Note: Descriptions are shown in the official language in which they were submitted.
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Droplet Deposition Apparatus and Methods of Manufacture thereof
The present invention relates to droplet deposition apparatus, in
particular an inkjet printhead, which comprise a channel communicating with a
supply of droplet liquid and an opening for ejection of droplets therefrom, at
least one channel side wall being displaceable in response to electrical
signals, thereby to effect ejection of droplets from the channel.
Figure 1 a is a cross-sectional view of the channels of the prior art inkjet
printhead construction according toW092/22429. Piezoelectric ceramic sheet
1o 12 is poled in its thickness direction 17 and formed in one surface with
channels 11 bounded on two sides lying parallel to the channel axis by
channel walls 13. By means of electrodes 23 formed on either side of each
wall 13, an electric field can be applied to the piezoelectric material of the
walls, causing them to deflect in shear mode in a direction transverse to the
channel axis. Pressure waves are thereby generated in the ink which result in
the ejection of an ink droplet. These principles are known in the art, e.g.
from
EP-A-O 364 136.
Channels 11 are closed along one side lying parallel to the channel
axis by the surface of a cover 14 having conductive tracks 16 at the same
pitch interval as the ink channels formed thereon. Solder bonds 28 are formed
between tracks 16 and the channel wall electrodes 23, thereby securing the
cover to the base and creating an electrical connection between the
electrodes and the track in a single step. To protect them from later being
corroded by the ink, electrodes and tracks are then given a passivant coating.
As shown in figurel b, which is a sectional view taken along the
longitudinal axis A of a single channel of the prior art printhead of figure
la, a
nozzle plate 20 having respective ink ejection nozzles 22 is mounted at the
front of the sheet 12 whilst an ink manifold 26 is defined at the rear by a
manifold structure 21. Tracks 16 are led to the rear of cover 14 for
connection
to a drive circuit, typically embodied in a microchip 27 which in turn is
driven
by signal received via input tracks 18.
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In printheads of this ilk, the channel walls - and in particular the
electrodes formed thereon - are often passivated so as to protect from
subsequent corrosion by the ink.
In the device discussed above, however, such conventional passivation
prior to attachment of the cover would inhibit the formation of solder bonds
between the electrodes and the tracks. On the other hand, passivation after
the cover has been attached can only be applied from the end of the channel,
resulting in low quality coating of the electrodes and tracks, especially at
the
midpoint of the channel remote from the channel ends.
The present invention has as an objective a printhead construction that
retains the connection advantages associated with the conductive tracks
formed on the cover of the prior art construction and yet is amenable to
passivation.
As embodied and broadly described herein, the invention provides a
droplet deposition apparatus comprising: at least one longitudinal, open-
topped droplet liquid channel defined by facing longitudinal side walls and a
bottom, longitudinal surface extending between the side walls; means for
applying an electric field to piezoelectric material in at least one of the
walls,
thereby to effect displacement of the wall relative to the longitudinal
channel
so as to eject a droplet from the channel; and a cover closing the open,
longitudinal top side of the channel; wherein the bottom longitudinal surface
of
the channel is formed with an opening for droplet ejection, and; the cover
incorporates two ports for supply of droplet liquid, the ports being spaced
along the channel on either side of the opening.
Since the sole point of electrical connection between the track and the
actuator in accordance with the present invention lies outside of the channel
and thus out of contact with the ink (with its potentially corrosive effects),
passivation of this point is no longer required. The channel itself can
therefore
be conventionally passivated via the open tops of the channels. Thereafter,
the cover can be attached and electrical contact established between the
conductive tracks on the cover and the actuator means associated with the
channel walls. Even in a printhead that - because of the type of ink it is
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designed to fire - does not require passivation, a point of electrical
connection
lying outside the channel as per the present invention is less likely to fail
in
fatigue than the channel-length solder bonds of the prior art device of
figures
1 a, lb.
The invention also relates to a method of manufacture of droplet
deposition apparatus comprising: providing a body including piezoelectric
material and having at least one longitudinal, open-topped channel, the
channel being defined by facing longitudinal side walls and a bottom,
longitudinal surface extending between the side walls; forming an opening in
the bottom longitudinal surface of the channel for ejection of droplet liquid;
providing means for applying an electric field to piezoelectric material in at
least one of the walls, thereby to effect displacement of the wall relative to
the
longitudinal channel so as to eject a droplet from the channel; and closing
the
open, longitudinal top side of the channel by means of a cover having two
droplet fluid supply ports arranged so as to lie spaced along the channel on
either side of the opening.
