Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
DROPLET DEPOSITION APPARATUS ~ 2 g 2 9 0 9
BACKGROUND OF THE INVENTION
This invention relates to pulsed droplet deposition
apparatus and more particularly to such apparatus including a
plurality of droplet deposition channels. Typical of this kind
of apparatus are multi-channel pulsed droplet ink jet printers,
often also referred to as "drop-on-demand" ink jet printers.
An existing technology for the production of
multi-channel drop-on-demand ink jet printers is known from, for
example, US-A-3,179,042; GB-A-2 007 162 and GB-A-2 106 039.
These patent specifications disclose thermally operated
printheads which, in response to an electrical input signal,
generate a heat pulse in selected ink channels to develop a
vapour bubble in the ink of those selected channels. This in
turn generates a pressure pulse having the pressure and time
characteristics appropriate for the ejection of an ink droplet
through a nozzle at the end of the channel.
Thermally operated printheads of this nature possess a
number of significant disadvantages. First, the thermal mode of
operation is inefficient and typically requires 10 to 100 times
the energy to produce an ink droplet as compared with known
piezo-electric printheads. Second, difficulties are found in
providing the very high levels of reliability and extended
lifetimes which are necessary in an ink ~et printhead. For
example, thermally operated printheads have A tendency for ink
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deposits to form on the heating electrodes. Such deposits have
an insulating effect sufficient to increase substantially the
electrical pulse magnitude necessary to eject an ink droplet.
Thermal stress cracks and element burn-out, as well as cavitation
erosion, have also proved difficult to eliminate. Third, only
ink specifically developed to tolerate thermal cycling can be
used and suitable ink formulations often proved to be of low
optical density compared with conventional inks.
Attempts have been made to produce multi-channel ink
jet printers using piezo-electric actuators and reference is made
in this connection to US-A-4,525,728; US-A-4,549,191 and
US-A-4,584,590 and IBM Technical Disclosure Bulletin Vol. 23 No.
10 March 1981. Piezo-electric actuators have the advantage,
compared with thermal processes, of low energy requirement.
However, the existing proposals have not achieved the levels of
printing resolution that are desired. A prime influence upon
printing resolution ig the number of channels, and thus nozzles,
per unit length in the direction transverse to paper movement
relative to the head. Existing piezo-electric printhead
technology as exemplified by the prior art referenced above, is
capable of achieving a maximum channel density of around 1 to 2
channels per mm. In terms of effective resolution, and by this
is meant the density at which the droplets can be deposited upon
paper, such nozzle density is for many applications insufficient.
It does not, for example, enable a transverse line to be printed
with ink droplets that are indistinguishable by the eye at normal
reading distance.
129Z909
Effective resolution can be increased, for example, by
angling the printhead in the plane of the paper so as to decrease
the inter-channel spacing in the transverse direction. However,
this necessitates sophisticated control logic and the use of
delay circuitry to ensure that all droplets associated with a
particular print line are deposited on the paper in a single
transverse line (or sufficiently close to the line to be
indistinguishable therefrom by the eye). An alternative approach
is to provide for movement of the printhead. As will be
understood, this introduces significant mechanical and control
complexities, and is not felt to be advantageous. A third
approach to increasing effective resolution is to provide two or
more banks of channels which are mutually spaced in the direction
of paper movement but which cooperate to print a single
transverse line. With only two such banks it may be possible to
configure the nozzles of both channels in a common print line.
With more banks, a significant nozzle spacing is built up in the
direction of paper movement and delay circuitry is required to
provide for the time spaced actuation of the channels necessary
for spatial coincidence. The provision of delay circuitry adds
to manufacturing costs by an amount which typically increases
with the amount of delay required.
It is useful to note at this point that colour printing
would typically require four banks of channels even if each bank
provided in itself sufficient single colour resolution. Where a
multiplicity of banks are required to produce the desired
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resolution for a single colour, it will be understood that colour
applications compound the problems outlined above.
The advantages of decreasing the inter-channel spacing
in the direction transverse to relative paper movement should now
be apparent. In many cases, typically where colour printing is
required, there are further advantages in reducing the
inter-channel spacing along the directlon of paper movement (that
is to say between banks). This reduces the bulk dimensions of
the printhead but more importantly reduces the time delays
necessary for spatial coincidence.
SUMMARY OF THE INVEN~ION
Broadly, it is an ob~ect of th~s invent~on to provide
improved multi-channel pulse droplet deposition apparatus
operating at low energy levels and providing relatively large
numbers of channels per unit length whether transverse to or
parallel with the direction of paper movement, or both. It is a
further object of this invention to provide such apparatus which
is economic in manufacture.
The present invention in one aspect consists in a high
density multi-channel array, electrically pulsed droplet depo-
sition apparatus, comprising a multiplicity of parallel droplet
liquid channels, mutually spaced in an srray dlrection normal to
the length of the channels, lack of said channels being sepa-
rated from a like channel by a side wall which is transversely
displaceable in respective opposite senses and which extends in
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the lengthwise direction of the channels, and in a direction
which is both normal to said lengthwise direction and normal to
the array direction, respective nozzles opening into said channels
for ejection therefrom of droplets of liquid, connection means
for connecting ~aid channels to a source of droplet deposition
liquid and electrically actuable means located in relation to
said channels for effecting in each channel selec~ed for actuation,
transverse displacement generally parallel to said array direction
of said transversely displaceable side wall of said selected
channel, to causP change of pressure in said selected channel to
effect droplet e~ection from the nozzle opening thereinto.
