Note: Descriptions are shown in the official language in which they were submitted.
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Description
Simplified Ink Jet Head
Technical Field
This invention relates to ink jet head arrangements
and, more particularly, to a new and improved ink jet
head arrangement having a simple and inexpensive struc-
ture.
Background Art
Conventional ink jet heads, in which ink received
from an ink reservoir is ejected selectively through a
series of orifices, have been made using thin plates of
l0 metal or ceramic material having appropriate passages
which are bonded together in adjacent relation in an as-
sembly, as described, for example, in the Roy et al. Pat-
ent No. 5,087,930 and the Hoisington et al. Patent No.
4,835,554. In such arrangements, each chamber or passage
15, in the flowpath leading from the ink inlet to the
orifice, through which the ink is ultimately ejected, is
provided in one or more of the several plates in the as-
sembly.. This requires an array of plates having differ-
ent thicknesses, each of which must be separately
20 machined to precise dimensions to produce the appropriate
chambers and passages, and also requires precise
positioning of all of the chambers and passages in the
plates. Moreover, the plates must be assembled and bond-
ed together and to a piezoelectric plate in highly pre-
25 cise alignment, and each plate must be flat and free from
burrs that would cause voids between adjacent plates.
Furthermore, because of differences in the coefficients
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of thermal expansion between the materials used in the
plates, bond stresses are generated by temperature varia-
tions which occur in connection with the manufacture and
use of the ink jet head which must be overcome.
In hot melt ink jet printheads, which operate at
elevated temperatures, the printhead materials must be
good conductors of heat so that the printhead will warm
up quickly and the temperature gradients during operation
will be small. The stresses created, when parts of dif-
ferent materials expand differently with changes in tem-
perature is another concern. The prime mover in a
printhead is usually a piezoelectric ceramic (PZT) which
has a relatively low thermal expansion coefficient. For
optimum printhead design, the challenge is to find other .
materials which are close to this expansion. If the
printhead materials cannot be matched, it is desirable to
have low-modulus materials to reduce the stresses.
The ink passages in an ink jet printhead are fine
features with tight tolerances. To maintain such tight
tolerances, the manufacture of the printhead requires low
machining forces, small tool deflection and small machin-
ing errors, no plastic deformation and no burrs. More-
over, it may be desirable, particularly in development,
that the manufacture should be carried out using standard
machining methods, such as grinding, milling, drilling
and shaping.
In addition, the printhead should be made of materi-
als which are chemically inert and da not change shape
over time when loaded or: oxidize or interact with organic
chemicals found in hot-melt and other inks or with pig-
ments or dyes in the inks.
Heretofore, some plates used in ink jet heads have
been photo-etched to provide the appropriate chambers and
passages, which has the advantage that the plates are
generally burr-free and can be made from Kovar, stainless
steel and other materials that have appropriate mechani-
cal and thermal expansion characteristics. The materials
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useful for photo-etching, however, have drawbacks when
used in connection with ink jet heads from which hot melt
ink is ejected since they generally have low thermal con-
ductivity. In addition, the photo-etching process has
the disadvantage of being a batch process with lot-to-lot
variations and, moreover, when used in this manner, pro-
duces a relatively large quantity of chemical waste.
Furthermore, conventional piezoelectric plates used
in ink jet heads are thin, fragile and susceptible to
damage during processing. Because of the greater likeli-
hood of damage to larger plates, the maximum size of pi-
ezoelectric plates is normally quite small, for example,
less than about 100mm, which correspondingly limits the
length of an array of orifices through which ink is
ejected as a result of the actuation of the piezoelectric
plate.
Disclosure of Invention
Accordingly, it is an object of the present inven-
tion to provide an ink jet head which overcomes the dis-
advantages of the prior art.
Another object of the invention is to provide an ink
jet head having a simple structure which is inexpensive
to develop, is convenient to manufacture and is capable
of providing high resolution ink jet printing.
These and other objects and advantages of the inven-
tion are attained by providing an ink jet head having at
least one or more components formed from a carbon member.
A preferred carbon component is one in which ink pressure
chambers and connecting passages from ink supply lines
and to ink jet orifices are formed. In one embodiment, a
carbon component is a plate having pressure chambers
formed on one side and flow-through passages to permit
continuous ink circulation through the pressure chambers
formed on the other side of the plate with connecting
passages leading to an orifice plate and to an ink supply
extending through the carbon plate. In addition, an ori-
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fice plate is affixed to one side of the carbon plate
with the orifices aligned with orifice passages in the
carbon plate and a piezoelectric plate is affixed to the
other side of the carbon plate with actuating electrodes
aligned with the pressure chambers to cause the
piezoelectric material to be deflected so as to apply
pressure to the corresponding pressure chamber and eject
a drop of ink through a corresponding orifice in the ori-
fice plate.
In another embodiment, a carbon pressure chamber
plate is formed on opposite sides with linear arrays of
pressure chambers having ink inlet and outlet passages at
opposite ends. Hoth sides of the carbon plate are cov-
ered by corresponding piezoelectric actuation plates and .
a plurality of such carbon pressure chamber plates are
retained in laterally adjacent relation in a carbon col-
lar member with the ink outlet passages therein
positioned in alignment with corresponding ink passages
extending through a carbon manifold plate. A manifold
plate has one side retained against the ends of the plu-
rality of pressure chamber plates and has lateral ink
passages formed in the opposite side leading to a line of
orifices in an orifice plate mounted on the opposite
side.