The step of closing the channel may result in the electrical connection
of the conductive track and the actuator means, thereby simplifying the
manufacturing process.
The invention also consists in droplet deposition apparatus comprising:
a bottom sheet of piezo-material poled droplet deposition apparatus
comprising: a bottom sheet of piezo-material poled in a direction normal to
said sheet and formed with a multiplicity of parallel, open-topped channels
mutually spaced in an array direction normal to the length of the channels and
defined each by facing side walls and a bottom surface extending between
said side walls; a top sheet facing said bottom surfaces of said channels and
bonded to said side walls to close said channels at the tops thereof;
respective nozzles communicating with said channels for the ejection of
droplets of liquid therefrom; connection means for connecting said channels
with a source of droplet deposition liquid; wherein each channel is formed
with
a forward part in which electrodes are provided on opposite sides of at least
one of the side walls defining the channel, thereby to form a shear mode
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actuator for effecting droplet expulsion from the channel; and wherein each
channel is formed with a rearward part having an electrically conductive
coating which is in electrical contact with the at least one electrode on the
channel-facing sides of the side walls in the forward part;
sealing means separating the forward part from the rearward part; and
wherein the apparatus further comprises conductive tracks formed on that
surface of said top sheet that is bonded to said side walls, the conductive
tracks being in electrical contact with the electrically-conductive coating in
said rearward part.
A corresponding method comprises forming a bottom sheet with a layer
of piezo-material poled in a direction method of manufacture of a droplet
deposition apparatus comprising: forming a bottom sheet with a layer of
piezo-material poled in a direction normal to said sheet; forming a
multiplicity
of parallel, open-topped channels mutually spaced in an array direction
normal to the length of the channels, each channel being defined by facing
side walls and a bottom surface extending between said side walls, each
channel further having a forward part and a rearward part; forming electrodes
on opposite sides of at least one of the side walls defining the forward part
of
each channel, thereby to form a shear mode actuator for effecting droplet
expulsion from the channel; and forming in the rearward part of each channel
an electrically-conductive coating in electrical contact with a respective
electrode; providing a top sheet having a surface formed with conductive
tracks thereon; and bonding that surface of the top sheet having conductive
tracks thereon to said side walls so as to close said channels at the tops
thereof; establishing electrical contact between said tracks and the
respective
electrically-conductive coating of each channel; andproviding sealing means
separating the forward and rearward parts of each channel.
The invention furthers provides a droplet deposition apparatus
comprising droplet deposition apparatus comprising: at least one longitudinal,
open-topped droplet liquid channel defined by facing longitudinal side walls
and a bottom, longitudinal surface extending between the side walls; means
for applying an electric field to piezoelectric material in at least one of
said
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walls, thereby to effect displacement of the wall relative to said
longitudinal
channel so as to eject a droplet from the channel; and a cover closing the
open, longitudinal top side of the channel; wherein said bottom longitudinal
surface of the channel is formed with an opening for droplet ejection, and;
the
cover incorporates two ports for supply of droplet liquid, the ports being
spaced along the channel on either side of the opening.
Such a construction again simplifies the manufacture of known
printheads, particularly those of the "top shooter" kind. Figure 2 shows a
sectional view along the channels of such a prior art printhead, with those
1o features that correspond to figure 1 being denoted by corresponding
reference numbers. Droplet ejection takes place from a nozzle 22 formed in
the channel cover component 60whiist droplet liquid is supplied to the channel
via ports 33 formed in the channel base and which are typically connected in
their turn to ink supply conduits (not shown) formed in a base component 35
that is separate from the piezoelectric channelled component 12.
In accordance with the invention, an opening communicating with a
droplet ejection orifice is formed in the bottom surface of the channel,
thereby
allowing the cover component to incorporate ports for supply of ink into the
channel. A further, separate base component is consequently no longer
required.
The invention also provides a droplet deposition apparatus comprising:
at least one longitudinal, open-topped droplet liquid channel defined by
facing
longitudinal side walls and a bottom, longitudinal surface extending between
the side walls; means for supplying droplet liquid to the channel; means for
applying an electric field to piezoelectric material in at least one of said
walls,
thereby to effect displacement of the wall relative to said longitudinal
channel
so as to eject a droplet from the channel; and a cover closing the open,
longitudinal top side of the channel; wherein the bottom longitudinal surface
of
the channel is formed with two openings for droplet ejection, the openings
being spaced along the channel.
