Note: Descriptions are shown in the official language in which they were submitted.
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PRINTHEAD WITH MATCHED RESONANT DAMPING STRUCTURE
FIELD OF THE INVENTION
The present invention relates to the field of inkjet printing and in
particular, inkjet
printers with pagewidth printheads.
BACKGROUND OF THE INVENTION
The Applicant has developed a wide range of printers that employ pagewidth
printheads instead of traditional reciprocating printhead designs. Pagewidth
designs increase print
speeds as the printhead does not traverse back and forth across the page to
deposit a line of an image.
The pagewidth printhead simply deposits the ink on the media as it moves past
at high speeds. Such
printheads have made it possible to perform full colour 1600dpi printing at
speeds in the vicinity of 60
pages per minute; speeds previously unattainable with conventional inkjet
printers.
Printing at these speeds consumes ink quickly and this gives rise to problems
with supplying
the printhead with enough ink. Not only are the flow rates higher but
distributing the ink along the
entire length of a pagewidth printhead is more complex than feeding ink to a
relatively small
reciprocating printhead.
The high print speeds require a large ink supply flow rate. This mass of ink
is moving
relatively quickly through the supply line. Abruptly ending a print job, or
simply at the end of a
printed page, means that this relatively high volume of ink that is flowing
relatively quickly must also
come to an immediate stop. However, suddenly arresting the ink momentum gives
rise to a pressure
pulse in the ink line. The components making up the printhead are typically
stiff and provide almost
no flex as the column of ink in the line is brought to rest. Without any
compliance in the ink line, the
pressure spike can exceed the Laplace pressure (the pressure provided by the
surface tension of the ink
at the nozzles openings to retain ink in the nozzle chambersand flood the
front surface of the printhead
nozzles. If the nozzles flood, ink may not eject and artifacts appear in the
printing.
Resonant standing waves in the ink occur when the nozzle firing pattern
matches a resonant
frequency of the ink supply line. Again, because of the stiff structures that
define the ink line, a large
proportion of nozzles for one color, firing simultaneously, can create a
standing wave in the ink line.
For example, printing spaced black lines for, say, a table of data, will fire
many, if not most, of the
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black nozzles at a particular frequency. If this particular frequency matches
a resonant frequency of the
ink supply structure, a standing wave can start oscillating back and forth.
This can result in nozzle
flooding, or conversely nozzle deprime because of the sudden pressure drop
after the spike, if the
Laplace pressure is exceeded.
The Applicant has addressed these issues by incorporating non-priming cavities
into the
printhead. A detailed description of the non-priming cavities is provided in
the Applicant's
U.S. Patent No. 8,025,383. Briefly, the stiff structures that define the ink
line have air pockets
distributed long the length of the printhead. A pressure pulse from a resonant
standing wave in
the ink will compress the air in the cavity as it passes that point in the ink
line. Compressing
the air in the cavity damps and dissipates the pressure pulse. The reduced
pulse amplitude is
less likely to flood the nozzles.
Unfortunately, the lowest resonant frequencies of the ink line have the
highest pressure
amplitudes. To damp these pressure waves, the non-priming cavities need to be
impractically
large. A series of large air pockets positioned along the ink line is counter
to compact design.
Furthermore, diurnal heating and cooling of big air cavities would either pump
a large volume
of ink out through the nozzles, or deprime the nozzles by drawing ink back
into the support
molding.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a printhead for an inkjet printer,
the printhead
comprising:
at least one printhead integrated circuit (IC) with an array of nozzles for
ejecting ink;
a support structure for supporting the printhead IC, the support structure
having an ink conduit
for supplying the array of nozzles with ink, the conduit having a set of
resonant frequencies at which
ink in the conduit generates a standing wave in response to certain operating
modes of the array of
nozzles; and,
a fluidic damper having a selected resonant frequency that damps the standing
waves associated
with each of the set of resonant frequencies such that they have an amplitude
less than a maximum
threshold.
