Note: Descriptions are shown in the official language in which they were submitted.
7~i
~1DF 002 P2 -l-
FLUID JET PRINT H~AD
Background of the Invention
The present invention relates to a fluid jet
print head and, more particularly, to a print head and
method for generating at least one stream of drops in
which construction and operation of the print head ar~
facilitated.
Jet drop printers and coating devices operate by
generating streams of small drops of ink or coating fluid
and controlling the deposit of the drops on a print
receiving medium. Typically, the drops are electrically
charged and then deflected by an electrical field. The
drops are formed from fluid filaments which emerge from
small orifices. The orifices communicate with a fluid
reservoir in which fluid is maintained under pressure.
Each El~1id Eilament tends to break apart at its tip to
form a stream of drops. In order to deELect drops
acc~lrateLy by means of an el.ectrical field and produce
selective cleposition of the drops on the print receiving
nlc~diuln, iL i~. r1ece~sary lor th~ drop~q to be sub.qtantial1y
uniEorm in size and ln interdroE~ spacin~ within each
stream. 'L'he brealc up of the fiLaments into streams of
drops is Eacilitated by mechar1ical vibration of some
portion or all of the print head structure in a process
termed "stimulation".
One prior art stimulation technique, as shown in
tl.S. patel1t No. 3,739,393, is~ued June 12, 1973, to Lyon
et al, is to provide the fluid oriEices in a relatively
thin, flexible wall of the fluid reservoir and to
stimulate this wall, known as an "orifice plate", by
causing a series of bending waves to travel along the
MDF 002 P2 -2- ~2~6
plate. This technique results in substantially uniform
drop size and spacing but the timing of break up of the
fluid filaments varies along the length of the orifice
plate.
Another approach is to vibrate the entire print
head, including the ink manifold structure and the orifice
plate structure, together. This is shown in U.S. patent
No. 3,586,907, issued June 22, 1971, to Beam et al. Such
an arrangement will necessarily fatigue the print head
mounting structure, since the mounting structure
experiences the same vibrations which are applied to the
manifold and the orifice plate. Further, the amplitude
and phase of the vibratory motion are difficult to control
at the frequencies commonly used for jet drop printer
operation.
A further approach to filament stimulation is
disclosed in U.S. patent No. 4,095,232, issued June 13,
197~, to Cha. U.sing the tecllllique discl.osed in this
patent, stimulators mounted in the upper portion oE a
Eluid reservoir generate pressure waves which are
transmitted downward through the Eluid. ~ach stimulator
ir)cludes a pair oE piezoelectric crystal.s which vibrate in
pllase and which are mounted on opposite sides oE a
mountillg plate which is coincident with a nodal plane. A
reaction mass is positioned at the end oE each stimulator
opposite the stimulation member which is coupled to the
fluid. The reaction mass ensures that the nodal plane is
properly positioned.
In British patent specification 1~22388, a print
head is disclosed in which a piezoelectric crystal forms
one wall of a single-jet ink jet print head. When a drop
MDF 002 P2 -3- ~2~9~
is to be emitted from the orifice, the piezoelectric
transducer is electrically actuated, causing it to distort
and thereby forcing a drop from the orifice.
In British patent specification 1293980,
published October 25, 1972, and U.S. patent No. 4,19~,643,
issued April 15, 1980, to Cha et al, print heads are
disclosed in which a pair of piezoelectric crystals are
bonded to opposite sides of a support plate. A print head
manifold structure is bonded to one of the piezoelectric
crystals and a counterbalance is bonded to the other of
the crystals. The weight of the counterbalance is
selected so as to offset the weight of the print head
manifold. By this balanced arrangement, the support plate
is placed in a nodal plane when tlle two piezoelectric
transducers are energized in svnchronism. It will be
appreciated, however, that the construction Oe such a
print head is relatively complicated and, Eurther, that it
is diEEiclllt to design such a print heacl to be resonant at
a desire.l Erequellcy. r~'he print head m~lst be tuned
subsecluell~ to construction, therecore, such that the
resonant Erequency oE t~e print llead equals che desired
operating Erequency.