In accordance with one aspect of the invention, the
carbon pressure chamber plate has an ink deaeration pas-
sage through which ink is supplied to the inlet passages
leading to the pressure chambers and an internally sup-
ported, thin-walled tubular member made of air-permeable,
ink-impermeable material~connected at one end to a source
of reduced pressure is inserted into the ink deaeration
passage to provide a unitary ink deareating and pressure
chamber carbon plate. This arrangement accomplishes the
necessary deaeration of ink immediately before it is sup-
plied to the pressure chambers with minimal space
requirements and without necessitating recirculation of
ink to an ink reservoir.
1
r
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In connection with the assembly of carbon plate com-
ponents of the above type in an ink jet head, it has been
learned surprisingly that it is not necessary to cement
or otherwise physically bond together carbon plate compo-
nents having communicating a passages or to provide a
gasket between them. Because the engaging surfaces of
such carbon plate components can be made very smooth and
flat and carbon plates are sufficiently rigid to avoid
flexing, such plates can be mechanically fastened togeth-
er by screws or the like without causing ink to flow be-
tween the components and, if desired, a filter layer may
be interposed between the surfaces of such fastened com-
ponents.
Because the carbon body can be machined precisely
without causing burrs using conventional machining tech-
niques and, since carbon has a low thermal coefficient of
expansion, dimensional variations resulting from thermal
expansion during machining are minimized on the plate.
In addition, the carbon expansion coefficient is
especially compatible with the piezoelectric plate which
is affixed to i.t, thereby reducing or eliminating stress-
es between the plates which might otherwise be produced
by temperature variations such as occur when the ink jet
head is used with hot melt ink. Moreover, carbon is
chemically inert with respect to materials in which it
comes in contact in an ink jet head. It does not oxidize
nor does it interact with organic chemicals found in hot-
melt and other inks or with pigments or dyes used in
inks.
According to another aspect of the invention, the
piezoelectric plate has actuating electrodes on only one
side of the plate and is prepared by a photo-etching
technique in which a piezoelectric plate coated on one
side with electrode material is affixed to the pressure
chamber side of the carbon plate with the electrode mate-
rial-coated side exposed. The exposed side of the plate
is coated with a photoresist material and is then exposed
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to a desired electrode pattern in precise alignment with
the pressure chamber pattern in the carbon plate, after
which the photoresist is developed, the exposed electrode
material is etched away, and the remaining photoresist is
removed to produce an electrode pattern conforming exact-
ly in shape and position to the pattern of pressure cham-
bers in the carbon plate. In addition, the electrode
pattern thus formed on the piezoelectric plate can
include other electrical elements such as a heater to
heat ink in the passages in the carbon plate.
In accordance with a further aspect of the
invention, the carbon plate is porous, preferably being
about 80-90% dense, and the porosity and a vacuum source
communicating with the surface of the plate can extract
dissolved air from ink in the ink passages separated from'
the porous carbon material by an air-permeable, ink-im-
permeable layer.
If desired, a page-size carbon plate can be prepared
with a raw of separate piezoelectric plates affixed to
one side of the plate. Moreover, the carbon plate may
have orifice passages formed in an edge of the plate
rather than in one of the sides of the plate.
Since engineering grade carbon is friable, i.e.,
microscopic grains are readily broken away from a carbon
body, it is easily shaped without producing burrs. As
described in "Graphite Machinery Made Easy", EDM Today,
Sept./Oct. 1993 pp. 24ff, the relative softness and lack
of ductibility of such carbon allows it to be cut at high
feed rates with little distortion and low tool wear.
These characteristics permit carbon blocks to be readily
formed into components of ink jet heads by conventional
or unique machining techniques.
In one example, the formation of an array of closely
adjacent pressure chambers for an ink jet head which have
a long aspect ratio and require highly precise and uni
form channel dimensions, would require prohibitively long
machine cycle times using a conventional end mill. In
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accordance with another aspect of the invention, however,
the process for manufacturing a carbon plate component of
an ink jet head is greatly simplified by shaping the car-
bon plate using a series of linear motions against the
surface of the carbon plate with a shaping tool having
the desired profile. For the pressure chambers of a car-
bon pressure plate, for example, a tool having a series
of closely spaced short teeth is scraped across the sur-
face in several strokes to produce a series of precise
l0 channels of the required dimensions. To make a row of
small diameter holes of substantial depth in one end of a
body, two carbon plates may be shaped in a similar way
with matching arrays of grooves having a depth equal to
half the diameter of the desired holes and then cemented
together with the grooves in alignment. Using certain
tool shapes the holes may have a hexagonal shape rather
than a circular shape.
Other machining techniques especially useful in
shaping carbon bodies are electric discharge machining,
which facilitates convenient formation of complex shapes,
and laser machining, which can be used effectively for
through holes and slots.
According to still another aspect of the invention,
improved directionality of ink drop ejection from orific-
es supplied from nonaxial orifice passages is achieved by
providing orifice plate orifices having cylindrical out-
let nozzle passages, larger diameter cylindrical inlet
passages, and a conical intermediate section joining the
outlet and inlet passages.