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The invention recognizes that particular resonant frequencies are more
problematic than others.
Typically, the lowest frequency harmonic causes an oscillating pulse with the
highest amplitude.
However, tuning the fluidic damper precisely to the frequency of the lowest
harmonic changes the
amplitude of the standing waves at the other frequencies and the next lowest
harmonic can then be a
problem. Tuning the damper to resonate at a frequency between the two lowest
resonant frequencies
can sufficiently damp the pressure amplitudes at all the resonant frequencies.
The fluid damper uses a
single thin tube of ink acting against a compliant structure such as an air
cavity. The tube of ink and
the air cavity are far more compact than a line of large air cavities along
the length of the printhead.
Similarly, expansion and contraction of the single small air cavity due to
diurnal temperature changes
are not problematic.
Preferably, the selected resonant frequency of the fluidic damper is between
the two lowest
resonant frequencies in the set of resonant frequencies. In a further
preferred form, the selected
resonant frequency is the root mean square of the two lowest resonant
frequencies in the set of resonant
frequencies ¨ that is, the square root of the product of the lowest two
frequencies.
Preferably, the fluidic damper has a cavity of compressible fluid connected to
the ink conduit
via a tube configured to at least partially prime with ink when the printhead
primes. In a further
preferred form, the compressible fluid is air trapped when the printhead is
primed with ink. In
particular embodiments, the printhead is a pagewidth printhead for printing on
A4-sized media, the ink
line having a main channel extending longitudinally along the length of the
printhead between the inlet
and the outlet, the ink line also having a series of non-priming air cavities
positioned along its length.
In specific embodiments, the support structure has an inlet for connecting the
ink line to an ink
supply, and an outlet for connecting the ink line to a waste ink reservoir,
the fluidic damper being
connected to the ink line adjacent the outlet. Preferably, the fluidic damper
has less than 0.4 ml of air.
Optionally, the maximum threshold pressure is less than 4 kPa. Optionally, the
ink pressure at
the array of nozzles is maintained above -3 kPa to avoid deprime and keep
ejected drop volumes above
a minimum volume.
In a particularly preferred form, the printhead is configured to print
different colored inks, each
ink color having a respective fluidic damper, the fluidic damper for one color
having a resonant
frequency that differs from at least one of the other colors.
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BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described by way of example
only,
with reference to the accompanying drawings, in which:
Figure 1 is a schematic representation of a prior art printer's fluidic
system;
Figures 2A, 2B and 2C show the standing waves for the lowest three resonant
modes of the
printhead ink supply line shown in Figure 1;
Figure 3A shows the peak pressures in the ink as a function of frequency for
the ink line
without a fluidic damper;
Figure 3B shows the peak pressures in the ink as a function of frequency for
the ink line with a
fluidic damper tuned to resonate at the first resonant frequency of the ink
line;
Figure 3C shows the peak pressures in the ink as a function of frequency for
the ink line with a
fluidic damper tuned to resonate at root mean square of the two lowest
resonant frequencies of the ink
line;
Figure 4 is a schematic representation of the printhead assembly with fluidic
damper according
to the present invention;
Figure 5 shows the printhead cartridge of the present invention installed the
print engine of a
printer;
Figure 6 shows the printhead cartridge of the present invention removed from
the print engine
of a printer;
Figure 7 is a perspective of the complete printhead cartridge according to the
present invention;
Figure 8 shows the printhead cartridge of Fig. 7 with the protective cover
removed;
Figure 9 is an exploded is a partial perspective of the printhead assembly
within the printhead
cartridge of Fig. 7;
Figure 10 is an exploded perspective of the LCP moldings within the printhead
cartridge of Fig.
7; and,
Figure 11A, 11B and 11C show the outlet manifold of the printhead cartridge.
DETAILED DESCRPITION OF THE PREFERRED EMBODIMENTS
Figure 1 is a schematic view of a prior art fluidic system of the type used in
the above
referenced U.S. Patent No. 8,025,383. The operation of the system and its
individual
components are described in detail in U.S. Patent No. 7,914,132.