Flnally, in U.S. pa~ell~ No. 3,972,47~, issued
~ugust 3, 1976, to i~eur, an ink drop writing system is
showll in whicll a vibratin-3 nozzle is used to produce a
stream of drops. The length oE the nozzle is selected so
that its mechanical resonallt frequency is much higher than
the frequency at which it is driven. The nozzle,
configured as a tube, is surrounded by a piezoelectric
ring which, when electrically driven, provides radial
contraction and expansion of the tube.
MDF 002 P2 -4~ 977~
There is a need for an improved fluid jet print
head in which uniform in-phase stimulation may be provided
for a plurality of jet drop streams, in which mounting oE
the print head is facilitated, and in which construction
and design of the print head are simplified.
Summary_of the Invention
A fluid jet print head for generatinq at least
one stream of drops comprises an elongated print head
body, the length of the body between first and second ends
thereof being substantially greater than its other
dimensions. The body defines a fluid receiving reservoir
in its first end and at least one orifice communicating
~ith the fluid receiving reservoir. Fluid is supplied to
the reservoir under pressure by appropriate means such
that it emeryes from the reservoir to Eorm a fluid
stream. ~ transducer means is mountec~ on the exterior of
the body and extends a substantial distance a:Long the body
in the direction of elongation Erom adjacent the support
means toward both the Eir~st ancl second ends oE the body.
Ihe tran-;~lucer means is responsive to an electrical
drivincJ sigrla:L Eor changillg dimcnsion in the direction of
elongatiorl oE the body, thereby causing mechanical
vibrcltion oE the body and break up oE the f:luid stream
into a stream oE drops.
rile transducer means compri.ses a pair oE
piezoelectric transducers bonded to opposite sides of the
hody and extending in the direction of elongation from
points adjacent the first end to points adjacent the
second end of the body. The piezoelectric transducers
provide alternate lengthening and contraction of the
elongated print head body in the direction of elongation
of the body.
MDF 002 P2 -5-
l9~;
The transducer means further comprises means for
electrically connecting the pair of transducers in
parallel, whereby the transducers operate in phase so as
to produce vibration which is in a direction substantially
parallel to the direction of elongation of the elongated
print head body. A support means for the print head
engages the print head body intermediate and substantially
equidistant from its first and second ends.
Alternatively, the transducer means may comprise
means for electrically connecting the transducers so that
they operate out of phase, thus producing flexure waves.
The support means for the print head engages the print
head body a distance from each end of the body
approximately equal to .23 of the overall length of the
body.
F'or vibration parallel to the direction of
elongation, the support means may comprise a pair of
mountinq Elanges, each inteqrally formed with the print
head body, and being relatively thin. The flanges extend
Erom the elongated print heacl hocly on opposite si.des
thereoE ancl are substantially equidis~ant from the first
ancl secolld encls o~ the bocly such that they support the
bocly alonc3 a noclaL plane. Alternatively, the support
means may compri.se a pair of support screws which engage
the body at opposite sides thereof at points substantially
ecTuidistant from the first and second ends of the print
head body.
The print head body includes means defining a
slot in the first end thereof and orifice plate means,
attached to the means defining a slot, and forming the
fluid receiving reservoir therewith. The orifice plate
~IDF 002 P2 -6- ~2~
means may define a plurality of orifices for production of
a plurality of drop streams. The print head body may
further define a Eluid supply opening and a fluid outlet
opening communicating with the slot. The fluid jet print
head may further lnclude fluid conduit lines connected to
the fluid supply opening and the fluid outlet opening.
The fluid conduit lines are formed of a material having a
substantially different vibrational impedance than the
print head body, whereby the conduit lines do not provide
a substantial power loss. The fluid conduit llnes may,
for example, be made of a poLymer material.