Brief Description of Drawings
Further objects and advantages of the invention will
be apparent from a reading of the following description
in conjunction with the accompanying drawings, in which:
Fig. 1 is a schematic perspective sectional view
illustrating a representative embodiment of a simplified
ink jet head arranged in accordance with the invention;
~ ~ ~i
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Fig. 2 is a plan view showing the pressure chamber
side of a typical carbon plate for a multicolor ink jet
head showing the arrangement of the pressure chambers and
the related ink passages formed in the carbon plate;
Fig. 3 is a view of the carbon plate of Fig. 2 from
the same side shown in Fig. 2, but illustrating the pas-
sages formed in the opposite side of the carbon plate;
Fig. 4 is a schematic view illustrating a typical
arrangement of electrodes on the exposed surface of a
piezoelectric plate used with the carbon plate shown in
Figs. 2 and 3;
Fig. 5 is a schematic plan view of a typical large-
size carbon plate having ~a series of piezoelectric plates
mounted on one surface in accordance with another embodi--
ment of~the invention;
Fig. 6 is a schematic perspective view illustrating
another representative embodiment of the invention;
Fig. 7 is a schematic perspective sectional view
similar to Fig. 6, illustrating another typical embodi-
ment of the invention;
Fig. 8 is a schematic perspective view illustrating
a further representative embodiment of the invention;
Fig. 9 is a fragmentary perspective view illustrat-
ing a modified form of the invention;
Fig. 10 is a perspective view showing a typical
shaping tool for shaping arrays of ink passages in a car-
bon body for use in an ink jet head;
FIG. 11 is a schematic view illustrating one
representative method for poling a piezoelectric plate;
FIG. 12 is a schematic view illustrating another
representative method for poling a piezoelectric plate;
FIG. 13 is a fragmentary view in longitudinal section
showing the shape of an orifice in an orifice plate in
accordance with the invention.
Fig. 14 is an exploded perspective view illustrating
another representative embodiment of a simplified ink jet
head arrangement in accordance With the invention;
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Fig. 15 is a side view illustrating a representative
carbon pressure chamber plate of the type used in the ar-
rangement shown in Fig. 14;
Fig. 16 is an end view of the representative carbon
pressure chamber plate shown in Fig. 15;
Fig. 17 is a top view of the carbon collar block
used in the arrangement shown in Fig. 14;
Fig. 18 is a side view of the collar block shown in
Fig. 17;
Fig. 19 is a plan view showing one side of a repre-
sentative carbon manifold plate of the type used in the
arrangement shown in Fig. 14; and
Fig. 20 is a plan view showing the opposite side of
the manifold plate of Fig. 19;
Best Mode for Carrying Out the Invention
In the typical embodiment of the invention schemati-
cally shown in Fig. 1, an ink jet head 10 includes a res-
ervoir 11 on one side containing ink which is to be se-
lectively ejected in the form of dz°ops through an array
of orifices 12 formed in an orifice plate 13 mounted on
the opposite side of the head. Ink from the reservoir 11
is supplied through a passage 14 to a deaerator 15 in
which an ink path 16 extends between air-permeable, ink-
impermeable membranes 17, each of which is backed by a
vacuum plenum 18 connected through ports a to a remote
vacuum source (not shown) to extract dissolved air from
the ink. Deaerated ink from the passage 16 is conveyed
through a passage 19 to a pressure chamber 20 from which
it is ejected on demand through an orifice passageway 21
and a corresponding orifice 12 in the orifice plate 13 in
response to selective actuation of the adjacent portion
22 of a piezoelectric plate 23.
The general arrangement of the ink jet head 10 and
the deaerator 15 is of the type described, for example,
in the Hine et al. Patent No. 4,937,598.
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The ink in the reservoir 11 may, if desired, be hot melt
ink which is solid at room temperature and liquid at
elevated temperatures, in which case heaters (not shown)
are mounted at appropriate locations in the ink jet
head 10.
In order to permit the ink supplied to the orif ices
to be deaerated continuously even though ink is not being
ejected through the orifices 12, the head includes a
f low-through passage 24 extending from each orifice pas-
sage 21 to a return passage 25 leading back to the
deaeration path 16 in the deaerator 15, and a continuous
slow circulation of ink through the duct 19, the chamber
20, the orifice passage 21, the flow-through passage 24
and the duct 25 back to the deaerator passage 16 is main-
tained by thermal convection, as described, for example,
in the Hine et al. Patent No. 4,940,995 issued July 10,
1990. For this purpose, a heater (not shown in Fig. 1) is
arranged to heat the ink near the lower end of the
flowpaths consisting of the passages 19, 20, 21, 24 and 25
above its normal temperature to cause a connective flow of
the ink through those passages, thereby conveying the ink
back to the deaerator 16.
In accordance with the invention, the passages 19,
20, 21; 24 and 25 are formed in a plate 26 made of
engineering carbon graphite, which is preferably about
80-90% dense, providing a slightly porous plate
structure. The carbon plate 26 is machined by
micromachining techniques from opposite sides to produce
the chambers and passages required for the ink jet head.
The carbon plate can be machined by milling, drilling,
broaching, grinding and the like, us:~_ng conventional
tools providing high material removal rates with minimum
tool wear, to produce openings with much closer toleranc-
es than the conventional metal plates of the type
described, for example, in the Hoisington et al. Patent
No. 4,835,554. Because the carbon material is friable,
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no burrs are produced during machining. Moreover, the
coefficient of thermal expansion of the carbon graphite
body is substantially the same as that of the ceramic
piezoelectric material of which the piezoelectric plate
23 is made so as to reduce or substantially eliminate
thermal stresses which occur between those components of
the head as a result of variations in temperature.
The preferred carbon material for use in forming
components on ink jet heads is polycrystalline graphite,
l0 which is a mixture of small crystals of graphite sintered
with amorphous carbon (lamp black). This produces an
amorphous matrix of small (0.025-5~,) grains and smaller
(0.005-0.2u) pores. This material, which is different
from powdered graphite and carbon fiber materials, offers_
many benefits, including good thermal conductivity, coef-
ficient of thermal expansion close to ceramic piezoelec-
tric materials, good machinability, dimensional stability
and chemical inertness.
