Note: Descriptions are shown in the official language in which they were submitted.
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INKJET PRINTHEAD WITH ADJUSTABLE BUBBLE IMPULSE
FIELD OF THE INVENTION
The present invention relates to inkjet printers and in particular, inkjet
printheads that generate vapor
bubbles to eject droplets of ink.
BACKGROUND TO THE INVENTION
The present invention involves the ejection of ink drops by way of forming gas
or vapor bubbles in a bubble
forming liquid. This principle is generally described in US 3,747,120 to
Stemme. These devices have heater
elements in thermal contact with ink that is disposed adjacent the nozzles,
for heating the ink thereby forming gas
bubbles in the ink. The gas bubbles generate pressures in the ink causing ink
drops to be ejected through the
nozzles.
The resistive heaters operate in an extremely harsh environment. They must
heat and cool in rapid
succession to form bubbles in the ejectable liquid, usually a water soluble
ink. These conditions are highly
conducive to the oxidation and corrosion of the heater material. Dissolved
oxygen in the ink can attack the heater
surface and oxidise the heater material. In extreme circumstances, the heaters
'burn out' whereby complete
oxidation of parts of the heater breaks the heating circuit.
The heater can also be eroded by 'cavitation' caused by the severe hydraulic
forces associated with the
surface tension of a collapsing bubble.
To protect against the effects of oxidation, corrosion and cavitation on the
heater material, inkjet
manufacturers use stacked protective layers, typically made from Si3N4, SiC
and Ta. Because of the severe
operating conditions, the protective layers need to be relatively thick. US
6,786,575 to Anderson et al (assigned
to Lexmark) is an example of this structure, and the heater material is ¨0.1 m
thick while the total thickness of
the protective layers is at least 0.71tm.
To form a vapor bubble in the bubble forming liquid, the heater (i.e. the
heater material and the protective
coatings) must be heated to the superheat limit of the liquid (-300 C for
water). This requires a large amount of
energy to be supplied to the heater. However, only a portion of this energy is
used to vaporize ink. Most of the
'excess' energy must be dissipated by the printhead and or a cooling system.
The heat from the excess energy of
successive droplet ejections can not raise the steady state temperature of the
ink above its boiling point and
thereby cause unintentional bubbles. This limits the density of the nozzles on
the printhead, the nozzle firing rate
and usually necessitates an active cooling system. This in turn has an impact
on the print resolution, the
printhead size, the print speed and the manufacturing costs.
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Attempts to increase nozzle density and firing rate are hindered by
limitations on thermal conduction out of
the printhead integrated circuit (chip), which is currently the primary
cooling mechanism of printheads on the
market. Existing printheads on the market require a large heat sink to
dissipate heat absorbed from the printhead
IC.
Inkjet printheads can also suffer from a problem commonly referred to as
`decap'. This term is defined
below. During periods of inactivity, evaporation of the volatile component of
the bubble forming liquid will
occur at the liquid-air interface in the nozzle. This will decrease the
concentration of the volatile component in
the liquid near the heater and increase the viscosity of the liquid in the
chamber. The decrease in concentration of
the volatile component will result in the production of less vapor in the
bubble, so the bubble impulse (pressure
integrated over area and time) will be reduced: this will decrease the
momentum of ink forced through the nozzle
and the likelihood of drop break-off. The increase in viscosity will also
decrease the momentum of ink forced
through the nozzle and increase the critical wavelength for the Rayleigh
Taylor instability governing drop break-
off, decreasing the likelihood of drop break-off. If the nozzle is left idle
for too long, these phenomena will result
in a "decapped nozzle" i.e. a nozzle that is unable to eject the liquid in the
chamber. The "decap time" refers to
the maximum time a nozzle can remain unfired before evaporation will decap the
nozzle.
OBJECT OF THE INVENTION
The present invention aims to overcome or ameliorate some of the problems of
the prior art, or at least
provide a useful alternative.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides an inkjet printhead for printing a
media substrate, the
printhead comprising:
a plurality of nozzles;
a plurality of heaters corresponding to each of the nozzles respectively, each
heater being configured for
heating printing fluid to nucleate a vapor bubble that ejects a drop of the
printing fluid through the corresponding
nozzle; and,
drive circuitry for generating an electrical drive pulse to energize the
heaters; wherein,
the drive circuitry is configured to adjust the drive pulse power to vary the
vapor bubble nucleation time.
The power supplied to each heater determines the time scale for heating it to
the 309 C ink superheat
limit, where film boiling on the surface of the heater spontaneously nucleates
a bubble. The time scale for
reaching the superheat limit determines two things: the energy required to
nucleate the bubble and the impulse
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delivered by the bubble (impulse being pressure integrated over area and
time). By varying the power of the
pulse used to generate the bubble, the printhead can operate with small,
efficiently generated bubbles during
normal printing, or it can briefly operate with large high energy bubbles if
it needs to recover decapped nozzles.
In preferred embodiments, the power supplied to the heaters in printing mode
is sufficient to cause
nucleation in less than 1 s, and more preferably between 0.4 s and 0.5 s, and
the power supplied to the heaters
in maintenance mode results in nucleation times above 1 s.