BRIEF DESCRIPTION OF THE DRAWINGS
The lnvention wlll now be deacribed, by way of example, wlth
reference to the accompanying, dlagrammatic drawings, in which:-
FIGURE l(a) is a schematic perspective view of ageneralised form of multi-channel pulsed droplet
deposition apparatus, namely, a drop-on-demand
ink-~et array printhead, according to the
invention, with parts (particularly a cover
plate) omitted to reveal structural details;
FIGURE l(b) is a cross-sectional view taken normal to the
axes of the channels of the generallsed printer
illustrated in Figure l(a)
FIGURE l(c) ls a sectional plan view taken on the line
l(c)-l(c) of Figure l(b);
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FIGURE 2(a) is a fragmentary cross-sectional view similar to
that of Figure l¦b) but to a larger scale and
showing a specific printhead according to the
invention;
FIGUR3 2(b) is a fragmentary sectional plan view of the
printer of Figure 2(a) illustrating electrical
connections thereof;
FIGURE 2(c) is a view similar to Figure 2(a) of a modified
form of the embodiment of Figures 2(a) and 2(b),
FIGURE 2(d) shows voltage waveforms employed for ejecting
droplets from the printhead of Figures 2(a) and
2~b) or that of Figure 2(c);
FIGURE 3(a) is a cross-sectional view showing a further
specific form of printhead according to the
invention providing a two dimensional array of
channels;
FIGURE 3(b) is a fragmentary sectional plan view of the
printhead of Figure 3(a) illustrating electrical
connections thereof;
FIGURE 3(c) shows voltage wave forms for operating the
~:~ printhead of Figures 3(a) and 3(b);
FIGURES 4 to 7 are cross-sectional views similar to Figures
2(a) and 3(a) showing further embodiments of the
invention;
FIGURE 8 is a sectional plan view of a modification
applicable to the embodi=ents of Figures 2(a)
7 lZg2909
and 2(b), Figures 3(a) and 3(b), Figures 4, 5, 6
7 and 9;
FIGURE 9 is a cross sectional view similar to Figures
2(a) and 3(a) illustrating a further embodiment
of the invention; and
FIGURE 10 is a series of graphs illustrating the effect of
compliance changes on pressure changes in
neighbouring channels.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the drawings, like parts have been accorded the same
numerical references.
Referring first to Figures l(a), ltb) and l(c3, a planar
high-density array, drop-on-demand ink ~et printer comprises a
printhead 10 formed with a multiplicity of parallel ink channels
2, nine only of which are shown and the longitudinal axes of
which are disposed in a plane.
By "high-density array" in this context is meant an array in
which the ink channel density along a line intersecting the
channel axes perpendicularly, is at least two per millimetre.
The channels 2 contain ink 4 and terminate at corresponding ends
thereof in a nozzle plate 5 in which are formed nozzles 6, one
for each channel. Ink droplets 7 are ejected on demand from the
~ ;
channels 2 and deposited on a print line 8 of a print surface 9
between which and the printhead 10 there is relative motion
; normal to the plane of the channel axes.
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The printhead 10 has a planar base part 20 in which the
channels 2 are cut or otherwise formed so as to extend in
parallel rearwardly from the nozzle plate 5. The channels 2 are
long and narrow with a rectangular cross-section and have
opposite side walls 11 which extend the length of the channels.
The side walls 11 are displaceable transversely relatively to the
channel axes along substantially the whole of the length thereof,
as later described, to cause changes of pressure in the ink in
the channels to effect droplet ejection from the nozzles. The
channels 2 connect at their ends remote from the nozzles, with a
transverse channel 13 which in turn connects with an ink
reservoir (not shown) by way of pipe 14. Electrical connections
(not shown) for activating the channel side walls 11 are made to
an LSI chip 16 on the base part 20. By designing the working
parts for the multiplicity of parallel channels of the printhead
in a planar configuration, the manufacture of printheads with
very large numbers of parallel print channels can be performed in
a sequence of parallel operations, as hereinafter described,
working on jigs supporting a large number of base parts at one
time.
High density of packing of the ink channels 2 and,
therefore, of the nozzles 6 is achieved by a number of features
not found in prior art array printheads. First, the ink channels
2 are rectangular in the cross-section thereof viewed normal to
the channel axes, the side walls 11 (which form the longer edge
of each channel cross-section) extending normal to the plane
containing the channel axes. The aspect ratio of the channel
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cross-sections i.e. the ratio of the dimensions normal and
parallel to thè plane of the channel axes, is substantial,
typically 3 to 30. The channels particularly are separated by
transversely displaceable side walls 11 which are electrically
actuated to effect printing.
In ce.tain prior art arrays, see for example United States
Patents 4,525,728 (Koto), 4,549,191 (Fukuchi and Ushioda) and
4,584,590 (Fishbeck and Wright), the channels employ droplet
ejection actuators not in walls between the channels thereof but
in the top walls bounding the respective channels. The use of
such "roof" actuators limits the channel density, even after
optimisation, to 1 to 2 channels per millimetre. With channels
having displaceable side walls and high aspect ratio
cross-sections disposed with their longer dimension perpendicular
to the plane of the channel axes it is possible to provide
printheads of linear density greater than, and indeed
substantially greater than, 2 per millimetre. This represents a
substantial advance in the competitive pursuit for low cost per
channel, high resolution array printheads not subject to the
disadvantages referred to of thermal bubble operated devices.
The array disclosed in IBM Technical Disclosure Bulletin
Vol.23 No.10 March l9ôl has a piezo-electric actuator apparently
of disc form mounted in the wall between two adjacent cha~bers
and disposed so as to actuate one chamber upon flexural
displacement in one sense and to actuate the other chamber of the
pair upon displacement in the opposite sense. The chamber width
and inter-chamber spacing are substantial with the result that
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the chambers are required to converge (in a region away from the
actuators) so as to reduce the inter-nozzle spacing.