The thermal properties of polycrystalline graphite
(Grade DFP-1 available from POLO Graphite, Inc., Decatur,
Texas) and other materials which might be considered for
printheads are compared with those of lead zinc titanate
(PZT) ceramic piezoelectric material in ,Table 1 below:
Table 1
Thermal
Conductivity Expansion Modulus
Material (W/CmK) (~e/m/degC) (x105k
g/cm_)
PZT .015 2 to 4 7
Thermoplastic
(Ultem) 0.0022 56 0.15
Aluminum (6000) 1.7 ' 23.4 7
Carbon (DFP-1) .75 ~ 8.4 1.1
The Ultem (as well as other thermoplastics) has both
poor conductivity and a very high thermal expansion coef-
ficient. The conductivity of aluminum is attractive, but
its high thermal expansion and modulus are problems.
Polycrystalline carbon offers a good combination of all
three properties.
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A potentially prohibitive aspect of the use of car-
bon members for ink jet components is the forming of
closely spaced arrays of pressure chambers and other mul-
titudinous, long aspect ratio, channels with precise di-
mensions. Using an end mill for these features could be
expected to result in excessively long machine cycle
times. To overcome this problem, a desired array of ad-
jacent channel profiles is shaped in the surface of a
carbon body by a specially formed tool 95, shown in Fig.
10, in a series of repeated linear motions or "scrapes."
In the tool g5 for example, an array of 64 short uniform-
ly spaced teeth 96 may be provided at one end of the tool
to cut 64 parallel pressure chambers in the surface of a
carbon plate. If the tool cuts at 0.025mm per scrape, a
depth of 0.15mm can be achieved in 6 scrapes, which re-
quires only a few seconds of machine time. The tool 95
makes an array of channels equal to the width of the
tool, and can make a wider array by taking repeat adja-
cent passes. Tool irregularities are averaged out by
taking finish cuts in the reverse direction or at a one-
tooth offset. Reversing the tool also allows the forma-
tion of steeper channel ends when this is required. This
technique can be used to make pumping chambers, manifold
passages, flow-through passages, and the like. Finally,
channels of variable or tapered depth can be made as
well, by raising or lowering the tool as a cut is being
made.
For an array of closely spaced deep, small diameter
holes in a carbon body, where the depth is more than 3
times the drill diameter, drilling can become difficult
and expensive. To provide an array of closely spaced
holes having a diameter of, for example, 0.22mm through a
carbon plate 1.75mm thick would require a great deal of
time and a number of expensive drills. To form such an
array in a simple manner, matching arrays of channels
(which rnay be semi-hexagonal in shape) 0.14mm deep are
formed in two matching blocks of carbon which are then
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bonded together with adhesive to form a single block con-
taining an array of parallel, long 0.28mm diameter holes.
This block is then sliced perpendicularly to the axes of
the holes and ground flat to form the array bodies. The
adhesive joint down the middle of the array body has been
found to be very strong.
Polycrystalline carbon is easily bonded by a variety
of adhesives. Three systems are particularly compatible
with carbon in hot melt ink jet head applications. The
first is a simple dispensed epoxy which works well for
coarse scale joints. The second is a thermoplastic sheet
adhesive. Using this technique, a thin teflon (TFE)
sheet compressed at elevated temperature and pressure
provides a tenacious bond between flat surfaces of
polycrystalline carbon. Similarly, acrylic sheet adhe-
sives should perform well at lower temperature. In the
third technique, a dilute sprayed B-stage epoxy system is
applied to the surfaces to be bonded. This has been
shown to be adaptable to the complex geometries of ink
jet printheads, yet high in strength at elevated tempera-
tures.
There is some challenge in bonding a porous material
like carbon with a spray application technique. Since
_ excess_adhesive will clog small passages, and thin layers
may be drawn by capillary forces into the carbon body
pores, careful control of process variables is required.
In particular, the carbon pore structure must be uniform,
the spray-deposited layer thickness must be small
compared to the particle/pore size, and the heat cure
process must be tuned to the adhesive rheology.
Fig. 2 illustrates a representative arrangement of
pressure chambers 20 and orifice passages 21 as viewed
from the piezoelectric plate side of the typical carbon
monolithic plate 26 of Fig. 1, while Fig. 3 illustrates
the other ends of the orifice passages 21 and the flow-
through passages 24 which are formed in the opposite side
of the monolithic array 26. Fig. 3, while showing the
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passages on the side of the monolithic array 26 which
face the orifice plate 13, is illustrated with the pas-
sages seen as they would be viewed in the same direction
as Fig. 2.
Figs. 2 and 3 show supply passages for supplying
four different color inks, e.g., black, yellow, magenta
and cyan, to four different groups 27, 28, 29 and 30 of
the orifice passages 21. Since black ink is normally
used to a much greater extent than the colored inks, half
of the orifice passages 21 are arranged to supply black
ink, and one-third of each of the remaining passages are
arranged to supply each of the colored inks.
As illustrated in Fig. 2, the pressure chambers 20
of the array are alternately disposed on opposite sides
of the line of orifice passages 21 and are supplied from
grooves formed in the pressure chamber side of the graph-
ite plate 26, for example, from grooves 32 and 33 supply-
ing black ink, grooves 34 and 35 supplying magenta ink,
grooves 36 and 37 supplying yellow ink, and grooves 38
and 39 supplying cyan ink. The appropriate color ink is
supplied to these grooves thro;~gh corresponding apertures
40 and 41, 42 and 43, 44 and 45 and 46 and 47, which ex-
tend through the carbon plate 26 to corresponding sets of
grooves 50 and 51, 52 and 53, 54~and 55 and 56 and 57,
formed in the opposite side of the plate. Those grooves,
shown in solid lines in Fig. 3 and in dotted lines in
Fig. 2, communicate with further apertures 60 correspond-
ing to the passages 19 and 25 of Fig. 1, which extend
through the plate to convey ink from the deaerator ink
path 16 of Fig. 1 to the pressure chamber supply grooves
32-39. As shown in Fig. 3, the flow-through passages 24
convey ink between the orifice passages 21 and the grove
patterns 50-57 on the opposite side of the carbon plate
to complete the continuous path for circulation of ink
through the deaerator 15.
