Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DROPLET DEPOSITION APPARATUS
The present invention relates to droplet deposition apparatus, such as, for
example, ink jet printheads.
The current drive in drop-on-demand inkjet printing towards higher resolution
requires increased density both of ink ejection nozzles and the associated
drive circuitry. However, the increased density of the drive circuitry can
lead
to problems associated with overheating. Similarly, the trend towards ever
greater printhead widths places correspondingly greater demands on heat
management within printheads. Thermal ("bubble jet") printheads benefit in
this regard from having their drive circuitry in close contact with the ink,
which
has a cooling effect. This is offset, however, by the need for special
measures
to maintain the electrical integrity of the circuitry in the ink environment.
It is an object of at least the preferred embodiments of the present invention
to prevent, in a simple manner, the drive circuitry of a printhead from
overheating without at the same time risking its electrical integrity.
In a first aspect the present invention provides droplet deposition apparatus
comprising:
a fluid chamber having actuator means actuable by electrical signals to
effect ejection of droplets from the fluid chamber;
drive circuit means for supplying the electrical signals to the actuator
means; and
conduit means for conveying droplet fluid to or from said fluid chamber;
the drive circuit means being in substantial thermal contact with said
conduit means so as to transfer a substantial part of the heat generated in
said
drive circuit to the droplet fluid.
Arranging the drive circuit means in such a manner can conveniently allow the
ink in the printhead to serve as the sink for the heat generated in the drive
circuitry. This can substantially reduce the likelihood of overheating, whilst
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avoiding the problems with electrical integrity that might occur were the
integrated circuit packaging containing the circuitry allowed to come into
direct
contact with the ink.
The apparatus may comprise first conduit means for supplying droplet fluid to
said fluid chamber and second conduit means for leading droplet fluid from
said fluid chamber. If so, the drive circuit means may advantageously be
thermally connected to the second conduit means. This can provide the most
direct route out of the printhead for the heat generated in the chip of the
drive
circuit and, in the event that the heat produced by the chip varies
significantly
during operation, can minimise any variation in the temperature of the ink in
the fluid chamber itself. As is known, for example, from W097/35167, such
temperature variation can give rise to variations in droplet ejection velocity
and
consequent dot placement errors in the printed image.
Where the drive circuit is incorporated within an integrated circuit package
of
substantially cuboid form in which at least some of the faces are rectangles
each having a surface area, a face other than that face having the smallest
surface area may advantageously be arranged so as to lie substantially
parallel to the direction of fluid flow in that part of the conduit closest to
said
face, and to be in substantial thermal contact with the fluid. Such an
arrangement can ensure significant heat transfer to the droplet fluid.
Preferably, that face having the greatest surface area is arranged so as to
lie
parallel to the direction of fluid flow. Circuit architecture permitting, such
an
arrangement can maximise heat transfer from the circuitry.
A second aspect of the present invention provides droplet deposition apparatus
comprising:
at least one droplet ejection unit comprising a plurality of fluid chambers,
actuator means and a plurality of nozzles arranged in a row, said actuator
means being actuable to eject a droplet of fluid from a fluid chamber through
a respective nozzle; and
a support member for said at least one droplet ejection unit, said
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support member comprising at least one droplet fluid passageway
communicating with said plurality of fluid chambers and arranged so as to
convey droplet fluid to or from said fluid chambers in a direction
substantially
parallel to said nozzle row and to transfer a substantial part of the heat
generated during droplet ejection to said conveyed droplet fluid.
This can provide for substantially even distribution of heat along the length
of
the support member, which can lead to reduced thermally-induced strains that
might otherwise distort the printhead. Such distortion would become more
pronounced as the width of the printhead increased, for example, to that of a
page (typically 12.6 inches (32 cm) for the American "Foolscap" standard) and
would occur regardless of whether a plurality of narrow ejection units or a
single wide ejection unit were used in conjunction with the support member.
Advantageously, the droplet fluid passageway may occupy the majority of the
area of the support member when viewed in cross-section. Alternatively or in
addition, the passageway may comprise respective portions for the flow of
droplet fluid in to and out of each fluid chamber. Such flow can aid the
transfer of heat from the fluid chamber (where the main source of heat - the
actuator means - is located) to the remainder of the support, thereby reducing
temperature differentials.