The fluid jet print head may further include
means for applying an electrical driving signal of a
frequency substantially equal to fo = C/2L, where L is
the dimension of the body in the direction of elongation,
and C is the speed of sound through the body. In this
case the fluid ~et print head is driven at a Erequency
approximating its mechanical resonant: Erequency.
r~or Elexure wave vibration, the tran.sducers are
driven at a Erecl-lency Fo ~ /L2~ wh(?re a is the
transverse thickness oE the print head body and ~ -1.7.
In this case, two nodal mounting axes are established a
distance equal to approximately .23 oE the length oE the
print heacl body/ centerecl betwe~en the transducers.
The mettlod Eor stimulating the break up of a
fluid stream emanatLng from at least one orifice
communicating with the fluid reservoir in a fluicl jet
print head includes the steps oE:
(a) providing an elongated print head which
defines the reservoir and the orifice at
one end thereof;
MDF 002 P2 -7-
(b) applying fluid under pressure to the
reservoir so as to produce fluid flow
through the orifice;
(c) supporting the print head at points in a
plane substantially equidistant from the
ends of the elongated print head and normal
to the direction of elongation of the print
head; and
(d) alternately elongating and contracting the
print head substantially at the resonant
frequency of the print head, whereby the
print head is supported in a nodal plane and the
stream is eEfectively stimulated to break up
into drops.
The resonant frequency of the print head
may be substantially equal to the resonant frequency
o~ the fluid stream. The print head may be
elongated and contracted by means of piezoelectric
trans-lucers bonded to its exterior.
The stream may also be stimulated by
operatincJ the transducers ouk of phase, thereby
causing ~lexure of the print head. In this
stimuLation mode, the print head is mounted at
points which are a distance Erom each end which are
approximately equal to ~23 times the length of the
print head.
Accordingly, it is an object oE the present
invention to provide a fluid jet print head for
generating one or more streams of drops in which the
print head includes an elongated body which is
driven to elongate and contract in the direction of
MDF 002 P2 -8- ~9~7~
elongation of the body; to provide such a print head
and method in which the print head is driven by
means of thin piezoelectric transducers bonded to
the print head exterior; and to provide such a print
head in which support for the print head is provided
in a nodal plane.
Other objects and advantages of the
invention will be apparent from the following
description, the accompanying drawings and the
appended claims.
Brief Description of the Drawings
.
Fig. 1 is an exploded view, illustrating a
first embodiment of the fluid jet print head of the
present invention;
Fig. 2 is a plan view oE the print head oE
Fig. 1, with the orifice plate removed;
Fig. 3 is a side view oE the print head oE
Fig. l with the electrical drive circuitry
illustrated;
Fig. 4 is an enlarc3ecl partial sectional
view, taken c~enerally along line 4-4 in Fig. 2;
Fic~. 5 is a graph, use~ul in explaining the
operation oE the print hea~3 oE the present invention;
Flig~ 6 is a graph, useful in e~plaining
operation oE the print head of the present invention.
Fig. 7 is a schematic diagram illustrating
driving circuitry for the fluid print heacl; and
F`ig. ~ is a side view oE a second
embodiment of the fluid jet print head of the
present invention.
76
MDF 002 P2 -9-
Detailed Description of the Preferred Embodiments
The present invention relates to a fluid
jet print head oE the type which may be used for ink
jet printing, coating, textile dyeing, and other
purposes. As is known, such devices typicall~
operate by electrically charging the drops in one or
more jet drop streams and, thereafter, deflecting
the trajectories of some of the drops by means of
electrical fields. In order to produce the stream
or streams of drops, fluid is typically applied to a
fluid reservoir under pressure such that it then
flows through one or more orifices or nozzles which
communicate with the reservoir. The fluid emerges
from the orifices as fluid filaments which, i~ left
undisturbed, would brealc up somewhat irregularly
into drops of varying size and spacing. It is not
possible to charge and deflect such nonuniform drops
accurately and, as a consequellce, jet drop devices
have typically applied mechanical stimulation in
some Eashion to the f]uid Eilaments so as to cause
break up oE the filame~nts into clrops of generally
uniEorm size allcl spacin~ at a deqired clrop break up
frequency.