In the embodiments of the invention herein described
acoustic waves are employed in conjunction with electrically
actuated displaceable walls which are long, that is they extend
the whole cr substantially the whole length of the channels from
the nozzles 6 to the ink supply manifold. When actuated (as will
be seen), the displaceable side walls 11 on one or both sides of
a channel compress the ink in the channel. This pressure is
dissipated by an acoustic pressure wave travelling from the
nozzle. The condensation of the wave acts, for the period of
travel of the wave along the length of the channel, as a
distributed source the length of the channel which feeds ink
under pressure out of the nozzles to expel a drop.
Where a channel and the long narrow actuator, provided by
the whole or a part of a side wall 11 extending the length
thereof, is combined with an acoustic pump in this w~y, the
volume displacement of the actuator can be distributed so that
the wall displacement is small at any section. Typically the
actuator wall has an aspect ratio, i.e. the ratio of its width
between channels to its height, of 3-30 or more. At the same
time the layout is a planar parallel channel config~ration,
suitable for manufacture in quantity.
In practice the length of the channel along which the
acoustic wave travels is limited (only) by the period suitable
for drop expulsion, and by the growth of viscous boundary layers
in the ink channel. Typically, the length of the channel will be
1~2~i0~
more than 30 and preferably more than about 100 times its width
in the channel plane.
When the linear density of the channels in a planar array is
increased, it is the result of reducing both the narrow section
dimension parallel to the plane of the channel axes and the
thickness dimension in the same plane of the common displaceable
walls. This causes reduced compliance (CI) of the ink in the
channels and increased compliance (CW) of the displaceable walls
between channels.
High density of channels consequently means that the
compliance of the wall between ink channels is an important
aspect of the printhead design, which has not been considered in
prior art systems.
The wall compliance, for example, may effect the velocity of
sound in the ink along a channel, causing the acoustic velocity
to be lower in magnitude than for the ink solvent alone. At the
same time, when the displaceable side walls 11 are actuated, the
pressure in the ink in the actuated channels is lower with more
compliant walls than would be the case with less compliant walls.
Additionally, due to compliance, some change in pressure is
generated in neighbouring channels which are not actuated. Means
to compensate for what might otherwise be a disadvantage of a
printhead with displaceable walls are discussed below.
The embodiments of the invention illustrated in Figures
2ta), 2(b), 3(a), 3(b) and 4 to 7 show different possible ways of
; constructing and of operating the transversely displaceable,
inter-channel side walls 11. These will be considered in turn.
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In Figures 2(a) and 2(b) a printhead is shown which because
of its ease of manufacture and electromechanical efficiency is a
preferred embodiment of the invention. The array incorporates
displaceable side walls 11 in the form of shear mode actuators
15, 17, 19, 21 and 23 sandwiched between base and top walls 25
and 27 and each formed of upper and lower wall parts 29 and 31
which, as indicated by arrows 33 and 35, are poled in opposite
senses normal to the plane containing the channel axes.
Typically, the distance between adjacent side walls is 0.05mm and
the height of said side wall 0.30mm. The length of each channel
is typically lOmm or more. Electrodes 37, 39, 41, 43 and 45
respectively cover all inner walls of the respective channels 2.
Thus, when a voltage is applied to the electrode of a particular
channel, say electrode 41 of the channel 2 between shear mode
actuators 19 and 21, whilst the electrodes 39 and 43 of the
channels 2 on either side of that of electrode 41 are held to
ground, an electric field is applled in opposite senses to the
actuators 19 and 21. By virtue of the opposite poling of the
upper and lower wall parts 29 and 31 of each actuator, these are
deflected in shear mode into the channel 2 therebetween into
chevron form as indicated by broken lines 47 and 49. A pressure
is thus applied to the ink 4 in the channel 2 between the
actuators 19 and 21 which causes an acoustic pressure wave to
travel along the length of the channel and eject an ink droplet 7
therefrom. Alternative configurations of shear mode wall
actuators which can be employed are considered in co-pending
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application No. 556,137, filed by applicant in Canada on January
8, 1988.
It will be seen from Figure 2(b) that the electrodes 37 to
45, each specific to a channel, are individually connected to the
chip 16, to which are also connected a clock line 51,data line
53, voltage line 55 and ground line 57. The channels 2 are
arranged in first and second groups of alternate channels and
successive clock pulses supplied from clock line 51 enable the
first and second groups to be actuated in sequence. The da~a in
the form of multi-bit words appearing on data line 53 determines
which of the channels in each of the groups are to be activated
and causes, by the circuitry of the chip 16, the electrode of
each of those channels in the currently active group to have the
voltage V of the voltage line 55 applled to ~t. The voltage
signal actuates both of the actuable side walls of the selected
channel: consequently every sidewall is available to operate the
channels in each group of alternate channels. The electrodes of
the channels in the same group which are not to be activated and
the electrodes of all channels belonging to the other group are
held to ground.
Figure 2(d) shows two different voltage waveforms which can
be used for drop expulsion. In the mode of operation using the
first of these waveforms, the electrode of the activated channel
~s energised by the application of a positive voltage V for a
period L/a, where L is the channel length and "a" is the velocity
of sound in the ink. The voltage is then allowed to fall
relatively slowly to zero. The acoustic wave which travels along
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the channel from the nozzle end thereof durin6 the period L/a o~
application of the voltage V causeq condensation of the liquid
pressure and expels a drop from the nozzle of that channel whilst
the negative pressure in adjacent channels causes a rearward
movement of the meniscus. Thereafter, as the voltage signal
slowly falls to zero the actuated channel walls return to their
original positions whilst the original position of the ink
meniscus in the nozzle is restored by liquid feed to the channel
from the ink reservoir.