To provide effective support for the at least one droplet ejection unit, the
cross-
section of support member is preferably wider in the direction of ink ejection
from the nozzles than in the direction of the nozzle row.
In one embodiment, the apparatus comprises a plurality of said droplet
ejection
units, the support member supporting the droplet ejection units side by side
in
the direction of the nozzle rows, the support member comprising at least one
droplet fluid passageway communicating with at least two of said ejection
units
and arranged so as to convey droplet fluid to or from said ejection units in a
direction substantially parallel to said nozzle rows and to transfer a
substantial
part of the heat generated during droplet ejection to said conveyed droplet
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fluid.
Heat distribution may be -iacilitated by constructing the support member from
a material - such as aluminium - having a high thermal conductivity. Such a
material also has advantages as regards manufacture and cost. Problems
arise, however, where the ejection unit is made from material having a
coefficient of thermal expansion that is significantly different to that of
the
support. This will be the case with an ejection unit comprising channels
formed in a body of piezoelectric material (typically lead zirconium titanate,
PZT) described hereafter. As will be readily appreciated, differential
expansion
- particularly in the direction of the nozzle row in a "pagewide" device - may
lead to distortion and/or breakage of ink seals, actuator components,
electrical
contacts, etc.
Therefore, it is preferable to provide means for attaching said at least one
droplet ejection unit to the support member in order to substantially avoid
transferral of thermal deformation of the support member to said at least one
droplet ejection unit.
A third aspect of the present invention provides droplet deposition apparatus
comprising:
a fluid chamber, at least part of which is formed from a first material
having a first coefficient of thermal expansion, said chamber being associated
with actuator means actuable to eject a droplet from the chamber and having
a port for the inlet of droplet fluid thereto;
a support member for said fluid chamber and including a passageway
for supply of droplet liquid to said port, the support member being defined at
least in part by a second material having a second coefficient of thermal
expansion greater than said first coefficient; and
means for attaching the fluid chamber to the support member in order
to substantially avoid transfer of thermal deformation of the support member
to said fluid chamber.
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Preferably, the attachment means comprises resilient bonding means for
bonding the or each fluid chamber to the support member. In an example
described hereafter, an adhesive rubber pad is used to bond a support
member of extruded aluminium to a fluid chamber structure comprising a
channel formed in a body of PZT and closed by cover member of a material,
such as molybdenum, that is thermally matched to the PZT. Forming ink
supply ports in the cover and ink ejection nozzles in the channelled component
can provide a particularly compact design having a low component count.
Further advantageous embodiments of the invention are set out in the
description, drawings and dependent claims.
The invention will now be described by way of example by reference to the
following diagrams, in which:
Figure 1 is a perspective view from the front and top of a first embodiment of
the invention;
Figure 2 is a perspective view from the rear and top of the printhead of
figure
1;
Figure 3 is a sectional view of the printhead taken perpendicular to the
direction of extension of the nozzle rows;
Figure 4 is a perspective view from the top and above of one end of the
printhead of figure 1;
Figure 5 is a sectional view taken along a fluid channel of an ink ejection
module of the printhead of figure 1; and
Figure 6 is a sectional view of a second embodiment of droplet deposition
apparatus taken perpendicular to the direction of extension of the nozzle
rows.
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Figure 1 illustrates a first embodiment of droplet deposition apparatus
embodied by a
printhead 10. The embodiment shown is a "pagewide" device, having two rows of
nozzles 20,30 that extend (in the direction indicated by arrow 100) the width
of a piece
of paper, which allows ink to be deposited across the entire width of a page
in a single
pass. Ejection of ink from a nozzle is achieved by the application of an
electrical signal
to actuation means associated with a fluid chamber communicating with that
nozzle, as
is known e.g. from EP-A-0 277 703, EP-A-0 278 590. To simplify manufacture and
increase yield, the "pagewide" rows of nozzles are made up of a number of
modules,
one of which is shown at 40. Each module has associated fluid chambers and
actuation means and is connected to associated drive circuitry (integrated
circuit ("chip")
50) by means e.g. of a flexible circuit 60. Ink supply to and from the
printhead is via
respective bores (not shown) in endcaps 90.