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Briefly, the printer fluidic system has a printhead assembly 2 supplied with
ink from an
ink tank 4 via an upstream ink line 8 and waste ink is drained to a sump 18
via a downstream
ink line 16. A single ink line is shown for simplicity. In reality, the
printhead has multiple ink
5 lines for full colour printing. The upstream ink line 8 has a shut off
valve 10 for selectively
isolating the printhead assembly 2 from the pump 12 and or the ink tank 4. The
pump 12 is
used to actively prime or flood the printhead assembly 2. The pump 12 is also
used to establish
a negative pressure in the ink tank 4. During printing, the negative pressure
is maintained by
the bubble point regulator 6.
The printhead assembly 2 is an LCP (liquid crystal polymer) molding 20
supporting a
series of printhead ICs 30 secured with an adhesive die attach film (not
shown). The printhead
ICs 30 have an array of ink ejection nozzles for ejecting drops of ink onto
the passing media
substrate 22. The nozzles are MEMS (micro electro-mechanical) structures
printing at true
1600 dpi resolution (that is, a nozzle pitch of 1600 npi), or greater. The
fabrication and
structure of suitable printhead IC's 30 are described in detail in U.S. Patent
No. 7,744,195.
The LCP molding 20 has a main channel 24 extending between the inlet 36 and
the outlet 38.
The main channel 24 feeds a series of fine channels 28 extending to the
underside of the LCP
molding 20. The fine channels 28 supply ink to the printhead ICs 30 through
laser ablated
holes in the die attach film.
Above the main channel 24 is a series of non-priming air cavities 26. These
cavities 26
are designed to trap a pocket of air during printhead priming. The air pockets
give the system
some compliance to absorb and damp pressure spikes or hydraulic shocks in the
ink. The
printers are high speed pagewidth printers with a large number of nozzles
firing rapidly. This
consumes ink at a fast rate and suddenly ending a print job, or even just the
end of a page,
means that a column of ink moving towards (and through) the printhead assembly
2 must be
brought to rest almost instantaneously. Without the compliance provided by the
air cavities
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26, the momentum of the iffl( would flood the nozzles in the printhead ICs 30.
Furthermore,
the subsequent 'reflected wave' can generate a negative pressure strong enough
to deprime the
nozzles.
In the majority of cases, the air cavities 26 offer sufficient damping.
However, the
printhead can operate in modes that excite the ifflc to one of the resonant
frequencies of the iffl(
line. For example, printing black lines across a page at a particular spacing
(for a table, bar
code or the like) requires all the black nozzles to fire simultaneously for
brief periods. This
cyclic input to the ink line can quickly establish a standing wave oscillating
at a resonant
Figures 2A, 2B and 2C, show the three lowest harmonics for printhead assembly
shown in Figure 1. It should be noted that the main channel responds as if it
is a blind end
even though it has the outlet 38. Because it is a closed end, the main channel
resonates with a
quarter wave harmonic, a three quarter wave harmonic, a 1.25 wave harmonic and
so on. An
open end would resonate at 0.5 wave, full wave, 1.5 wave and so on. The lowest
harmonics
Figure 2A is the lowest frequency harmonic; the quarter wave, in which the
length L
of the LCP main channel is one quarter the wavelength. Testing on some of the
Applicant's
A4 printers has shown this to occur at about 12 Hz and has a peak amplitude of
about 9 kPa.
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amplitude of about 2 kPa at 60 Hz. As the frequency of the harmonic increases,
the amplitude
of the wave rapidly attenuates. Hence the higher frequency harmonics have peak
pressures
small enough for the non-priming air cavities to damp.
