Note: Descriptions are shown in the official language in which they were submitted.
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= PIEZOELECTRIC INK JET MODULE WITH SEAL
This is a divisional of Application Serial
No. 2,386,737, filed October 5, 2000.
Background of the Invention
This invention relates to piezoelectric ink jet
modules.
A piezoelectric ink jet module includes a module
body, a piezoelectric element, and an electrical connection
element for driving the piezoelectric element. The module
body, usually carbon or ceramic, is typically a thin,
rectangular member into the surfaces of which are machined a
series of ink reservoirs that serve as pumping chambers for
ink. The piezoelectric element is disposed over the surface
of the jet body to cover the pumping chambers and position
the piezoelectric material in a manner to pressurize the ink
in the pumping chambers to effect jetting.
In a typical shear mode piezoelectric ink jet
module, a single, monolithic piezoelectric element covers
the pumping chambers to provide not only the ink
pressurizing function but also to seal the pumping chambers
against ink leakage. The electrical connection is typically
made by a flex print positioned over the exterior surface of
the piezoelectric element and provided with electrical
contacts at locations corresponding to the locations of the
pumping chambers. An example of a piezoelectric shear mode
ink jet head is described in U.S. 5,640,184.
In one known ink jet module, available from
Brother, a resin diaphragm is provided next to each of the
pumping chambers. The central region of each diaphragm is
pumped by
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a piezoelectric feature. Electrodes are embedded in the
piezoelectric material.
Summary of the Invention
This invention relates to a piezoelectric ink jet
shead that includes a polymer, preferably a flex print,
located between the piezoelectric element and the pumping
chambers in the jet body. The polymer seals the pumping
chambers and also positions the electrodes on the side of the
piezoelectric element in which motion is effected, which can
reduce the magnitude of the drive voltage required for
operation. The compliant flex print material also can
provide electrical, mechanical, and fluidic pressure
isolation between pumping chambers, which improves jetting
accuracy.
IS Thus, in one aspect, the invention features a
piezoelectric element that is positioned to subject the ink
within an ink reservoir to jetting pressure. A flexible
material carries electrical contacts arranged for activation
of said piezoelectric element and is positioned between the
reservoir and the piezoelectric element in a manner to seal
the reservoir.
Implementations of the invention may include one or
more of the following features. The material may be a
polymer. The ink reservoir may be defined by a multi-element
module body. An ink fill flow path leading to the reservoir
may be sealed by the polymer. The polymer may include an
area that is not supported. The piezoelectric element may be
sized to cover the reservoir without covering the ink fill
flow path. The module may include a series of reservoirs all
covered by a single piezoelectric element, or in other
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examples by separate respective piezoelectric elements. The
module may be a shear mode piezoelectric module. The
piezoelectric element may be a monolithic piezoelectric
member.
In other general aspects of the invention, the
flexible material over the flow path contains an area that
is not supported; the piezoelectric element spans the ink
reservoir and is positioned to subject the ink within the
reservoir to jetting pressure; and electrical contacts are
located only on a side of the piezoelectric element adjacent
to the ink reservoir. In some implementations, the contacts
may be thinner than 25 microns, preferably thinner
than 10 microns.
According to one aspect of the present invention,
there is provided an ink jet module, comprising: a
reservoir; a flow path in fluid communication with the
reservoir; a piezoelectric element positioned to subject
fluid within the reservoir to jetting pressure; an
electrically insulating flexible material that carries an
electrical contact configured to activate the piezoelectric
element, the flexible material being positioned between the
reservoir and the piezoelectric element in a manner to seal
the reservoir; and a flexible element positioned in a manner
to form a flexible wall of the flow path.
According to another aspect of the present
invention, there is provided an ink jet module, comprising:
a reservoir; a piezoelectric element positioned to subject
fluid within the reservoir to jetting pressure; an
electrically insulating element positioned between the
piezoelectric element and the reservoir; and a flow path in
fluid communication with the reservoir, the flow path having
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a flexible wall comprising an electrically insulating
flexible material.