In the mode of operation employing the second of the
waveforms shown in Figure 2(d), a negative voltage V is
relatively gradually applied, as shown over a period L/a, to the
side walls of the actuated channel, this rate of application
being less than will cause drop ejection from the channel. The
voltage is now held for a period o~ about 2L/a when the residual
wave pressure in the activated channel, because of flow of ink
thereto from the adjacent channels, becomes positive. The
voltage -Y is then instantaneously removed so that the pressure in
the channel is increased and a droplet is e~ected as the walls
thereof are rapidly restored to their original positions. In
this mode of operation some of the initial energy is retained in
the acoustic pressure waves to assist droplet ejection. Also,
the side wall elasticity, which resists the actuator movement
during application of the voltage provides energy to generate
droplet expulsion following removal of the voltage signal. Wall
compliance coupled with the ink further helps to eject the ink
droplet during travel of the acoustic wave.
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In certain circumstances it may not be appropriate to have a
nozzle plate directly abutting the channel ends. Where, for
example, two banked arrays of channels are required to print on a
single line or where two side-by-side array modules are required
to produce constant drop spacing across the module boundary, it
may be necessary to have short connecting psssages between each
channel and its associated nozzle. It is believed important that
the volume of any said connecting passage should be 10% or less
of the volume of the channel.
Referring now to Figure 2(c), the embodiment of the
invention herein illustrated differs from that of Figures 2(a)
and 2(b) inasmuch as the upper and lower wall parts 29 and 31 of
side walls 11 taper from the adjoining top wall 27 and base wall
25. The width - transversely to the channels - of the roots of
the wall parts 29 and 31 is wider than in the case of the
previous embodiment whereas the tips are narrower.
So this feature is one way of reducing the compliance of the wall
actuators 15-23 or, equally, reducing the mean width that would
be occupied by the walls for the same compliance. It will be
apparent that the electrical arrangements for operating the
embodiment of Figure 2(c) are the same as illustrated in and
described with reference to Figure 2(b).
The constructions illustrated in Figures 2(a), 2(b) and 2(c)
can be further modified and operated differently from the mode of
operation described. To this end, alternate actuators, say,
actuators 15, 19, 23 are made active by having electrodes applied
thereto whilst the remaining actuators 17 and 21 are kept
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inactive either by being de-poled or by not having electrodes
applied thereto. With such an arrangement, the electrical
arrangement and method of operation is the same as that described
below for Figures 3(a) and 3(b).
It will be observed that in Figures 2(a) and 2(c) the
nozzles of alternate channels are slightly offset perpendicularly
of the plane of channel axes. This is to compensate for the time
difference in droplet ejection from the nozzles of first and
second groups of nozzles so that the droplets from both groups
are deposited in predetermined locations, suitably on a
rectilinear printline.
The method of manufacture of the embodiments of the
invention illustrated in Figures 2(a), 2(b) and 2(c) involves
poling each of two sheets of piezo-electric ceramic material in
the direction normal to the sheet and laminating the sheets
respectively to the base and top wall~ 25 and 27 which are of
inactive material, suitably, glass. The direction of poling is
in both cases towards the glass. Parallel grooves are then cut
in the sheets of piezo-electric ceramic material by rotating,
parallel, diamond cutting discs or by laser cutting. These
grooves extend through to the top or base wall, as the case may
,~
be, such grooves each providing half a channel of the finished
printhead. In the case of the version illustrated in Figure
2(c), the grooves are cut by laser or by profiled cutting discs.
The parallel grooves are arranged to open to one end of the
; corresponding ceramic sheet but stop short of the other end. At
the inner groove ends a transverse groove is cut to form an ink
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manifold. A hole is now drilled in a side of one of the ceramic
sheets to receive the pipe 14 for the connection of the ink
manifold with an ink reservoir. The exposed areas of the
piezo-electric ceramic material and adjoining top or bottom wall
surfaces are coated in known manner with metal in a metal vapour
deposition stage to form electrodes. In the case where
electrodes are not applied to all channel walls, selective metal
coating is effected by masking. The metal on the top surfaces of
the side walls, that is to say the surfaces disposed parallel to
the channel axes, is now removed and those surfaces of the
respèctive halves of the structure are then bonded together to
form the channels 2 between the integral side walls 11 ~o formed.
At a suitable stage in the manufacturing procedure, a passivating
insulator layer is applied over the electrode coating in the
channels. The nozzle plate 5 is then secured in position at one
end of the channels whilst, at the other end of the channels the
electrical connections are made to the chip 16 from the
electrodes coating side wall surfaces of the channels. The chip
16 is positioned in a recess cut in one of the ceramic sheets
rearwards of the cross channel 13 in the other of the ceramic
sheets.
A method of manufacture of the embodiments of Figures 1 and
2 above uses operations working simultaneously on large numbers
of parallel chains in an array plane. As explained above this
enables production costs per channel to be reduced.
In certain product configurations, however, it may be
convenient to assemble the arrays using a sandwich construction.
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For example, where multiple banks of channels are assembled in a
single printhead, each layer of the ~sandwich~ may provide one or
two channels of each bank. Embodiments showing each method of
working are described in this document but it will be understood
that each method can be adapted to any of the constructions
described.
With reference to Figures 3(a) and 3(b), there will now be
described an embodiment which exemplifies the sandwich form of
construction in a multiple bank printhead. As shown in Figure
3(a), inactive layers 61 alternate with layers of piezo-electric
material ~3 in a sandwich construction. The piezo-electric
material is poled in the thickness direction, that is to say in
the direction of arrow 65. The stack of layers is closed by a
top inactive layer 69 and a bottom inactive layer 71. A series
of parallel groove~ 73 are cut in the lower surface of each
inactive layer 61 and of the top inactive layer 69. Similarly, a
series of parallel grooves 75 is cut in the top surface of each
inactiv0 layer 61 and in the top surface of inactive bottom wall
71. It will be understood that in this way, rectangulsr channels
77 are formed which are bounded on three sides by inactive
material and on the fourth side by piezo-electric material.