In addition, the carbon plate 26 is especially ad-
vantageous for ink jet heads used with hot melt ink.
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Because of its high thermal conductivity, the carbon
plate provides excellent heat conduction from heaters
mounted at relatively remote locations in the head to all
of the ink passages in the head. This assures that the
hot melt ink at each of the orifices 12 is at the same
temperature and therefore has the same viscosity, thereby
providing good uniformity of operation throughout the
length of the array of orifices.
A typical carbon plate 26 may be about 2mm thick and
have orifice passages 21 about 0.2mm in diameter, pres-
sure chambers about 9mm long, 0.5mm wide and 0.2mm deep,
supply grooves about 1.0 to l.5mm wide and 0.5mm deep,
flow-through passages about 4mm long, O.lmm wide and
0.05mm deep, and apertures 60 about l.5mm in diameter.
With this arrangement, a 96-aperture linear array of the
type shown in the drawings can be provided in a carbon
plate 26 having dimensions of about 4cm by 7cm with the
orifice passages 21 spaced by about 0.5mm. When oriented
at an appropriate angle with respect to the scanning di-
rection, an ink jet head using an array of this type can
produce a resolution of about 300 dots per inch (120 dots
per cm) in the subscanning direction and, when actuated
at a rate of about 14 Khz at a scanning rate of lm/sec to
produce 100 picoliter drops, can produce the same resolu-
ta_on in the scanning direction.
In certain high-frequency ink jet applications, the
rigidity of the walls of the pressure chambers 20 formed
in the carbon plate may be less than desired, requiring a
higher operating voltage,for the piezoelectric actuating
plate. To alleviate this, the surfaces of the pressure
chambers 20 formed in the carbon plate may be coated with
a thin layer, such as 0.01 to O.lmm thick, of a very hard
(i.e., high modulus of rigidity) material such as a car-
bide or nitride, e.g., silicon carbide or nitride, boron
carbide or nitride, tungsten carbide or nitride, tantalum
carbide or nitride, or the like. Preferably, the coating
is applied by chemical vapor deposition.
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In order to actuate the piezoelectric plate 23 so as
to selectively eject ink from the pressure chambers 20
through the orifice passages 21 and through corresponding
orifices 12 in the orifice plate I3, the piezoelectric
plate 23, which is mounted on the pressure chamber side
of the carbon plate, has no electrodes on the carbon
plate side and is patterned with an electrode array of
the type shown in Fig. 4 on the exposed side. In the
array shown in Fig. 4, a common electrode 65 extends
along the portion covering the orifice passage array in
the carbon plate and also extends laterally into regions
66 over the carbon plate surface portions between the
pressure chambers.
Interlaced between the lateral extensions 66 is a
spaced array of individual electrodes 67 which are posi-
tioned directly over the pressure chambers in the carbon
plate so that, when selectively actuated by application
of appropriate potential to a corresponding terminal 68,
the piezoelectric plate 23 is mechanically distorted in
the shear mode in the direction toward the adjacent pres-
sure chamber 20 so as to cause ejection of an ink drop
from the orifice with which that pressure chamber commu-
nicates. Shear-mode operation of a piezoelectric plate
is described, for example, in the Fischbeck et al. Patent
No. 4,584,590. Such shear-mode operation does not require
any electrode on the opposite side of the piezoelectric
plate but, if desired, the carbon plate 26, being
conductive, can be used to provide an electrode on the
opposite side of the plate.
The electrode pattern shown in Fig. 4 also includes
a heater conductor 70 having a thermistor temperature
control switch 71 extending between two terminals 72 and
73 and arranged to heat the ink in the passages in the
lower portion of the carbon plate 26 so as to cause cir-
culation of the ink in the manner described above by
thermal convection. Because the carbon material in the
.)
CA 02263783 2002-08-26
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plate 26 has a high thermal conductivity, the plate acts
as a thermal conductor between the heater and the adja-
cent ink passages in the plate.
In order to form an electrode pattern of the type
shown in Fig. 4 on the piezoelectric plate 23, the plate,
' which is initially provided with a continuous conductive
coating on the exposed side, is permanently afffixed by an
epoxy adhesive to the pressure chamber side of the carbon
plate 26. Since the carbon plate is slightly porous, an.
epoxy adhesive can be used to mount~,not only the piezo-
electric plate 23, but also the orifice plate 13, to the
opposite surfaces of the carbon body. For this purpose,
one .of the surfaces of the plates to be joined is prefer-
ably spray-coated with a layer of B-stage epoxy adhesive .
about 2 microns thick before the piezoelectric plate 23
or the orifice plate is applied to it. Such a thin layer
of epoxy adhesive provides excellent seals between the
plates, including the very narrow portions between. the
orifice passages, but does not flow into the passages or
apertures in such a way as to interfere with the opera-
tion of the head.
In order to ground the surface of a the piezoelec-
tric plate which .is bonded to the carbon body, the epoxy
adhesive may be doped with conductive particles. Alter-
natively, the clamping force applied during bonding of
the piezoelectric plate to the carbon body is increased
until the epoxy adhesive is driven into the carbon body
to provide a large number of point contacts between the
plate and the carbon body.