According to still another aspect of the present
invention, there is provided an ink jet module, comprising:
a reservoir; a flow path in fluid communication with the
reservoir; a piezoelectric element positioned to subject
fluid within the reservoir to jetting pressure; an
electrically insulating flexible material positioned between
the reservoir and the piezoelectric element in a manner to
seal the reservoir, the electrically insulating flexible
material extending beyond the piezoelectric element in a
manner to form a flexible wall of the flow path.
Other features and advantages will become apparent
from the following description and from the claims.
Description
We firstly briefly describe the drawings.
Fig. 1 is an exploded view of a shear mode
piezoelectric ink jet print head;
Fig. 2 is a cross-sectional side view through an
ink jet module;
Fig. 3 is a perspective view of an ink jet module
illustrating the location of electrodes relative to the
pumping chamber and piezoelectric element;
Fig. 4A is a graph of the field lines in a
piezoelectric element, while Fig. 4B illustrates element
displacement when a driving voltage is applied;
Fig. 5 is an exploded view of another embodiment
of an ink jet module;
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Fig. 6 is a graph of jet velocity data for a 256 jet
embodiment of the print head.
Referring to Fig. 1, a piezoelectric ink jet head 2
includes multiple modules 4, 6 which are assembled into a
scollar element 10 to which is attached a manifold plate 12,
and an orifice plate 14. Ink is introduced through the
collar 10 to the jet modules which are actuated to jet ink
from the orifices 16 on the orifice plate 14. An exemplary
ink jet head is described in US 5,640,184,
loand is available as Model CCP-256 (Spectra, Inc.,
Hanover, New Hampshire).
Each of the ink jet modules 4, 6 includes a body 20,
which is formed of a thin rectangular block of a material
such as sintered carbon or ceramic. Into both sides of the
is body are machined a series of wells 22 which form ink pumping
chambers. The ink is introduced through an ink fill passage
26 which is also machined into the body.
The opposing surfaces of the body are covered with
flexible polymer films 30, 301 that include a. series of
20 electrical contacts arranged to be positioned over the
pumping chambers in the body. The electrical contacts are
connected to leads, which, in turn, can be connected to a
flex print 32, 32' including driver integrated circuit 33,
T
33'. The films 30, 30' may be flex prints (Kapton) available
25from Advanced Circuit Systems located in Franklin, New
Hampshire. Each flex print film is sealed to the body 20 by a
thin layer of epoxy'. The epoxy layer is thin enough to fill
in the surface roughness of the jet body so as to provide a
mechanical bond, but also thin enough so that only a small
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amount of epoxy is squeezed from the bond lines into the
pumping chambers.
Each of the piezoelectric elements 34, 34', which may
be a single monolithic PZT member, is positioned over the
sflex print 30, 30'. Each of the piezoelectric elements 34,
34' have electrodes that are formed by chemically etching
away conductive metal that has been vacuum vapor deposited
onto the surface of the piezoelectric element. The
electrodes on the piezoelectric element are at locations
iocorresponding to the pumping chambers. The electrodes on the
piezoelectric element electrically engage the corresponding
contacts on the flex print 30, 30'. As a result, electrical
contact is made to each of the piezoelectric elements on the
side of the element in which actuation, is effected. The
ispiezoelectric elements are fixed to the flex prints by thin
layers of epoxy. The epoxy thickness is sufficient to fill
in the surface roughness of the piezo electric element so as
to provide a mechanical bond, but also thin enough so that it
does not act as an insulator between the electrodes on the
2opiezoelectric element and the electrodes on the flex print.
To achieve good bonds, the electrode metallization on the
flex print should be thin. It should be less than 25
microns, and less than 10 microns is preferred.
Referring to Fig. 2, the piezoelectric elements 34,
2534' are sized to cover only the portion of the body that
includes the machined ink pumping chambers 22. The portion
of the body that includes the ink fill passage 26 is not
covered by the piezoelectric element. Thus the overall size
of the piezoelectric element is reduced. Reducing the size of
30 the piezoelectric element reduces cost, and also reduces
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electrical capacitance of the jet, which reduces jet
electrical drive power requirements.