Figure 3A shows these pressure peaks as function of frequency. If the deprime
and
flood thresholds are set at, say, - 3 kPa and 4 kPa respectively, it can be
seen that the quarter
wave and three quarter wave harmonics have peak pressures that will be
problematic for
printer operation. However, incorporating a damper that resonates at the
quarter wave
frequency does not solve the problem. Figure 3B shows the change in the
frequency response
curves when a fluidic damper tuned to the quarter wave is added to the end of
the main
channel 24 (see Fig. 1). Essentially the main channel now responds as if it
were an open
channel and the half wave, full wave etc harmonics become relevant. One or
more of these
harmonics may also generate excessive peak pressures.
Figure 3C shows the frequency response when the fluidic damper is tuned to a
frequency between the quarter and half wave harmonics. This attenuates both
the quarter and
half wave harmonics. The Applicant has found that the optimum resonant
frequency for the
fluidic damper is approximately the root mean square of the quarter wave
frequency and the
half wave frequency; that is, the square root of the product of the quarter
wave resonant
frequency and the half wave resonant frequency. In reality, it is necessary to
test several
frequencies around the root mean square frequency to find to the optimum
resonant frequency
for the fluidic damper. Irregularities such as ink filters, bends and
elasticity in the ink supply
line and so on shift the actual pressure response curves from the theoretical
curves.
Figure 4 is a schematic representation of the printhead assembly 2 according
to the
present invention. The LCP molding 20 has a fluidic damper 40 that resonates
at a frequency
selected to attenuate potentially problematic standing waves at any of the
resonant frequencies
of the main channel 24. The fluidic damper 40 has a thin tube 32 filled with
ink connecting
the main channel 24 to a small cavity of compressible fluid 34 ¨ most
typically air. The thin
tube of ink has an inertance proportional to its length, cross sectional area
and density of the
ink. The air cavity is a compliance against which the ink in the thin tube 32
can oscillate.
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In the printhead assembly shown, the fluidic damper is tuned to a frequency at
or near
the root mean square of the quarter wave and the half wave resonant frequency
of the main
channel 24 in the LCP molding 20. As discussed above, the impedance provided
by the
damper at the quarter and half wave harmonics is sufficient to keep both of
them less than the
predetermined pressure threshold. Positioning the fluidic damper 40 adjacent
the outlet 38 of
the main channel 24 is most effective as it transmits the majority of the
standing wave and the
reflected wave is small.
The invention will now be described with reference to the Applicant's
printhead cartridge and
print engine shown in Figures 5 and 6. A printhead cartridge recognizes that
individual ink ejection
nozzles may fail over time and eventually there are enough dead nozzles to
cause artifacts in the
printed image. Allowing the user to replace the printhead maintains the print
quality without requiring
the entire printer to be replaced. The print engine 3 is the mechanical heart
of a printer which
can have many different external casing shapes, ink tank locations and
capacities, as well as
different media feed and collection trays.
Figure 5 shows a printhead cartridge 2 installed in a print engine 3. The
printhead
cartridge 2 is inserted and removed by the user lifting and lowering the latch
126. The print
engine 3 forms an electrical connection with contacts on the printhead
cartridge 2 and fluid
couplings 120 are formed at the inlet and outlet manifolds, 48 and 50
respectively.
Figure 6 shows the print engine 3 with the printhead cartridge removed to
reveal the
apertures 122 in the fluid couplings 120. The apertures 122 engage spouts on
the inlet and
outlet manifolds (48 and 50 of Fig. 5). The fluid couplings 120 connect the
inlet manifold to
an ink tank, and the outlet manifold to a sump. As discussed above, the ifflc
tanks, media feed
and collection trays have an arbitrary position and configuration relative to
the print engine 3
depending on the design of the printer's outer casing.
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Figure 7 shows the printhead assembly 2 as a printhead cartridge for user
insertion and
removal from the printer body (see Fig. 6). The printhead cartridge 2 has a
top molding 44
and a removable protective cover 42. The top molding 44 has a central web for
structural
stiffness and to provide textured grip surfaces 58 for manipulating the
cartridge during
insertion and removal. The base portion of the protective cover 42 protects
the printhead ICs
(not shown) and line of contacts (not shown) prior to installation in the
printer. Caps 56 are
integrally formed with the base portion and cover the ink inlets and outlets
(see 54 and 52 of
Fig. 9).