Figure 2 is a perspective view of the printhead of figure 1 from the rear and
with
endcaps 90 removed to reveal the supporting structure 200 of the printhead
incorporating ink flow passages 210,220,230 extending the width of the
printhead. Via
a bore in one of the endcaps 90 (omitted from the views of figures 2 and 3),
ink enters
the printhead and the ink supply passage 220, as shown at 215 in figure 2. As
it flows
along the passage, it is drawn off into respective ink chambers, as
illustrated in figure 3,
which is a sectional view of the printhead taken perpendicular to the
direction of
extension of the nozzle rows. From passage 220, ink flows into first and
second parallel
rows of ink chambers (indicated at 300 and 310 respectively) via aperture 320
formed in
structure 200 (shown shaded). Having flowed through the first and second rows
of ink
chambers, ink exits via apertures 330 and 340 to join the ink flow along
respective first
and second ink outlet passages 210,230, as indicated at 235. These join at a
common
ink outlet (not shown) formed in the endcap located at the opposite end of the
printhead
to that in which the inlet bore is formed.
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Each row of chambers 300 and 310 has associated therewith respective drive
circuits 360, 370. The drive circuits are mounted in substantial thermal
contact
with that part of structure 200 acting as a conduit and which defines the ink
flow passageways so as to allow a substantial amount of the heat generated
by the circuits during their operation to transfer via the conduit structure
to the
ink. To this end, the structure 200 of the embodiment of figures 1-3 is made
of a material having good thermal conduction properties. Of such materials,
aluminium is particularly preferred on the grounds that it can be easily and
cheaply formed by extrusion. Circuits 360,370 are then positioned on the
outside surface of the structure 200 so as to lie in thermal contact with the
structure, thermally conductive pads or adhesive being optionally employed to
reduce resistance to heat transfer between circuit and structure.
In the embodiment shown, the cuboid drive circuit dies 360,370 are arranged
such that a largest (rectangular or square) surface of each die lies
substantially
parallel to the direction (indicated at 235) of fluid flow in the respective
parts
of the conduits 210,230 lying closest to those surfaces. This helps maximise
heat transfer between circuit and ink, which is also facilitated by minimising
the
thickness of the structure separating the ink channel and the circuit, as well
as
by making the structure of a material having good thermal conduction.
Reference is now made to figure 4, which is a perspective view from the top
and above of one end of the printhead with all but one of the modules 40
having been removed to show external and internal details of structure 200
more clearly. The structure includes recesses 500 to accommodate drive
circuits 370 and lips 510,520 to retain further circuit boards 530 populated
with
those components not suited to incorporation into the drive circuits 370.
Forming rear lip 520 on a separate component 540, as shown in figure 4,
allows these boards to be clamped into place by the action of fastening
means, for example screws inserted through holes 240 shown in figure 2 and
engaging with a bar (not shown) residing in channel 550. Preferably the bar
is made of a strong material, such as steel, able to accommodate screw
threads and reinforce aluminium structure 200, particularly against the forces
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generated when installing and connecting the printhead.
In the present embodiment, further circuit board is also formed with pins
(figure
3, 420) for supply of power and data into the printhead and with posts 560 for
supplying power and data - suitably processed - to the drive circuits 370 via
flexible connectors 570. Such connection techniques are well known in the art
and will not therefore be discussed in further detail.
As explained above, heat generated in the drive circuits is transferred to the
ink whence it is distributed about the structure 200 as a result of the
aforementioned ink flow paths. Heat generated in the ink chambers by the
associated actuator means is also distributed in this manner. As a result, any
temperature differentials that arise within structure 200 are small and do not
give rise to significant internal forces and/or distortion.
However, the overall warming of the printhead during operation may lead to
differential expansion of the structure 200 and the body in which the fluid
chambers 300,310 are formed where these two members are of materiais
having significantly differing coefficients of thermal expansion, CTE. This is
the
case in the present embodiment having fluid chambers formed in a body of
piezoelectric material in accordance with the aforementioned UK application
number 9721555.