The flex prints provide chemical isolation between
the ink and the piezoelectric element and its electrodes,
sproviding more flexibility in ink design. Inks that are
corrosive to metal electrodes and inks that may be adversely
affected by exposure to electrical voltages such as water
based inks can be used.
The flex prints also provide electrical isolation
iobetween the jet body and the ink, on one hand, and the
piezoelectric element and its electrodes on the other hand.
This allows simpler designs for jet drive circuitry when the
jet body or the ink in the pumping chamber is conductive. In
normal use, an operator may come into contact with the
is orifice plate, which may be in electrical contact with the
ink and the jet body. With the electrical isolation provided
by the flex print, the drive circuit does not have to
accommodate the instance where an operator comes in contact
with an element of the drive c ^uit.
20 The ink fill passage 2E ..s sealed by a portion 31,
31' of the flex print, which is attached to the exterior
portion of the module body. The flex print forms a non-rigid
cover over (and seals) the ink fill passage and approximates
a free surface of the fluid exposed to-atmosphere. Covering
2s the ink fill passage with a non-rigid flexible surface
reduces the crosstalk between jets.
Crosstalk is unwanted interaction between jets. The
firing of one or more jets may adversely affect the
performance of other jets by altering jet velocities or the
3o drop volumes jetted. This can occur. when unwanted energy is
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transmitted between jets. The effect of providing an ink
fill passage with the equivalent of a free surface is that
more energy is reflected-back into the pumping chamber at the
fill end of a pumping chamber, and less energy enters the ink
5fill passage where it could affect the performance of
neighboring jets.
In normal operation, the piezoelectric element is
actuated first in a manner that increases the volume of the
pumping chamber, and then, after a period of time, the
lopiezoelectric element is deactuated so that it returns to its
original position. Increasing the volume of the pumping
chamber causes a negative pressure wave to be launched. This
negative pressure starts in the pumping chamber and travels
toward both ends of the pumping chG:nber (towards the orifice
is and towards the ink fill passage as suggested by arrows 33,
33'). When the negative wave reaches the end of the pumping
chamber and encounters the large area of the ink fill passage
(which communicates with an approximated free surface), the
negative wave is reflected back into the pumping chamber as a
2opositive wave, travelling towards the orifice. The returning
of the piezoelectric element to its original position also
creates a positive wave. The timing of the deactuation of
the piezoelectric element is such that its positive wave and
the reflected positive wave are additive when they reach the
25orifice. This is discussed in US 4,891,654.
Reflecting energy back into the pumping chamber
increases the pressure at the orifice for a given applied
voltage, and reduces the amount of energy transmitted into
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the fill area which could adversely affect other jets as
crosstalk.
The compliance of the flex print over the fill area
also reduces crosstalk between jets by reducing the amplitude
s of pressure pulses that enter the ink fill area from firing
jets. Compliance of a metal layer in another context is
discussed in US 4,891,654.
Referring to Fig. 3, the electrode pattern 50 on the
flex print 30 relative to the pumping chamber and
iopiezoelectric element is illustrated. The piezoelectric
element has electrodes 40 on the side of the piezoelectric
element 34 that comes into contact with the flex print. Each
electrode 40 is placed and sized to correspond to a pumping
chamber 45 in the jet body. Each electrode 40 has an
iselongated region 42, having a length and width generally
corresponding to that of the pumping chamber, but shorter and
narrower such that a gap 43 exists between the perimeter of
electrode 40 and the sides and end of the pumping chamber.
These electrode regions 42, which are centered on the pumping
20 chambers, are the drive electrodes. A comb-shaped second
electrode 52 on the piezoelectric element generally
corresponds to the area outside the pumping chamber. This
electrode 52 is the common (ground) electrode.