The invention will now be described by way of example by reference to
the following diagrams, of which:
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Figure 3 is a sectional view taken along the channel axis of a printhead
according to a first embodiment of a first aspect of the present invention;
Figures 4a and 4b show detail of the rear part of the printhead of figure
3 before and after attachment of the cover respectively;
Figure 5 is a sectional view taken along the channel axis of a printhead
according to a second embodiment of a first aspect of the present invention;
Figure 6 is a sectional view taken along the channel axis of a printhead
incorporating both first and second aspects of the present invention;
Figure 7 is a sectional view taken along the channel axis of a printhead
1o according to a second embodiment of a second aspect of the present
invention;
Figure 8 is a detail perspective view of the end of the piezoelectric
body of the printhead of figure 7.
Figures 9 and 10 are sectional and detail sectional views respectively
of an alternative embodiment of the printhead shown in figure 7.
Figure 3 illustrates a printhead according to a first embodiment of the
first aspect of the present invention, with those features that are common to
figure 3 and the prior art printhead of figures 1 and 2 being designated by
common reference numerals.
As in the prior art device, a piezoelectric ceramic body 12 poled in the
thickness direction is formed with channels 11 separated by channel walls 13.
As known from EP-A-0 364 136, electrodes 23 are formed along each wall 13
in the ink-containing channel 11 as well as extending along a rearward groove
100 to the rear face 130 of the body. In addition, there is provided a cover
14,
a surface 15 of which closes the open side of each of the channels 11, a
nozzle plate 20 with nozzles 22 for droplet ejection and a manifold for supply
of ink into the channel in the form of a transverse cut in the body 12.
Surface
15 of cover 14 has tracks 16 formed thereon (suitable processes are well
know) which in turn are connected to microchip 27 (which is illustrated
figuratively in figure 3 and not to scale) which in turn receives input
signals
from input tracks 18.
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Detail of the rear part of the printhead prior to attachment of the cover
is shown in figure 4a: a passivation layer 140 (not shown in figure 3 but
indicated by dashed hatching in figure 4a) is applied over the entirety of the
electrodes 23 (indicated by solid hatching in both figures 3 and 4a) in the
channel and part way along the rearward groove 100. In contrast to the prior
art construction, passivation is carried out before attachment of the cover.
A mechanical bond between body and surface 15 of cover 14 is
achieved by means of adhesive layer 160, applied to the end surfaces of the
walls 13 in the region of the channels 11 prior to assembly of cover and body.
Figure 4b illustrates the assembled printhead, with the adhesive bond being
indicated at 220. Such a bond may indeed be tougher and have a longer
fatigue life than the corresponding solder bond of the prior art construction
described above.
Electrical connection between the conductive tracks 16 on the cover
and that part of the electrode 23 in the rearward groove 100 is achieved by a
protrusion 170 of a malleable, deformable, conductive material such as solder
affixed to the end 180 of track 16. On assembly of the cover to the body, as
illustrated in figure 4b, protrusion 170 comes into contact with electrode 23
and is deformed, thereby providing an effective electrical contact 200 between
electrode 23 and track 16.
A bead 190 of a sealing paste or high viscosity glue is also applied so
as to form on assembly an ink seal 210 between the end of the ink channel 11
and the electrical contact 200. Such a seal protects the electrical contact
from
later corrosion by ink. Preferably, the seal is positioned so as to straddle
the
free end 150 of the passivation layer 140, thereby preventing the seepage of
ink under the passivation layer from where it might otherwise attack the
electrode material 23.
Figure 5 illustrates a second embodiment of the first aspect of the
present invention. A ceramic piezoelectric body 290 is, as in the previous
embodiment, poled in the thickness direction and formed with channels 11
separated by channel walls 13 which in turn have an electrode 23 formed on
each side. Ink ejection, however, takes place from a centrally located nozzle
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320 formed either directly in the cover 350 or, as shown, in anozzle plate 330
communicating with the channel via an aperture 340 formed in the cover.
Body 290 is additionally formed with two manifolds 310 for supply of from both
ends of the channel, as indicated by arrows 300. A further structure (not
shown) will supply the manifolds with ink from a reservoir.
Such a "double-ended" printhead configuration has advantages in
terms of a lower operating voltage over the "single-ended" configuration
described above. Furthermore, the configuration of base 290 is suited to
manufacture by moulding - a technique that is potentially more attractive from
the point of view of manufacturability than conventional sawing techniques.