Within each channel 77, a central electrode strip 79 is
deposited on the facing surface of the piezo-electric material.
Further eIectrodes 81 are established on each piezo-electric
layer surface at the lands of inactive material intermediate the
c~annels. In one example, the electrodes 81 are all connected to
ground.
B
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12~Z909
The channels 77 can be regarded as grouped into pairs in the
vertical array direction. The channels of each pair are then
divided by a common displaceable side wall formed by the
intervening piezo-electric layer. The central electrode 79 for
both channels of the pair are interconnected and it will be seen
that the application of a positive or negative voltage to these
electrodes will establish an electric field transverse to the
direction of poling of the piezo-electric material which will
deflect upwards or downards as appropriate to increase pressure
in the selected channel.
In this configuration, where channels are grouped into pairs
sharing the common actuating wall that divides them, there is
more than one way of assigning channels into groups. One option
is to assign, by analogy with the previously described
embodiment, all even numbered channels in one vertical line to
one group and all odd numbered channels to the other group. This
meets the requirement that both channels of one pair are never
slmultaneously called upon to e~ect a droplet. This requirement
can be met in other ways, however, and there is some advantage in
a scheme in which each group of channels is formed from
alternately left and right hand channels of successive channel
pairs.
~ ~ For example:
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GROUP CHANNEL NUMBERS
1 1 4 5 8 9
2 2 3 6 7 10 11
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An advantage of this scheme is that if, for example,
channels 2 and 3 are actuated simultaneously, they will apply
equal and opposite pressure to the inactive wall between them.
The simultaneous actuation of two such neighbouring channels 2
and 3 does not of course happen every time, but the event is
sufficiently common for the described advantage to be
significant.
The nozzles for the channels 77 are not shown in the
drawings. If necessary, an offset can be introduced between
alternate channels in a vertical direction to compensate for the
time difference between drop ejection from the channels of the
two groups. The spatial offset will be in the direction of
relative movement between the print surface and the described
array; this direction may be a vertical, horizontal or oblique.
Figure 3(b) shows how the electrodes are connected at the
channel ends remote from the nozzles, in the case of electrodes
81, by way of conductors 78 to ground and in the case of
electrodes 79 by way of conductors 80 to the power chip 16. The
chip has voltage lines 82,83 and 84 of +V, -V and zero
respectively connected thereto as well as clock line 87 and data
line 89.
Becauss one actuator operates a pair of channels and this
pair is isolated by inactive layers 61 on either side from the
operation of the other channels in the vertical array, the
description is now confined to the operation of an adjacent pair
of channels marked A and B operated by the actuator therebetween
and isolated by the inactive walls on opposite sides thereof.
lZC`~2C.O9
The signals which operate these channels are initiated by a 2 bit
data word supplied in a particular print cycle via the data track
87 to the drive circuit chip 16. This in turn generates one of
four voltage pulse waveforms of voltage range -V and applies
them to the actuator via track 80.
The 2 bit data word causes the drive circuit chip to produce
one of four voltage signals depending on whether the channel pair
is to print from both, the upper, lower or neither channel. The
four alternative voltage signals are illustrated in Figure 3(c)
and are supplied to those of the alternatives of the channels to
be actuated in the first or second group of channels, the clock
pulses from line 87 determining which group is to be operational
at any particular instant.
When only the first channel A is to generate a drop, the
signal (i) is generated. This comprises a voltage pulse of
magnitude V applied for two consecutive periods L/a and then
restored to zero. The response of the actuator and the
travelling pre~sure waves in the ink channels in response to the
signal (i) is now considered, the description being limited to
the lossless (zero viscosity) case.
When the voltage pulse V is applied to the actuator in the
pair of channels A,B the resulting displacement generates
instantaneously at time zero a positive unit pressure (Ip) in one
channel and an equal negative unit pressure (-p) in the other.
These pressures are dissipated by travelling acoustic step
pressure waves which propagate along the channel from the ends.
A drop is consequently expelled in time L/a from the first
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129Z909
channel nozzle aperture: at the same time ink flows from the back
of this channel round into the channel A: and the ink meniscus in
the nozzle in the second channel is also drawn inward. After
period L/a the pressure in the first channel after expelling a
drop is a negative pressure and the pressure in the second
channel is a positive pressure of magnitude depending on the
reflection co-efficient of the pressure waves at the channel ends
and the acoustic wave attenuation.
In the second period, since the actuator wall remains
displaced during the second period L/a, the travelling pressure
waves continue to propagate in each channel. The ink meniscus in
the first channel is now drawn inward and at the same time ink
flows into the channel at the back end from the second channel
due to the prevailing negative pressure. Meanwhile ink flows out
refilling the aperture in the second channel and from its back
end so that after period 2L/a the pressures again become ~ve in
the first channel and -ve in the second.
The ink meniscus in the aperture of the first channel has
now withdrawn by approximately the volume of one drop from its
initial condition due to the expulsion of a drop. The ink
meniscus in the aperture of the second channel after receding has
returned after period 2L/a to its initial position.
At the time 2L/a the voltage signal i9 cancelled and the
actuator returns to its rest position. This substantially
extinguishes the pressures in each channel and arrests the
expulsion of further ink from either aperture. The wave form in
Figure 3(c)(i) therefore expels an irx drop only ~rom the first
- 23 - ~ZC~2~09
channel. After the refill period T the ink is drawn back to
equilibrium by surface tension so that the ink has recovered its
datum position in each channel and further printing may proceed.
Waveform (ii) is that used to expel a drop only from the
second channel B. This involves application of a negative
voltage pulse for period 2L/a and works identically with the
application of the signal in Figure 2(a) and does not require
full description.