A first ernbocliment of the print head of the
present invention is shown in Figs. 1-4. ~rhe print
head generally includes an elongated print head body
lO, the Length of which, L, is substantially greater
than its other dimensions a and b. The body lO
includes an orifice plate 12 and a block of material
14. The body lO defines a fluid receiving re,ervoir
16 in its first end, and at least one and preferably
~DF 002 P2 -10-
a number oE orifices 18 which are arranged in a row
across orifice plate 12. The orifice plate 12 is
bonded to block 14 of material, such as stainless
steel by means oE a suitable adhesive. Block 14
defines a slot 20 which, in conjunction with orifice
plate 12 defines the reservoir 16. The block 14
further defines a fluid supply opening 22 and a
fluid outlet opening 24, both of which communicate
with the slot 20.
The print head further includes means for
supplying fluid to the reservoir 16 under pressure
such that fluid emerges from the ori~ices 1~ as
1uid Eilaments which then break up into streams of
drops. This includes a pump 26 which receives fluid
from a tank 28 and delivers it, via fluid conduit
line 30, to the reservoir 16. A conduit 32 is
connected to fluid outlet 2~ such that fluid may be
removecl from the re~qervoir L6 at shut down of the
print head or during cross-Elushill~ oE the reservoir
L6. As wlll become apparent, the end oE the print
head to which conduits 30 and 32 are attached, as
well as the opposite encl of the print head, i;
subjected to mechanical vibrations which cause the
fluid filalnents to break up into streams of drops of
uniform si~e and spacing. The conduits 30 and 32
are selected from among a number of materials, such
as a polymeric material, which have a vibrational
impedance substantially different from that oE the
stainless steel block 14. As a consequence, power
loss through the conduits 30 and 32 and the
resulting damping of the vibrations are minimized.
r~lDF 002 P2 ~ 77~
The print head further includes support
means, such as mounting flanges 34. Flanges 34 are
relatively thin and are integrally formed with the
block 14. The flanges 34 extend from opposite sides
of the elongated print head body 10 and are
substantially equidistant from the first and second
ends of the body. As a result, the flanges may be
used to support the body 10 in a nodal plane. The
flanges 34 are therefore not subjected to
substantial vibration.
The print head further comprises a
transducer means, including thin piezoelectric
transducers 36 and 38. The transducers are bonded
to the exterior oE the body of block 14 and extend a
substantial distance along the body in the direction
of elongation thereof, from ad~aeent the support
means toward both the first and seconcl ends of the
body. The transdueers 36 and 3~ respond to an
electrical drivlncJ sicJnal, provided by power supply
~0 on lin~ ~2, by eharlcJiny dimension, thereby
causing meehallieal vibration oE the body and break
up oE the Elllid streams into streams of drops.
'I'ht~ piezoeleetric transclueers 36 and 3~
have electrically eonduetive eoatings on their outer
~urfaces, that is the sur~aces away frorn the print
head block 1~, whieh define a first eleetrode for
each such transducer. The metallie print head bloek
14 typieally grounded, provides the seeond electrode
for eaeh of the transdueers. The piezoeleetric
transducers are seleeted such that when driven by an
a.e. drive signal, they alternately expand and
~IDF 002 P2 -12~
contract in the direction of elongation oE the print
head. As may be seen in Fig. 3, transducers 36 and
38 are electrically connected in parallel. The
transducers are oriented such that a driving signal
on line 42 causes them to elongate and contract in
unison. Since the transducers 36 and 3~ are bonded
to the block 14, they cause the block to elongate
and contract, as well.