The use of single sided piezoelectric plates
requires special techniques for poling the plates. Ac-
cording to one technique, schematically shown in Fig. i1,
a piezoelectric plate 140 is compressed between two elec-
trodes 143 and 144 separated from the plate 140 by two
slightly conductive rubber sheets 141 and 142 to provide
intimate electrical contact throughout the surfaces of
the plate while limiting the current available for arcing
CA 02263783 1999-03-15
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if a breakdown occurs. When this procedure is carried
out in two steps to minimize piezo stresses, high yield
poling of unmetalized piezoelectric plates is achieved.
The other poling technique, shown in Fig. 12, uses a
corona discharge to set up a poling field across the pi-
ezoelectric plate. A piezoelectric plate 150 is laid on
a flat ground plate 151, and a corona discharge device
152 rains charges 153 down onto the surface. When the
applied charge is sufficient to create an occasional
breakdown through the plate, which is non-destructive
because of the high surface resistance of the piezoelec-
tric material, the plate is poled. This process is pref-
erably carried out at an elevated temperature, such as
100-150°C, to ameliorate poling stresses.
The orifices 12 in the orifice plate 13 of Fig. 1,
which may be a stainless steel plate about 0.05mm thick,
are preferably about 0.05mm in diameter and are formed by
electrical discharge machining. By selecting the appro-
priate size wire and controlling the current/voltage pro-
file, the size and shape of the orifice can be controlled
accurately. Bonding of the orifice plate to the surface
of the carbon body is accomplished in the same way as the
bonding of the piezoelectric plate.
With conventional bell-mouthed shaped orifices in an
orif ice plate of the type shown, for example, in the
Hoisington et al. Patent No. 5,265,315 in which the ori-
f ice diameter decreases at a continuously decreasing rate
from a large diameter on the side of the orifice plate
facing the ink jet head to a smaller diameter about one-
third that of the large diameter on the side of the ori-
fice plate through which the ink drop was ejected, the
direction of ink drop ejection is very sensitive to asym-
metries in the ink path near the periphery of the
entrance to the bell-mouthed orifice. Moreover, it is
not possible to space such bell-mouthed orifices as
closely as desired for high resolution printing.
CA 02263783 1999-03-15
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To overcome this problem, electrical discharge ma-
chining is used to form orifices 97 having the shape
shown in Fig. 13 with a cylindrical inlet section 98, a
smaller diameter cylindrical nozzle section 99 providing
the outlet from the orifice plate and a tapered section
100 having a conical surface joining the inlet section
and the nozzle section. It has been found that with an
orifice design of this type, the non-axial velocity com-
ponent of an ejected drop, i.e. the extent of deviation
from the axial arrow 101 resulting from asymmetry of the
passage leading to the orifice is reduced by more than
50$.
In a typical orifice plate 13 having a thickness of
0.05mm, a nozzle 99 having a diameter of 0.054mm and a
length of 0.01mm, a tapered section 100 having a cone
angle of 30° and a length of O.Olmm and an inlet section
98 having a diameter of O.llmm and a length of 0.03mm, a
substantial improvement in axial projection of drops sup-
plied from an asymmetric ink path leading to the orifice
was obtained. For an orifice plate 13 having a thickness
of 0.075mm and having the same nozzle and tapered section
dimensions described above, and having an inlet section
o.055mm long, a similar improvement in the direction of
projection of drops was obtained at a slightly increased
pressure drop. Preferably, the diameter of the inlet
portion 98 of the orifice is no more than twice the diam-
eter of the nozzle portion 99 and the length of the inlet
portion 98 is greater than that of the nozzle portion.
Such orifice shapes having successive conical and tapered
cylindrical sections can'be obtained by appropriate con-
ventional electrical discharge machining techniques.
In accordance with one aspect of the invention, af-
ter the piezoelectric plate 23 has been affixed to the
carbon plate, a layer of photoresist material is coated
on the exposed surface and, using the precisely known
positions of the pressure chambers from a reference edge
in the carbon body, the photoresist is exposed to produce
CA 02263783 1999-03-15
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a pattern which corresponds exactly with the locations of
the pressure chambers, and the unexposed resist is
removed in the usual manner. Thereafter, the conductive
layer is etched away from the exposed surface of the pi-
ezoelectric plate and the remaining resist is then
removed to provide the final electrode pattern.
In this way, the piezoelectric plate 23, which is
preferably only about 0.1 to 0.25mm thick and is quite
fragile, is protected from damage during the formation of
the electrode pattern and other head-manufacturing steps.
Consequently, substantially large piezoelectric plates,
for example, 50mm by 100mm or more, can be used without
substantial risk of damage during processing. Moreover,
large-scale production is facilitated since a large-size .
carbon plate can be machined with multiple identical or
similar patterns, and a corresponding number of
piezoelectric plates can be bonded to the pattern loca-
tions on the large sheet and simultaneously exposed and
etched to form electrode patterns corresponding precisely
to the structures of the adjacent portions of the carbon
plate, after which the large-size plate is separated into
individual plates.
Furthermore, instead of separating a large-size car
bon plate into smaller plates, a single carbon plate 20cm
wide, or even 150cm wide, if appropriate, may be made to
provide a page-width ink jet head by mounting a row of
piezoelectric plates to one surface and simultaneously
processing the piezoelectric plates in the manner
described above. A typical page-width ink jet head is
shown in Fig. 5, in which a carbon plate 26 has a row of
adjacent piezoelectric plates 23 affixed to one side.
The ink jet head of Fig. 5 has internal passages arranged
to supply ink to an orifice plate 74 mounted on one edge,
as described hereinafter with respect to Fig. 8. Alter-
natively, if desired, the large-size plate 26 of Fig. 5
may have internal passages of the type described above
with respect to Fig. 1 leading to an orifice plate (not
CA 02263783 1999-03-15
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shown in Fig. 5) mounted on the opposite side of the car-
bon plate.