The flex print has electrodes 50 on the side 51 of
25 the flex print that comes into contact with the piezoelectric
element. The flex print electrodes and the piezoelectric
element electrodes overlap sufficiently for good electrical
contact and easy alignment of the flex print and the
piezoelectric element. The flex print electrodes extend
3o beyond the piezoelectric element (in the vertical direction
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in figure 3) to allow for a soldered connection to the.flex
print 32 that contains the driving circuitry. It is not
necessary to have two flex prints 30, 32. A single flex
print can be used.
s Referring to Figs. 4A and 4B, a graphical
representation of the field lines in a piezoelectric element
and the resulting displacement of the piezoelectric element
are shown for a single jet. Figure 4A indicates theoretical
electric field lines in the piezoelectric element, and Fig.
lo 4B is an exaggeration of the displacement of the
piezoelectric element during actuation for illustration
purposes. The actual displacement of the piezoelectric
element is approximately 1/10,000 the thickness of the
piezoelectric element (1 millionth of an inch). In Fig. 4A,
is the piezoelectric element is shown with electrodes 70, 71 on
the lower surface next to the jet body 72, and air 74 above
the piezoelectric element 76. For simplicity, the kapton
flex print between the piezoelectric element and jet body is
not shown in this view. The drive electrodes 70 are centered
20on the pumping chambers 78, and the ground electrode is
located just outside the pumping chambers. Application of a
drive voltage to the drive electrode results in electric
field lines 73 as shown in Fig. 4A. The piezoelectric
element has a poling field 75 that is substantially uniform
2s and perpendicular to the surface containing the electrodes.
When the electric field is applied perpendicularly to the
poling field, the piezoelectric element moves in shear mode.
When the electric field is applied parallel to the poling
field, the piezoelectric element moves in extension mode. In
30 this configuration with ground and drive electrodes on the
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side of the piezoelectric element that is next to the pumping
chambers, for a given applied voltage, the displacement of
the surface of the piezoelectric element adjacent to the
pumping chamber can be substantially greater than if the
selectrodes were on the opposite surface of the piezoelectric
element.
The bulk of the displacement is due to the shear mode
effect, but in this configuration, parasitic extension mode
works to increase the displacement. In the piezoelectric
ioelement, in the material between the common and the drive
electrodes, the electric field lines are substantially
perpendicular to the poling field, resulting in displacement
due to shear mode. In the material close to the electrodes,'
the electric field lines have a larger-component that is
lsparallel to the poling field, resulting in parasitic
extension mode displacement. In the area of the common
electrodes, the piezoelectric material extends in a direction
away from the pumping chamber. In the area of the drive
electrode, the component of the electric field that is
20 parallel to the poling field is in the opposite direction.
This results in compression of the piezoelectric material in
the area of the drive electrode. This area around the drive
electrode is smaller than the area between the common
electrodes. This increases the total displacement of the
25 surface of the piezoelectric element that is next to the
pumping chamber.
Overall, more displacement may be achieved from a
given drive voltage if the electrodes are on the pumping
chamber side of the piezoelectric element, rather than on the
30opposite side of the piezoelectric element. In embodiments,
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this improvement may be achieved without incurring the
expense of placing electrodes on both sides of the
piezoelectric element.
Referring to Fig. 5, another embodiment of a jet
5module is shown. In this embodiment, the jet body is
comprised of multiple parts. The frame of the jet body 80 is
sintered carbon and contains an ink fill passage. Attached to
the jet body on each side are stiffening plates 82, 82',
which are thin'metal plates designed to stiffen the assembly.
io Attached to the stiffening plates are cavity plates 84, 84',
.which are thin metal plates into which pumping chambers have
been chemically milled. Attached to the cavity plates are
the flex prints 30, 30', and to the flex prints are attached
the piezoelectric elements 34, 34'. All these elements are
is bonded together with epoxy. The flex prints that contain the
drive circuitry 32, 32', are attached by a soldering process.