Figure 8 shows the printhead assembly 2 with its protective cover 42 removed
to
expose the printhead ICs on the bottom surface and the line of contacts 33 on
the side surface.
The protective cover is discarded to the recycling waste or fitted to the
printhead cartridge
being replaced to contain leakage from residual ink. Figure 9 is a partially
exploded
perspective of the printhead assembly 2. The top cover 44 has been removed
reveal the inlet
manifold 48 and the outlet manifold 50. The inlet and outlet shrouds 46 and 47
have been
removed to better expose the five inlet and outlet conduits, 52 and 54
respectively. The inlet
and outlet manifolds 48 and 50 form a fluid connection between each of the
individual inlets
and outlets and the corresponding main channel 24 (see Fig. 11) in the LCP
molding 20. As
discussed above, the main channels extend beneath the line of non-priming air
cavities 26.
Figure 10 is an exploded perspective of the printhead assembly without the
inlet or
outlet manifolds or the top cover molding. The main channels 24 for each ink
color and their
associated air cavities 26 are formed in the channel molding 68 and the cavity
molding 72.
Adhered to the bottom of the channel molding 68 is a die attach film 66. As
discussed above
in relation to Fig. 1, the die attach film 66 mounts the printhead ICs 30 to
the channel molding
such that the fine channels on the underside of the are in fluid communication
with the
printhead ICs 30 via small laser ablated holes through the film.
Flex PCB 70 is adhered to the side of the air cavity molding 72 and wraps
around to
the underside of the channel molding 68. The printer controller connects to
the lines of
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contacts 33. At the other side of the flex PCB 70 is a line of wire bonds 64
to electrically
connect the conductors in the flex 70 to each of the printhead ICs 30. The
wire bonds 64 are
covered in encapsulant 62 which is profiled to have a predominantly flat outer
surface. On the
other side of the air cavity molding 72 is a paper guide 74 to direct sheets
of media substrate
5 past the printhead ICs at a predetermined spacing.
Figures 11A, 11B and 11C show the outlet manifold 50 detached from the rest of
the printhead
cartridge. Interface plate 76 has outlet spouts 54 for connection to the ink
sump housed in the
printer body. The coupling 60 connects to each of the main channels 24 in the
channel
10 molding 68 (see Fig. 10). As shown in Figures 11B and 11C, the inner
side of the interface
plate 76 supports the thin inks tubes 32 and the air cavities 34 for the
respective main
channels. The ink line outlets 38 connect to the thin tubes 32 immediately
before the air
cavities 34. The air cavities 34 and the thin tubes 32 are sealed from each
other with the heat
sealable foil 78 applied to the back of the outlet manifold 50. The foil 78 is
heat sealed around
the entire perimeter of the five air cavities and ink tubes as it is essential
that they are
completely sealed from each other. To ensure the seal is not compromised
during use, the heat
seal resists internal pressure to 100kPa.
When the printhead assembly primes, the ink flows through the thin tube 32 as
far the
outlet 38 only. The length of the ink column in the thin tube, the diameter of
the tube and the
properties of the ink determine an inertance for the ink in the tube. The
inertance is equates to
the dash-pot in the equivalent mechanical damper and the inductor in an
electrical damper.
The volume of the air cavity is relatively small; less than 0.4m1, and
typically between 0.15m1
and 0.3m1. This provides to the spring in a mechanical damper or the capacitor
in the
corresponding electrical circuit.
As the main channels 24 of the channel molding 68 have slightly different
configurations, the resonant frequencies are likewise different. Accordingly,
the fluidic
dampers for each main channel 24 are tuned to resonate at different
frequencies for optimum
damping of each ink line.
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The invention has been described herein by way of example only. Skilled
workers in
this field will readily recognize many variations and modifications that do
not depart from the
spirit and scope of the broad inventive concept.