As an alternative to the deaerator arrangement 15
shown in Fig. 1, the use of a carbon plate 26 which is
Slightly porous permits the plate to act as a conduit
between the vacuum plenum and the ink in the passages
within the carbon plate in the manner shown in the alter-
native embodiment of Fig. 6 so that dissolved air can be
extracted. For this purpose, the surfaces of the plate
passages are coated with a layer 75 of an air-permeable,
ink-impermeable epoxy resin and one or more openings 76
are provided in the piezoelectric plate 23 to expose the
adjacent surface of the carbon plate 26 to a vacuum
source 77 which replaces the deaerator 15, the other ex-
posed surfaces of the carbon plate 26 being sealed to
prevent entry of air into the porous plate. The vacuum
source 77 may be connected to a remote vacuum supply
through the port 18a, or it may be a replaceable vacuum
reservoir of the type described in the PCT Application No.
W095/12109 published May 4, 1995.
In another modified deaerator arrangement shown in
Fig. 7, ink passages 80 extending between the passages 24
and 25 are formed in a plate 81 which is mounted on the
front surface of the carbon plate 26 and an air-
permeable, ink-impermeable membrane 82, similar to the
membranes 17 of Fig. I, is positioned between the carbon
plate 26 and the plate 81. In this case, the coating 75
applied to the various passages within the carbon plate
26 is impermeable to air and only the portion of the
plate 26 adjacent to the membrane 82 is used to extract
air from the ink in the passages 80. If desired, a fil-
ter may also be incorporated in the plate 81 in the path
of the ink between the passages 24 and 25. Otherwise,
the arrangement of Fig. 7 is the same as that shown in
Fig. 6.
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Because the high thermal conductivity of the carbon
plate 26 assures heat conduction from relatively remote
heaters through the carbon plate to hot melt ink adjacent
to an orifice plate, a hot melt ink jet head according to
the invention may be arranged so that the ink is ejected
from an orifice plate mounted on an edge of a carbon
plate rather than from an orifice plate mounted on one
side of the carbon plate. Moreover, even if the ink used
in the ink jet head is not hot melt, ink, the easy~machin-
ability of the carbon plate provides a distinct advantage
in an arrangement of this type in contrast to a conven-
tional laminated plate arrangement, in which edges of the
plates adjacent to the orifice plate cannot be perfectly
aligned, leading to irregularities in the mounting of the
orifice plate.
This arrangement is shown in Fig. 8, in which a car-
bon plate 85 has pressure chambers 86 formed in one side
and a piezoelectric plate 87 affixed to that side of the
plate, and a bottom cover plate 88 affixed to the oppo-
site side of the plate. A row of orifice passages 89,
which are drilled into one edge 90 of the carbon plate
85, communicate with the pressure chambers 86 through
perpendicular passages 91 extending through the plate 85.
With a~carbon plate 85 of this type, the end surface 90
can be ground perfectly flat and the plate can then be
drilled to form the passages 89 and 91 to connect with
the pressure chambers 88, after which an orifice plate 92
is affixed to the edge 90 by epoxy adhesive in the manner
described above.
An ink jet head made in this way is especially ad-
vantageous, not only because it requires only a very nar-
row strip for the orifice plate 92, but also because it
permits the bulk of the printhead to be spaced from the
paper path and also permits stacking of multiple
printheads.
If desired, the ink jet head of Fig. 1 can be modi-
fied to provide similar advantages by forming the carbon
CA 02263783 1999-03-15
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plate 26 with a projecting portion 94 through which ori-
fice passages 93 extend to an orifice plate 92, as illus-
trated in Fig. 9.
In a further embodiment of the invention shown in
Figs. 14-20, an ink jet head is assembled from a plurali-
ty of carbon components. In this embodiment, as illus-
trated in the exploded view of Fig. 14, two carbon pres-
sure plate assemblies 102, described in greater detail
hereinafter, are assembled in a carbon collar 103 so that
their end surfaces, which contain ink outlet passages,
are aligned with corresponding openings in a manifold
plate I04 to which the pressure chamber plate assemblies
102 are affixed by screws 105 extending through the mani-
fold plate and into an adjacent pressure chamber plate
106. An orifice plate 107 has a linear array of closely
spaced orifices 108 which are aligned with the ends of
arrays of passages 109 (Fig. 20) in the manifold plate
104 so as to eject ink in response to selective actuation
of the pressure plate assemblies.
Interposed between the ends of the pressure plate
assemblies 102 and the manifold plate 104 is a filter
layer 110 having pores or openings slightly smaller than
the orifices 108 in the orifice plate 10? so as to pre-
vent potentially orifice-clogging solid material from
reaching the orifices 108 but large enough to permit par-
ticles of solid material smaller than the size of the
orifices to pass through the filter layer.
An ink reservoir 111, mounted against one side of the
collar 103 has an ink supply opening 112 which supplies ink
to the collar 103. As best seen in Fig. 16, a
corresponding opening 113 in the collar is aligned with the
opening 112 to receive ink from the reservoir 111. In
addition, if hot melt ink is to be used in the ink jet
CA 02263783 1999-03-15
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head, a cartridge heater 114 is mounted in a groove 115
formed in the side of the reservoir and in a correspond-
ing groove 116 (Fig. 18) in the side of the collar 103
and is controlled so as to maintain the ink within the
assembled ink jet head at a desired temperature during
operation of the system.
Each pressure chamber assembly 102 includes a pres-
sure plate 106 having arrays 117 of closely spaced pres-
sure chambers formed on opposite sides of the plate 106
and each of those arrays is covered by a piezoelectric
plate 118 of the type described previously with respect
to Fig. 4, having an array of electrodes 119 arranged
with respect to the array of pressure chambers 117 to
change the volume of the corresponding pressure chamber
in response to appropriate electrical signals.