The connection of the channel electrode 23 to conductive tracks 370
formed on that surface of cover 350 facing body 290 is as already described
with regard to figures 3, 4a and 4b, however, and is located in groove 360
formed at one side of the body 290. Similarly, in the region of the channel
itself (the channel walls of which are passivated prior to assembly) and at
that
end 380 of the body not occupied by an electrical connection, cover 350 is
attached to the piezoelectric ceramic body by a conventional adhesive bond
(not shown).
In order to minimise the distance travelled by the ink from the channel
proper 11 to the outlet of the nozzle 320 - thereby reducing pressure losses
and consequent reductions in droplet ejection velocity - the nozzle 320 may
be formed in the cover 350 itself. Advantageously the nozzle is formed by
laser ablation, and to this end the cover may be made of an easily ablatable
material, suitably a polymer such as polyimide, polycarbonate, polyester or
polyetheretherketone, typically of 50pm thickness.
The stiffness of a cover plate formed of such an easily ablatable
material may be increased by application of a coating of stiffer material to
the
inner and outer surfaces of the ablatable cover plate. Particularly suitable
for
this purpose is silicon nitride: it can also be used as a passivant coating in
the
process of the aforementioned W095/07820, is deposited as a smooth coating
suitable for the subsequent application of a non-wetting coating, and will not
short out electrodes of adjacent channels due to its non-conducting
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properties. Two layers of such a material placed either side of thepoiyimide
cover and each having a thickness of around 5% of that of the cover(2.5cm in
the case of a50calm thick cover) will typically increase bending stiffness by
a
factor of 5-10 (based on standard compound beam theory and assuming a
value of Young's Modulus for the stiffening material approximately 100 times
greater than that of the polymer and good adhesion between the stiff and
polymer materials). Such a thin layer has no significant effect on the ease
with
which the cover plate can be ablated to form a nozzle, particularly if the
material of the layer itself is to some degree ablatable.
Expressed in broad terms, the cover plate for an inkjet printer
comprises a layer of a first, easily ablatable, material having further layers
bonded on opposite sides thereof, the further layers each being of a material
having a stiffness at least an order of magnitude greater than that of the
first
material and being of a thickness at least an order of magnitude less than
that
of the first layer.
Referring now to figure 6, there is shown a printhead incorporating both
first and second aspects of the present invention. Piezoelectric ceramic body
400 is formed with channels 11, channel-separating walls 13 and electrodes
23 which are supplied with actuating signals via conductive tracks 410
connected to drive circuitry (not shown). Unlike previous embodiments,
however, droplet ejection takes place from a nozzle 420 communicating with
an opening 430 formed in the body 400 at the closed, bottom surface 440 of
the channel 11 - this is in contrast to figure 5 where the nozzle 320 is
located
in a cover 350 closing the open, top side of the channel 11.
Moulding is again the preferred method of manufacture of the
channelled body 400, and the arrangement of figures 4a and 4b is again
employed for electrical connection between the electrodes 23 and conductive
tracks 410. Communication hole 430 may also be formed during the moulding
process or may be formed subsequently, e.g. by means of a laser. Cover 450
no longer incorporates a nozzle but is instead formed with ink inlet ports
460.
Such an arrangement has a lower component count than embodiments
discussed earlier and has consequential manufacturing advantages.
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Alternatively, ink supply ports could be formed in the channelled component,
e.g. at the channel ends.
The printhead of figure 7 also employs a cover component 500 having
ink inlet ports 520, 522 and 524 located at either end and in the middle of a
channel 11 formed in a piezoelectric body 530. Channel walls are separated
by a gap 540 into two sections 550,560 supplied by ports 520,522 and
522,524 respectively, with each section being independently actuable by
means of respective electrodes 570, 580 driven by drive circuits (not shown)
via conductive tracks 650,660. For each section there is provided a respective
1o nozzle 610,620 formed in a nozzle plate 615 and communicating with a
section of the channel 11 via communication holes 630,640 formed in the
bottom surface of the channel at points located midway between the
respective inlet ports for that section.
Such a configuration results in a printhead having two parallel rows of
independently actuable printing elements that is compact and which has a
reduced actuating voltage per unit droplet ejection velocity due to the
"double-
ended" ink supply to each channel section.
Unlike earlier embodiments, the conductive tracks 650,660 that
electrically connect the channel electrodes to the drive chips are formed on
the piezoelectric body itself, advantageously in the same step in which the
electrodes 570,580 are deposited on the channel walls. Such an arrangement
is known from EP-A-0 397 441, and consequently will not be described in
further detail here. Connection between track 650,660 and drive chip 590,600
may be achieved by any conventional method, including wire bonding or gold
ball connection.
