehicle drop 68 is
ejacted onto t~a same pixel area a~ the toned ink drop 64
and thi~ produces a certain quanti~y o~ vehicle volume plus
"1" toned ink volum~ in area 69 a3 indicated in 4~ and
having an optical density pro~ 70 which has a maximum
35 valu2 in the center portion 72 ther~of and then decreases
out to the ~dges o~ the tonod area 69.
Referring now to Figures 5A and 5B, there i5
shown, in iso~etric and plan view~ respectively, a ~hree-
color, eight-level halftone printhead 73 including three
foam filled reservoir~ 74, 76 & 78 of the type disclosed and
claimed in U.S. Patent No. 4,771,295
fiO
of Jeffrey Baker, assign d to the present
assignee. These foam filled reservoirs 74, 76 and 78 are
5 filled with inks having dye colors of cyan, magenta and
yellow, respectively for subtractive col3r printing. These
reservoirs 74, 76 and 78 are connected using known fluid
interconnect technique~ to an outer orifice plate 82 with
the cyan, magenta and yellow orifice patterns 84, 86 and 88,
respectively. The orifica pattern 84, 86 and 88 are shown
in an enlarged plan view in Figure 5~ and include n-rows of
the "4", "2", "1" binary drop generators in each of the
cyan, magenta and yellow column8 a~ indicated. Thus, each
of the cyan, magenta and yellow column~ may have a~ many n-
15rows o~ "4", "2", "l" binary drop generators as may benecessa~y to provide a de~ired gua~tity oP these side -by
side ink dispen ers for an eight-levol three-color
halftoning printing operation. ~dditionally, each o~ the n-
rows may be modifi2d to include a ~ingla clear fluid or ink
20 vehicle drop generator (no~ sh~wn) if it becomes desirable
for some reason to di~fuse on~ or more of the colors
produced in order to r~duGs edge sharpness of isolated,
color printed dots. This may b2 d~sirable for example when
printing color dot~ (in the ~anner described previously to
provida a more uniform coverag~ of a colored region within a
pixel).
Various ~odifications may be mad~ in the above-
describ~d embodim~nts without departing from the spirit and
30 scop~ of thi~ invention. For example, the present invention
is not limited ~pec~ically to tho "4", "2", -1t~ binary
sequence indicated, and instead m~y employ vàriations of
this binar,v sequence in accordance with tha particular type
of grey level i~aging operation being performed. In
35 addition, multicolor printing may be produce~ by individual
color pen~ (cyan, yellow, magenta, black) registered and
aligned with respect to one another 90 a~ to print succes-
sively within the boundary o~ a single pixel to produce
mixing of drop~ of various volumes and color~. Furthermore,
the present invention is not limited to the particular 3-
color printhead of Figure 5 wherein th~ differen~ colors are
``` ~ 27~
all located in adj acent columr~s in an integrat~d ink storage
- and orifice plate structure. In~tead, the cyan, magenta,
5 yellow and black inks may be spaced apart by a chosen
distance compatible with a particular color printing
appl ication .