The pressure chamber plate 106, which is illustrated
in greater detail in Figs. 15 and 16, has a longitudinal-
ly extending opening 120 which, in the illustrated
embodiment receives ink through an internal passage 123
terminating at an end surface 124 which faces the mani-
fold plate 104 as seen in Fig. 14. As shown in Fig. 19,
the surface of the manifold plate 104 facing the pressure
chamber plate, has an opening 125 which receives ink from
the collar passage t13 and supplies it to a groove 126
(Fig. 20) on the opposite side of the manifold plate,
from which ink'passes through two further openings 127 in
the manifold plate to the passages 123 in the pressure
plates 106 so that the ink is distributed through the
longitudinal opening 120 to all of the pressure chambers
in both of the arrays 117 in each plate.
In order to extract dissolved air from the ink as it
is being supplied to the arrays 117 of pressure chambers,
a deareator 128 consisting of a tubular member 129 made
of air-permeable, ink-impermeable material, such as ex-
truded polytetrafluoraethylene having a O.Imm wall thick-
ness and a l.5mm internal diameter, extends through an
opening 130 in the edge of each pressure chamber plate
CA 02263783 1999-03-15
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106 and into the longitudinal opening I20. A plug 131
closes the inner end of the tube and the end projecting
out of the opening 130 in the plate 106 is connected to a
vacuum source 132 supplying sufficient negative pressure,
such as 0.7 atmosphere, to reduce the dissolved air con-
tent of the ink being supplied to the pressure chambers
below the level at which air bubbles can form in the
pressure chamber during operation of the ink jet system.
In order to prevent the tube 129 from collapsing in re-
l0 sponse to application of negative pressure, a porous sup-
port, such as a rod of porous carbon or a helical wire
having a diameter substantially equal to the internal
diameter of the tube, is inserted into the tube.
As shown in Fig. 16, the end surface 124 of the car-.
bon plate 106 has two arrays of ink passages 133 which
extend perpendicularly to the end surface 123 and each of
those passages communicates internally with the adjacent
end of a corresponding pressure chamber in the arrays
117. Consequently, upon actuation of one of the pressure
chamber, ink is forced out of the plate 106 through a
corresponding one of the passages 133.
After passing through the filter layer 110, ink from
each of the passages 133 is supplied through a
corresponding passage 134 in an adjacent surface of the
manifold plate 104 shown in Fig. 17 and, as shown in Fig.
20, the arrays of passages 109 in the opposite surface of
the manifold plate extend horizontally along the surface
of that plate to convey the ink supplied through the pas-
sages I34 in a lateral direction toward the center of the
manifold plate. Those passages terminate in a central
line 135 extending longitudinally along the manifold
plate so as to be in line with the line of ink jet ori-
fices 108 in the orifice plate 107.
Although carbon is the preferred material for the
manifold plate 104, especially for ink jet heads used
with hot melt ink, other materials which can be formed
with a sufficiently flat surface and which have a thermal
CA 02263783 1999-03-15
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expansion coefficient compatible with adjacent components
may also be used. For example, steel and ceramics such
as alumina and glass, in which appropriate passages can
be formed by photoetching, may also be used to form the
manifold plate.
In a typical embodiment of the type shown in Figs.
14-20, each pressure chamber plate 106 is approximately
75mm long, 22mm wide and 2.5mm thick and each pressure
chamber array 117 contains 64 pressure chamber approxi-
mately 9mm long, lmm wide and 0.15mm deep and the mani-
fold plate 104 is approximately l.4mm thick. _
Heretofore it was believed that the total length of
the descender, which is the ink path leading from the end
of the pressure chamber to the orifice in the orifice
plate, should be as short as possible, i.e. no more than
about lmm. Although it is clear that each descender
should have a constant cross-section similar to that of
the pumping chamber so that its acoustic properties do
not result in undesirable reflections, and that it should
be short enough that viscus flow losses are not excessive
and that it should also be fluidically stiff so that
pressure energy losses from the surrounding structure are
not excessive, it has now been determined that, in ink
jet heads made of carbon components, the descender need
not be~so limited in length and can consist of a plurali-
ty of passages such as those in the manifold plate and
within the pressure chamber plate which total as much as
7mm in length without loss of performance. This permits
greater flexibility in the design of ink jet heads in
several respects. For eXample, the piezoelectric plate,
which is quite fragile, can be spaced a significantly
greater distance away from the substrate being printed by
the head and the body of the ink jet array may be made
thick enough to be mechanically robust and to provide
s5 good thermal uniformity. Moreover, laterally spaced
pressure chambers such as those in the arrays 117 may be
connected through laterally spaced passages 133 to supply
CA 02263783 1999-03-15
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ink to a single line of orifices 108 by using an arrange-
ment of laterally directed passages 109 such as that in-
corporated into the manifold plate 104.
With the simplified ink jet head according to the
invention, the problems caused by burrs and dimensional
variations resulting from heat produced in machining, by
differences in temperature coefficient of expansion of
the materials used in the ink jet head, and by the neces-
sity for assembling a number of previously formed plates
in precise relation, and the problems of bond stresses
during temperature cycling are effectively eliminated in
a convenient and inexpensive manner. Moreover, the num-
ber of steps required for the formation of the electrode
pattern on the piezoelectric plate and application of the.
plate to the ink jet head is substantially reduced and
variations in electrode positioning with respect to the
pressure chamber positions are eliminated.
Although the invention has been described herein
with reference to specific embodiments, many
modifications and variations therein will readily occur
to those skilled in the art. Accordingly, all such vari-
ations and modifications are included within the intended
scope of the invention.