Waveform tiii) is that used to expel drops from the
apertures in both channels. The waveform is simply the two
previous waveforms (i) and (ii) applied one after the other, and
is complete after period 4L/a. The trivial case that no drop is
expelled from either channel when no actuation signal is applied
is shown for completeness as waveform (iv). The period L/a is
comparatively short so that the refill period T has greater
significance in defining the minimum period of the print cycles
than the period L/a of the travelling waveform.
Referring now to Figure 4, there is illustrated an
embodiment which operates broadly in the same way as is described
in connection with Figures 2(a) and 2(c), and therefore uses the
electrical arrangement of Figure 2(b), but employs shear mode
actuators generally of the form discussed in relation to Figure
3(a). The actuators are provided in every wall of the array
between the top and bottom walls 27 and 25 which, suitably, are
of glass. The electrodes take the form of two stiff metal,
suitably, tungsten blocks 95. One block 95 is provided at the
tip of the actuator wall part 97 extending from top wall 27 and
- 24 ~ 12~Z~09
the other at the tip of actuator wall part 99 extending from
bottom wall 25. Electrodes 103 and 105 (equivalent to electrodes
81 of Figure 3(a)) are located, as to electrodes 103, between the
wall parts 97 and top wall 27 and, as to electrodes 105, between
wall parts 99 and bottom wall 25. The poling direc~ion of the
wall parts 99 and 97 is parallel with the bottom and top walls
and is indicated by arrow 107. Accordingly, the electric field
applied to the poled wall parts is normal to the bottom and top
walls 25 and 27, The electrode connections are made at the ends
of the channels remote from the nozzles 6 by three point
connections via connectors 109, 110. As shown, connectors 109
connect a line at potential zero to electrodes 103 and 105 of one
actuator wall and to the blocks 95 of an adjacent actuator wall.
Connectors 110 connect a line at potential V to electrodes 103
and 105 of one actuator wall and also to blocks 95 in the next
ad~acent actuator wall.
The channels 2 are, as in the case of Figure 2(a) and 2(b)
arranged in first and second group of alternate channels, the
electrical connections providing as described for that embodiment
for switching of voltage Y or zero to selected channels of each
group in order to operate both side walls of each actuated
channel.
The manufacture of the embodiment of Figure 4 is performed
in the array plane in a generally similar fashion to that of the
embodiments of Figures 2(a) and 2(c). First each of the bottom
and top walls 25 and 27 has applied thereto a layer of metal
comprising the electrodes 105 and 103 using a masking technique
- 25 ~ 1 2 9 2 ~ O 9
to limit metal deposition to the places required. A layer of
piezo-electric ceramic poled in the direction o~ arrows 107 is
then bonded to each of the bottom and top wall~. To each of
said piezo-electric layers is then bonded a plate of tungsten or
other suitable stiff metal. Parallel grooves are cut into each
of the two multi-layered structures so formed and a transverse
groove is formed to unite common ends of the channel grooves.
The surfaces of the metal plates parallel with the bottom and top
walls are then bonded together to form the channels 2. The
nozzle plate 5 is thereafter secured at one end of the channels
and at the other end thereof the three point electrical
connectors are attached and leads are taken therefrom as before
described to the chip.
Refsrring now to Flgure 5, there is illustrated an
alternative embodiment in which walls 152 to 157 are assembled in
a sandwich construction by parallel strips 158, 159 of
piezo-electric ceramic. Each channel 2 is bounded by ad~acent
side walls and by a pair of piezo-electric Strip-Q 158 and 159.
The walls are conducting or have conducting electrodes applied to
their surfaces in contact with the piezo-electric strips so as to
form field electrodes. Poling of the piezo-electric strips is in
the direction of the arrows 160, that is to say in the field
direction. According, application of a field causes the
piezo-electric strips to expand or contract in thickness
~depending upon the polarity) and thus either draw together or
force apart the ad~oining walls.
- 26 -
12~2~09
To take the example in which it is desired to eject an ink
droplet from the channel marked A, the opposing walls 154 and 155
(or the electrodes on both faces thereof) are connected
respectively to the ~V and -V rails as shown in the Figure. Also
as shown, the other walls 152, 153, 156 and 157 are connected to
the ground rail. In this way a potential of ~2V is applied in
the same sense across both the piezo-electric strips associated
with channel A causing these to contract and pull together the
adjoining walls 154 and 155. A positive ink pressure is
therefore generated in the desired channel. Since the
piezo-electric strips between walls 153 and 154 and between walls
155 and 156 (that is to say the piezo-electric strips in the
channels at either side of the channels of interest) receive a
potential -V , they expand to permit movement of the walls 154
and 155 with no net change in overall dimension of the
printhead,
If a droplet is required to be e~ected simultaneously from
the next channel to A in the same group, for example channel C,
wall 156 is connected to the IV rail and wall 157 to the -V rail.
In this case the piezo-electric strips between walls 155 and 156
receive a potential at -2V so that they expand to accommodate
both the leftward movement of wall 155 and the rightward movement
of wall 156. The behav~our of the remaining walls is as
described above. This embodiment is another where every
sidewall is available to actuate the channels in each group.
Whilst this embodiment utilises the expansion or contraction
of piezo-electric elements in the 3-3 mode, the skilled man would
- 27 -
~2g2~09
appreciate that an alternative arrangement could be employed
utilising the 3-1 mode of deformation. In each case the
employment of a sandwich construction is favoured.
A still further embodiment of this invention is illustrated
in Figure 6. This employs bimorph walls 172 to 177 of thickness
poled piezG-electric material. These walls are separated by
conducting spacer blocks 170 and 179 which are electrically
connected to ground. Each channel 2 is defined between adjacent
bimorph walls and the interposed spacer blocks. Each bimorph
piezo-electric wall has a central electrode 180 to which voltages
of +V, 0, or -V can be applied. By way of example, if it is
desired to eject a droplet from the channel marked A, voltages of
~V and -V respectively are applied to the central electrodes 180
of the actuator walls 174 and 175. These accordingly distort in
flexure in opposite senses inwardly of the channel A, This is
illustrated in dotted outline in the Figure. There is
accordingly a positive ink pressure generated in the channel A to
e~ect a droplet.