Describing the embodiment shown in Fig. 5 in more
detail, the jet body is machined from sintered carbon
approximately 0.12 inches thick. The stiffening plates are
20 chemically milled from 0.007 inch thick kovar metal, with a
fill opening 86 per jet that is 0.030 inches by 0.125 inches
located over the ink fill passage. The cavity plates are
chemically milled from 0.006 inch thick kovar metal. The
pumping chamber openings 88 in the cavity plate are 0.033
25 inches wide and 0.490 inches long. The flex print attached
to the piezoelectric element is made from 0.001 inch Kapton,
available from The Dupont Company. The piezoelectric element
is 0.010 inch thick and 0.3875 inches by 2.999 inches. The
drive electrodes on the piezoelectric element are 0.016
30 inches wide and 0.352 inches long. The separation of the
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drive electrode from the common electrode is approximately
0.010 inches. The above elements are bonded together with
epoxy. The epoxy bond lines between the flex print and the
piezoelectric element have a thickness in the range of 0 to
s15 microns. In areas were electrical connection must be made
between the flex print and the piezoelectric element, the
thickness of the epoxy must be zero at least in some places,
and the thickness of the epoxy in other places will depend on
surface variations of the flex print. and the piezoelectric
i.oelement. The drive circuitry flex print 32 is electrically
connected to the flex print 30 attached to the piezoelectric
element via a soldering process.
Referring to Fig. 6, velocity data is shown for a 256
jet print head of the design in Fig. 5. The velocity data is
is presented normalized to the average velocity of all the jets.
Two sets of data are overlaid on the graph. One set is the
velocity of a given jet measured when no other jets'are
firing. The other set of data is the velocity of a given
jet when all other jets are firing. The two sets of data
20 almost completely overlaying one another is an indication of
the low crosstalk between jets that this configuration
provides.
Other Embodiments
In another embodiment, the piezoelectric elements 34,
2534' do not have electrodes on their surfaces. The flex
prints 30, 30' have electrodes that are brought into
sufficient contact with the piezoelectric element and are of
a shape such that electrodes on the piezoelectric material
are not required. This is discussed in US 5,755,909.
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In another embodiment, the piezoelectric elements 34,
34' have electrodes only on the surface away from the pumping
chambers.
In another embodiment, the piezoelectric elements
shave drive and common electrodes on the surface away from the
pumping chambers, and a common electrode on the side next to
the pumping chambers. This electrode configuration is more
efficient (more piezoelectric element deflection for a given
applied voltage) than having electrodes only on the surface
roof the piezoelectric element away from the pumping chambers.
This configuration results in some electric field lines
going from one surface of the piezoelectric element to the
other surface, and hence having a component parallel to the
poling field in the piezoelectric element. The component of
is the electric field parallel to the poling field results in
extension mode deflection of the piezoelectric element.
With this electrode configuration, the extension mode
deflection of the piezoelectric element causes stress in the
plane of the piezoelectric element. Stress in the plane of
20 the piezoelectric element caused by one jet can adversely
affect the output of other jets. This adverse effect varies
with the number of jets active at a given time, and varies
with the frequency that the jets are activated. This is a
form of crosstalk. In this embodiment, efficiency is traded
2s for crosstalk.
In the embodiment with electrodes on the surface of
the piezoelectric element adjacent to the pumping chambers,
no efficiency is gained from adding a ground electrode on the
surface of the piezoelectric element away from the pumping
30 chambers. Adding a ground electrode to the surface of the
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piezoelectric element away from the pumping chamber will
increase the electrical capacitance of the jet and so will
increase the electrical drive requirements.
In another embodiment, the piezoelectric elements 34,
s34' have drive and common electrodes on both surfaces.
Still other embodiments are within the scope of the
following claims. For example, the flex print may be made of
a wide variety of flexible insulative materials, and the
dimensions of the flex print may be any dimensions that will
ioachieve the appropriate degrees of compliance adjacent the
ink reservoirs and adjacent the fill passage. In regions
where the flex print seals only the fill passage and is not
required to provide electrical contact, the flex print could
be replaced by a compliant metal layer.
is What is claimed is:
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