If desired, an additional piezoelectric
transducer 44 may be bonded to one of the narrower
sides o e the print head to provide an electrical
output potential on line 46 which fluctuates in
correspondence with the elongation and contraction
of the print head block 14. The amp]itude of the
signal on line 46 is proportional to the amplitude
oE the mechanical vibration of the block 14.
rrhe mechanism by which the Eirst embodiment
Oe the print head oE the present invention functions
may be deseribc-.~d as Eollows. rl'hc elongated print
heacl bocly is som~what analogous to an ordinary
hellcal speing. If such a spring i5 COmpreS5ed alld
then quickly released, it will oscillate about its
center at a frequency eO, callec~ its Eundamental
longitudinal resonant Erequency. In this eondition,
both ends of the spring move toward and away from
the center of the spring, while the center remains
at rest. Therefore, if one fixes the center of the
spring and repeats the above described operation,
the spring will oscillate in the same manner at the
Erequency Fo.
MDF 002 P2 -13- ~Z~97~6
The steel block 14 which forms a part of
the print head body can be considered to be a very
stiff spring. If properly mechanically stimulated,
it may therefore be held at its center, as by
flanges 34, while both ends of the block 14
alternately move toward and away from the center.
Since the center of the block lies in a nodal plane,
the flanges 34 are not subjected to substantial
vibration and the support for the print head does
not interfere with its operation. As the end of the
print head body 10 which defines the fluid receiving
reservoir 16 is vibrated, the vibrations are
transmitted to the fluid filaments which emerge frorn
the orifices 16, thus ca~lsing substantially
simultaneous uniform drop break up. Note that the
reservoir 16 is small in relation to the overall
size oE the block 14 and is centered in the end of
the block. As a consequence, the reservoir 16 does
not interE~re si~niEicantly with the vibration of
the bLock ]4, nor a~Eect the resonant ~requency of
the print llead substantially.
rt'he resonant frequency of the block 14 can
~enerally be said to be given by
~0 = C/2L =VE/P/2L
where C is the speed of sound through the print head
block 14 material, L is the length of the print head
body in the direction of elongation, E is the
modulus of elasticity of the material forming block
14 and p is the density of the material forming the
block 14. Preferably the print head is designed to
operate at or near its resonant frequency, and this
~DF 002 P2 -14~ ~2~9~7~
frequency, in turn, is selected within an
appropriate fluid jet stimulation frequency range,
e.g., 50KH~ to ]OOKH~.
By providing a pair of piezoelectric
transducers 36 and 38 on opposite sides of the block
14, the block 14 is elongated and contracted without
the flexure oscillations which would otherwise
result if only one such piezoelectric transducer
were utilized. Additionally, the use oE two
piezoelectric transducers allows for a higher power
inpu~ into the print head for a given voltage and,
consequently, for a higher maximum power input into
the print head, since only a limited voltage
differential may be placed across a piezoelectric
transducer without break down of the transducer.
As is well known, E, p and L are
temperature dependent and, as a con.sequence, the
resonant Eeequ~ncy oE the print head varies with
changes in temperature. The variation Af in Eo
for a temperature chanc~e oE Aq', at or near room
temperature, is given by AE = AfokAll/2~ where k is
approximateLy ~ x 10-4/C Eor stainless steel.
Whell the dimensions a and h are smal:L as
compared to L, the print head can be driven at a
Erequency off resonance. ~ig. 5 illustrates the
changes in the driving voltage applied to the
transducers which are required in order to drive a
single jet print head for a constant nominal
filament length oE 16.5 x 10-3 . In general, the
nominal filament length is a function of both the
driving voltage and the driving frequency. At any
given driving frequency the nominal filament length
decreaes with increases in the driving voltage.
MDF 002 P2 -15-
~L9776
From Fig. 5, it is clear that at resonance,
83 KHz, the print head requires a drive voltage of
approximately 20 volts peak-to-peak. When driven by
an oscillator at a frequency to either side oE the
resonant frequency, the driving voltage must be
increased substantially in order to maintain the
filament length at 16.5 x 10-3 . On either side
of the resonant frequency, the voltage required
rises approximately linearly with frequency. There
is, however, a maximum voltage which may be applied
to the piezoelectric transducers and, so long as the
maximum voltage is not exceeded, the transducers may
be driven on the positive slope portion of the curve
of Fig. 5, or the negative slope portion of the
curve. Assuming that the resonant frequency remains
constant, the driving frequency may be varied in
synchronization with Eluctuations in speed of the
print receiving mediurn upon which drops from the
print head are to be deposit:ed, thereby compensating
~or SUCtl Eluctuations. In such an instance, the
frequency oE the drive signal i9 monitored, however,
and the volta~e oE the drive signal adjusted
accordingly in order to compensate for the frequency
shiet and thereby maintain the desired fluid
filament length.