Turning now to Figure 7, two sheets of piezo-electric
ceramic 190 and 191 are thickness poled and support between them
a parallel stack of walls 192 to 197. Adjacent walls serve to
define the channels 2. Each piezo-electric sheet 190,191 is
provided with an array of electrodes 198 formed, for example, by
parallel saw cuts in the piezo-electric ceramic being filled with
metal. The electrodes l9ô are arranged to lie at the
wall/channel interfaces and corresponding electrodes in the upper
- 28 - 1292~09
and lower sheets 190 and 191 are lnterconnected in a suitable
manner.
The mode of operation of the embodiment of Figure 7 involves
the shear rotation of sections of the piezo-electric ceramic
applying bending moments to the walls on opposite sides of the
channel of interest, so as to flex the walls inwardly of the
channel. This operation will be described in more detail,
taking, as an example, the e~ection of an ink droplet from the
channel marked A which lies between walls 194 and 195. As shown
in Figure 7, the electrodes 198 at either edge of channel A are
held at -V; the next two outward electrodes are held at ~V whilst
all other electrodes are held at ground. Considering the
piezo-electric ceramic sections lying between walls 193 and 194,
these receive a potential of ~V and undergo a rotation in the
arrowed direction. The piezo-electric ceramic ~ections carrying
the wall 194 receive a potential of -2V and thus undergo a double
rotation in the opposite sense. The piezo-electric ceramic
sectionq between walls i94 and 195 are not sub~ect to a field and
accordingly do not rotate, although they are displaced outwardly
by the action of neighbouring sections. It will be seen in this
manner that upper and lower ends of wall 194 have bending moments
applied thereto causing the wall to flex towards the position
shown in dotted outline. In analagous fashion, wall 195 is
caused to flex in the opposite sense leading to a positive
pressure change in the channel A.
If it is required to e~ect a droplet simultaneously from the
next channel in the same group, for example the channel marked C,
B
- 29 ~ 1 2 ~ 2 ~ 0 ~
the electrodes on either side of the channel recei~e a potentialof
-V whereas the next two outward electrodes receive a potential of
~V. The wall behavour is anslogous with that just described
except that the piezo-electric section between walls 195 and 196
has zero rather than -V potential applied. Accordingly this
section no longer undergoes a rotation but - as would be expected
of the central section between two actuated channels - merely
moves laterally to accommodate the rotations of its neighbours.
It is convenient at this stage to compare the embodiments so
far described. Aside from the constructional variations, the
embodiments can be grouped into two broad classes according to
the manner in which selected channels are energised.
In the first class, comprising the embodiments of Figures 2
and 4 to 7, every wall in the channel array is displaceable and
the necessary pressure change in each selected channel is brought
about through transverse displacement of both side walls of the
channel. This is the so-called ~every line active~ mode, (ELA)
and provides a number of advantages. In the ex&mple of Figure
2, with the opposing electrodes of both side walls in each
channel remaining at the same potential, a co~mon electrode can
be formed for each channel by plating all internal surfaces of
the channel. In manufacturing terms, this is considerably
simpler than forming separate electrodes on opposing side walls
of the channel. A further advantage is that with both walls
participating in droplet e~ection from a channel, maximum use is
made of the piezo-electric material available in the printhead,
and the actuation energy is lowered.
- 30 -
1292909
An alternative mode of wall actuation is where each channel
has one displaceable side wall, the other side wall remaining
fixed or inactive. This is the so-called "alternate lines
active" mode (ALA). It is exemplified by the embodiment of
Figure 3 and by the described modification to the Figure 2
embodiment in which alternate actuating walls are rendered
inactive by, for example, de-poling. As with the ELA mode, the
ALA mode can be driven in a unipolar manner, that is to say with
connections to a ground and one voltage rail, or bipolar, with
ground, ~V and -V rails. Unipolar drive circuitry is simpler but
the number of track connectors in the ALA mode is reduced if a
bipolar drive arrangement is used.
It will be recognised that a particular wall construction
can usually be driven in either of the ELA or ALA modes and a
design choice will be made depending upon the circumstances.
It has been mentioned previously that the compliance of the
walls between channels becomes an increasingly important factor
as channel density is increased. By "compliance" i9 meant here
the mean displacement in response to ink pressure. The relative
compliance of the wall as compared to the compliance of the ink
affects operation of the printhead in a number of related ways.
The electro-mechanical coupling efficiency is critically affected
by the compliances, so also is the degree of cross-talk between
neighbouring channels. In terms of energy efficiency, it is
important to match the compliance of the ink (CI) with the
compliance of wall (CW) and to optimise these with regard to
other channel parameters, particularly the nozzle.
:
- 31 - 12~2~09
Energy efficiency is not, however, the only deslgn criterion
of importance to compliance considerations. It is found that
cross-talk between channels increases markedly as relative wall
compliance increases. Clearly, it ls important that an ink
droplet should be ejected from only those channels that are
selected and the pressure generated in neighbouring channels
through cross-talk must be kept safely below the levels
associated with drop ejection.
Prior to the making of this invention, the problem of cross
talk was a factor regarded as placing an upper limit upon channel
density. It i9 interesting to note, for example, that the array
disclosed in IBM Technical Disclosure Bulletin Vo.23 No.10 March
1981 was shown having a wall thickness between actuator-sharing
chamber pairs which is still greater than that of the wall
accommodating the actuator. This was a method of reducing cross
talk.