As illustrated in figure 5, which is a sectional view taken along a fluid
channel
of a module 40, channels 11 are formed in a base component 860 of
piezoelectric material so as to define piezoelectric channel walls
therebetween.
These walls are subsequently coated with electrodes to form channel wall
actuators as are known e.g. from the aforementioned EP-0-0 277 703, a break
in the electrodes at 810 allowing the channel walls in either half of the
channel
to be operated independently by means of electrical signals applied via
electrical inputs (flexible circuits 60).
Each channel half is closed along a length 600,610 by respective sections
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820,830 of a cover component 620 which is also formed with ports
630,640,650 that allow ink to be supplied to and from each channel half for
cleaning and heat removal purposes, as is generally known. As is also known,
cover component 620 is preferably made of a material that is thermally
matched to the piezoelectric material of the channelled component. Ink
ejection from each channel half is via openings 840,850 that communicate the
channel with the opposite surface of the piezoelectric base component to that
in which the channel is formed. Nozzles 870,880 for ink ejection are
subsequently formed in a nozzle plate 890 attached to the piezoelectric
component.
To avoid the distortion of the printhead that might otherwise occur as a
result
of the differing thermal expansion characteristics of the piezoelectric
material
of the fluid chambers and the aluminium of the structure 200, tie rods may be
inserted in bores 580 in the structure and tightened so as to keep structure
200 in compression. Although any material having a value of CTE less than
that of the structure - steel in the case of an aluminium structure - is
suitable
for the tie rods, it will be appreciated that low values of CTE are to be
preferred.
In addition, cover component 620 may be attached to structure 200 by means
of a resilient bond - adhesive coated rubber is shown at 430 in figure 3 - so
as to allow any relative expansion that may occur in spite of the presence of
tie rods (and which may be of the order of 0.3mm over a typical 12.6" (32 cm)
length of a printhead) to take place at this less critical interface rather
than
generating stresses and deformations in the printhead module 40 itself. As
shown in figure 4, cover 620 may be sat in a well 590 formed in structure 200
and may additionally extend to either side of the printhead to provide
mounting
surfaces for the printhead. Molybdenum, which has high strength and thermal
conductivity in addition to being thermally matched to PZT, has been found to
be a particularly suitable material for the cover.
Figure 6 shows a sectional view of a second embodiment of droplet deposition
apparatus taken perpendicular to the direction of extension of the nozzle
rows.
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Similar to the first embodiment shown in Figure 3, the supporting structure
900
of the printhead incorporates ink flow passages 910,920 extending the width
of the printhead. Ink enters the printhead and the ink supply passage 920 as
shown at 915 in figure 6. As it flows along the passage, it is drawn off into
respective ink chambers 925 via aperture 930 formed in structure 900. Having
flowed through the ink chambers, ink exits via apertures 940 and 950 to join
the ink flow along ink outlet passage 910 as indicated at 935.
A flat alumina substrate 960 is mounted to the structure 900 via alumina
interposer layer 970. The interposer layer 970 is preferably bonded to the
structure 900 using thermally conductive adhesive, approximately 100 microns
in thickness, the substrate 960 being in turn bonded to the interposer layer
970
using thermally conductive adhesive.
Chips 980 of the drive circuit are mounted on a low density flexible circuit
board 985. To facilitate manufacture of the printhead, and reduce costs, the
portions of the circuit board carrying the chips 980 are mounted directly on
the
surface of the alumina substrate 960. In order to avoid overheating of the
drive circuit, other heat generating components of the drive circuit, such as
resistors 990, are mounted in substantial thermal conduct with that part of
the
structure 900 acting as a conduit so as to allow a substantial amount of the
heat generated by these components 990 during their operation to transfer via
the conduit structure to the ink.
In addition to the alumina substrate and interposer layer, an alumina plate
995
is mounted to the underside of the structure 900 in order to limit expansion
of
the aluminium structure 900 at this position, thereby substantially preventing
bowing of the structure due to thermal expansion.
Each feature disclosed in this specification (which term includes the claims)
and/or shown in the drawings may be incorporated in the invention
independently of other disclosed and/or illustrated features.