If desired, the additional piezoelectric
transducer 44 may be utilized to monitor the
Erequency oE the drive signal and amplitude of
vibration of the print head. In Fig. 6, the voltage
output on line 46 is plot~ed against the frequency
of the driving signal for the maintenance of a
MDF 002 P2 -16- ~2~7~6
single jet print head nominal fluid filament of a
length equal to 16.5 x 10-3 , and a diameter of
approximately 1 x 10-3 . Assuming no change in
the resonant frequency o the print head or the jet,
a fluid filament of a desired length can be
maintained by monitoring the output voltage and
frequency on line 46 and adjusting the level of the
driving signal as needed to maintain the output
voltage on line 46 at a reference voltage level
specified by the curve of Fig. 6.
It will be appreciated that numerous
variations may be made in the disclosed print head
within the scope of the present invention. For
example, flanges 34 may be deleted. Another
arrangement, such as support screws may be provided
for attaching the print head body to appropriate
support structure, as long as the point or points of
attachment lie substantially in the nodal plane
intermecliate the ends oE print head body 10.
Reference is made to Fig. 7 which
illustrates a circuit which provides a means for
supplying an electrical driving signal. The output
oE a flxecl frequency osciLlator 48 is supplied to
transducers 36 and 38 via a voltage controlled
a~tenuator circuit 50, a power amplifier 52 and a
step-up transformer 54. The output from transducer
44 on line 4fi is used to control the amount of
attenuation provided by circuit 50. The signal on
line 46 is amplified by amplifier 56, converted to a
d.c. signal by converter 58, and then compared to a
selected reference signal by summing circuit 60 to
MDF 002 P2 ~ 2~76
produce a signal on line 62 which controls the
attenuation provided by circuit 50. By this
feedback arrangement, the amplitude of the driving
signal on line 42 and the amplitude of the
mechanical vibration of the print head are precisely
controlled.
Fig. 8 is a side view illustrating a second
embodiment of the present invention, with elements
corresponding to the print head of Fig. 1 being
labeled with identical reference numerals. In this
embodiment the transducers 36 and 38 are oriented on
the print head body such that a positive driving
signal on line 42 causes one of the transducers to
elongate and the other transducer to contract, while
a negative driving signal has the opposite effect.
As a consequence, as an a.c. driving signal is
supplied to line 42, the print head is caused to
vibrate in its first flexure mode. This vibrational
mode i5 iLlustrated in E'i~. ~ by medial lines 6~
which, alttlough greatly exaggerated in Elexure Eor
purpose~s oE clarity, indicate the extent of movement
of the center oE the print head body 1~. It should
be noted that lines 64 cross at points which are
approximately .23L inward from each end oE the print
head body, thus indicating nodal points. Mounting
hoLes 66 are drilled into body 14 at the nodal
points and a second corresponding pair of mounting
holes are drilled into the opposite side of the
print head body. By providing mounting pins which
extend into holes 66, pivot supports are provided
which do not interfere with flexure of the print
head.
~IDF 002 P2 -18- 12~
This flexure mode may be excited by driving
the transducers at a frequency
Fo = ~Ca/L2, where ~is approximately 1.76.
This is a simplification of the resonant frequency
equation
Fo = 9~CK/8L2, where K is the radius
of gyration, which for
the print head illus-
trated equals a/2.
It will be further appreciated that the
present invention is not limited to the precise
method and form of apparatus disclosed, and that
changes may be made in either without departing from
the scope of the invention.