Certain methods have been described earlier in this document
for reducing wall compliance. The shape of each wall can be
varied to increase stiffness and the thickness and nature of the
electrode layer applied to the walls can also usefully be varied
to increase stiffness. It is also practical to coat each
actuating wall with a rigid insulator such as silicon carbide or
tungsten carbide which are both about thirteen times as stiff as
PZT. A still further option to stiffen the actuator walls is to
corrugate them so that the channels are not straight, but
slightly sinuous. This modification is illustrated in Figure 8
which shows in schematic form, actuating walls 11 of sinuous form
- 32 - lZ92~09
arranged so that the channel 2 between them remains of constant
width. Such methods are particularly applicable to actuators
which deform in shear mode, since flexural rigidity is increased
independently. There is thus no increase in the voltage required
to produce a required displacement in shear mode.
As an alternative to reducing wall compliance, this
invention proposes techniques for increasing the compliance of
the ink. One such technique will now be described with reference
to Figure 9. In its operating characteristics, this embodiment
is very similar to that of Figure 2(a). However, the channels in
this case extend a significant distance into the glass substrate.
As will be apparent from the Figure 9, alternate channels are
extended into the bottom wall 25 and top wall 27 respectively.
This construction is achieved simply by increasing the depth of
cut of the disc, laser device or other cutting system used to
produce the channel in the piezo-electric sheet so as to cut a
slot not on~y in the sheet itself but also in the underlying
glass substrate.
By extending each channel laterally in this way the
compliance of the ink CI is increased with the same effect upon
the ratio CI/CW as is achieved by stiffening the walls. It will
be understood that methods spoken of as increasing relative wall
compliance may be used to reduce mean wall thickness for the same
compliance and therefore produce a printhead design of increased
linear channel density.
The influence of the ratio CI/CW is described with reference
to Figure 10. This is a graphical representation of the fluid
i29;~909
pressure arising in neighbouring channels upon energisation of a
single channel PO when both side walls are energised. The~
notation employed is that P 1 and Pl represent immediate
neighbour channels, P 2 and P2 next following channels, and
so on. In the theoretical case of entirely rigid walls, CI/CW is
infinite. As shown in Figure lO(a) a positive pressure at l2
arbitrary units is produced in channel PO and negative
pressures of -1 in neighbouring channels P 1 and Pl. There
is zero pressure change in channels P 2 and P2, which are of
course the immediately adjacent channels in the group containing
PO, so as would be expected there is no cross-talk. Figures
lO(b) to lO(d) illustrate the effect of varying CI/CW to assumed
values of, respectively, 18,8,3 and 1. It will be seen that as
the ratio CI/CW decreases, that is to say with the walls becoming
increasingly compliant in relative term~, the relative pressure
increases in group neighbour channels P 2 and P2. The
influence of compliance i8 also to reduce the pressure PO and
energy stored in the ink and to increase energy stored in the
walls. It will be recognised that size and velocity of a
droplet being ejected from say the P2 channel is reduced
particularly if channels PO and P4 are actuated
simultaneously. It should be noted, however, that the cross-talk
effect is substantially restricted to immediate group neighbours,
even at a wall compliance equal to the compliance of the ink.
This somewhat surprising result enables high density arrays to be
produced with the problem of cross-talk remaining of manageable
proportion.
129Z909
- 34 -
A still further method of compensation will be explained
with reference to Figure 9. If extended channel 254
is actuated, a positive pressure P will result in a negative
pressure -P/a in the physically neighbouring channels 253 and
255. The group neighbour channels 252 and 256 will be subject,
to negative pressures -P/b. Now, upon suitable choice of
material, dimension and the like, it can be arranged that the
cantilever beam substrate portions lying between channel 254 and
its group neighbours 252 and 256, will deform under the action of
the pressure differential between channels, so as to generate a
pressure ~P/b and compensate the negative pressure -P/b. In this
way the problem of cross-talk can be eliminated, thereby removing
the disadvantage that may be considered to arise from an array
with compliant walls. A design configuration can accordingly be
selected whlch is based on considerstions of channel density and
energy efficiency wlthout regard to interchannel cross-talk
within a group of channels.
It should be understood that this invention has been
described by way Or example and a wide variety of modificatlons
sre possible without departing from the scope of the claims.
With regard to piezo-electric material, for example, PZT is
preferred although lt would be posslble to use other ceramic
materials such as barium titanate, or piezo-electric crystalline
substances such as gadolinium molybdate or Rochelle salt. The
piezo-electric msterial may be used as a layer upon a substrate
of which glass has been described as an example but for which
numerous alternatives will appear to the skilled man.
B
129Z909
- 35 -
Alternatively, blocks of piezo-electric material can be employed
in place of the described layered or laminate structures with the
piezo-electric walls then being integral with the supporting base
wall. An advantage of the structure in which a piezo-electric
side wall is mounted upon a glass or other electrically insulated
substrate is that electrical cross talk between channels of the
array is reduced as is the problem of stray fields causing
unwanted distortion of a base wall formed of piezo-electric
material.
It should be understood that the channels or apparatus
according to this invention whilst parallel, need not have their
axes lying precisely in a common plane. It has been described
how offset channels can offer advantages. Generally, the
parallel channels should be spaced in an array direction. In
apparatus affording a two-dimensional array channels, it should
be noted that the array direction need not necessarily be normal
to the direction of relative movement. Indeed, the advantages
have been explained of increasing channel density in an array
direction which is parallel to the direction of relative movement
of the print aurface.
The specific description of this invention has been confined
largely to pulsed droplet ink ~et printers. Whilst references
have been made to "paper", it should be understood that this term
has been used generically to cover a variety of possible print
surfaces. More genérally, the invention embraces other forms of
pulsed droplet deposition apparatus. For example, such apparatus
may be used for depositing photo-resist, sealant, etchant,
dilutent, photo-developer, dye and the like.