Piezoelectric body 530 may be moulded: in addition to having clear
manufacturing advantages, such a process permits the end of the channel 11
to be formed as illustrated in figure 8, namely with a smooth, continuous
transition 700 from the top surface 720 of the body to both the channel wall
730 and the bottom, longitudinal surface 710 of the channel. This in turn
avoids discontinuities in the subsequently-deposited electrode material and
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the associated heating effects which might have a deleterious effect on the
operational life of the printhead as a whole.
Alternatively, channels may be formed in the piezoelectric component
by sawing using a disc cutter - as described e.g. in EP-A-0 309 148 - and
illustrated in the sectional and detail sectional views of figures 9 and 10.
It
follows that the depth of the channel 11 will run out more gradually at each
end, as shown at 800, and that the piezoelectric channel wall defined between
adjacent sawn channels 11 will run continuously between the two active
sections 550,560. However, a break 810 in the electrodes on the channel
walls at a location between the two sections ensures that each the wall in
active section can be actuated independently by signals supplied via
electrical
input 820. Such a break may be achieved e.g. by masking during deposition
of the metal plating or by removal of the plating by a laser.
Connection between the electrodes on the channel walls and the
electrical input 820, whilst not shown in detail, may be achieved by any of
the
known techniques including wire bond between tracks formed in shallow "run-
out" grooves formed in the area 900 rearward of the channel 11 (described in
the aforementioned EP-A-0 364 136) or conductive adhesive (e.g. anisotropic
conductive adhesive) between conductive tracks formed in area 900 on the
surface of the piezoelectric sheet itself and (described in EP-A-0 397 441).
As in the embodiment of figure 7, each channel 11 is closed along its
two active sections 550,560 by appropriate lengths 820,830 of a cover
component 500 which is also formed with ports 520,522,540 that allow ink to
be supplied to each channel active section and, optionally, allow ink to be
circulated through each channel section for cleaning purposes, as is generally
known. Ports may be positioned so as to define the edge of an active section,
as in the case of port 522, in which case manufacture is simplified. In the
example shown, the width of cover port 552 and the cover closing lengths
820, 803 are of the same order of magnitude, typically 2mm.
Ink ejection from each active section is again via openings that
communicate the channel with the opposite surface of the piezoelectric
component (sheet 860) to that in which the channel is formed. In the present
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embodiment, these openings take the form of slots 840,850 which extend
some distance - typically200m - in the longitudinal direction of the channel
so
as to allow some leeway in the placing of the respective nozzles 870,880 in
nozzle plate 890. Offsetting of nozzles is generally necessary whenever
simultaneous droplet ejection from adjacent channels is not possible e.g. in
"shared wall" printheads of the kind illustrated, is generally known e.g. from
EP-A-0 376, and will not therefore be discussed in any greater detail.
Printheads according to the present invention may also be made in a
modular format as described in the aforementioned W091/17051, each
1o module being formed in opposite end surfaces thereof with respective
channel
parts so that, upon butting together of modules, further channels are formed
between respective pairs of butted modules. In such arrangements, the
respective channel parts may include at least part of a slot formed in the
channel base and of sufficient length that, even if a pair of butted modules
and their respective slot parts are not perfectly aligned, there remains an
overlap between the two slot halves sufficient to accommodate a nozzle.
As in the previous embodiment, nozzles 870,880 are formed in a
nozzle plate 890 which, as illustrated, may extend over the substantially the
entire length of piezoelectric sheet 860 so as to provide a suitably large
area
for engagement e.g. of a capping and/or wiping mechanism.
It should be understood that this invention has been described by way
of examples only and that a wide variety of modifications can be made without
departing from the scope of the invention. Features shown in the context of
the first aspect of the invention may be equally applicable to the second
aspect and vice versa.
The piezoelectric channel walls, for example, can be polarised in
opposite directions normal to the plane of the channel axes as known, for
example, from EP-A-0 277 703. Alternatively, polarisation of the channel walls
can be parallel to the plane of the channel axes with electrodes formed in the
channel walls themselves as known, for example, from EP-A-0 528 647.
Nor is every channel in a printhead required to be capable of droplet
ejection: active channels capable of droplet ejection may be alternated in the
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printhead with inactive - so-called "dummy" channels - as described, for
example, in the aforementioned EP-A-O 277 703.
13