In some forms, the energy in each printing pulse is less than the maximum
amount of thermal energy
that can be removed by the drop, being the energy required to heat a volume of
the ejectable liquid equivalent to
the drop volume from the temperature at which the liquid enters the printhead
to the heterogeneous boiling point
of the ejectable liquid. In this form, the printhead is "self cooling", a mode
of operation in which the nozzle
density and nozzle fire rate are unconstrained by conductive heatsinking, an
advantage that facilitates integrating
the printhead into a pagewidth printer.
In some forms, the power delivered to each heater may be adjusted by changing
the voltage level of the
pulse supplied to the heater. In other forms, the power is adjusted using
pulse width modulation of the voltage
pulse, to adjust the time averaged power of the pulse.
Optionally, the drive circuitry is configured to operate in a normal printing
mode and a high impulse
mode such that the drive pulses are less than 1 microsecond long in the normal
printing mode and greater than 1
microsecond long in the high impulse mode.
Optionally, the high impulse mode is a maintenance mode used to recover
nozzles affected by decap.
Optionally, the high impulse mode is used to increase the volume of the
ejected drops of printing fluid.
Optionally, the high impulse mode is used to compensate for printing fluid
with higher viscosity than
other printing fluid ejected during the normal printing mode, to provide more
consistent drop volumes.
Optionally, each of the drive pulses has less energy than the energy required
to heat a volume of the
printing fluid equivalent to the drop volume, from the temperature at which
the printing fluid enters the printhead
to the heterogeneous boiling point of the printing fluid.
Optionally, the drive pulse power is adjusted in response to temperature
feedback from the array of
nozzles.
Optionally, the drive pulse power is adjusted by changing its voltage.
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Optionally, the drive pulse power is adjusted using pulse width modulation to
change the time averaged
power of the drive pulse.
Optionally, the maintenance mode operates before the printhead prints to a
sheet of media substrate.
Optionally, the maintenance mode operates after the printhead prints a sheet
of media substrate and
before it prints a subsequent sheet of media substrate.
Accordingly in a second aspect the present invention provides a MEMS vapour
bubble generator comprising:
a chamber for holding liquid;
a heater positioned in the chamber for thermal contact with the liquid; and,
drive circuitry for providing the heater with an electrical pulse such that
the heater generates a vapour
bubble in the liquid; wherein,
the pulse has a first portion with insufficient power to nucleate the vapour
bubble and a second portion
with power sufficient to nucleate the vapour bubble, subsequent to the first
portion.
If the heating pulse is shaped to increase the heating rate prior to the end
of the pulse, bubble stability
can be greatly enhanced, allowing access to a regime where large, repeatable
bubbles can be produced by small
heaters.
Preferably the first portion of the pulse is a pre-heat section for heating
the liquid but not nucleating the
vapour bubble and the second portion is a trigger section for nucleating the
vapour bubble. In a further preferred
form, the pre-heat section has a longer duration than the trigger section.
Preferably, the pre-heat section is at least
two micro-seconds long. In a further preferred form, the trigger section is
less than a micro-section long.
Preferably, the drive circuitry shapes the pulse using pulse width modulation.
In this embodiment, the pre-
heat section is a series of sub-nucleating pulses. Optionally, the drive
circuitry shapes the pulse using voltage
modulation.
In some embodiments, the time averaged power in the pre-heat section is
constant and the time averaged
power in the trigger section is constant. In particularly preferred
embodiments, the MEMS vapour bubble
generator is used in an inkjet printhead to eject printing fluid from nozzle
in fluid communication with the
chamber.
Using a low power over a long time scale (typically >> 1 s) to store a large
amount of thermal energy in
the liquid surrounding the heater without crossing over the nucleation
temperature, then switching to a high
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power to cross over the nucleation temperature in a short time scale
(typically < li.ts), triggers nucleation and
releasing the stored energy.
Optionally, the first portion of the pulse is a pre-heat section for heating
the liquid but not nucleating the vapour
5 bubble and the second portion is a trigger section for superheating some
of the liquid to nucleate the vapour
bubble.
Optionally, the pre-heat section has a longer duration than the trigger
section.
Optionally, the pre-heat section is at least two micro-seconds long.
Optionally, the trigger section is less than one micro-section long.
Optionally, the drive circuitry shapes the pulse using pulse width modulation.
Optionally, the pre-heat section is a series of sub-nucleating pulses.
Optionally, the drive circuitry shapes the pulse using voltage modulation.
Optionally, the time averaged power in the pre-heat section is constant and
the time averaged power in the trigger
section is constant.
In another aspect the present invention provides a MEMS vapour bubble
generator used in an inkjet printhead to
eject printing fluid from a nozzle in fluid communication with the chamber.
Optionally, the heater is suspended in the chamber for immersion in a printing
fluid.
Optionally, the pulse is generated for recovering a nozzle clogged with dried
or overly viscous printing fluid.
TERMINOLOGY
"Power" in the context of this specification is defined as the energy required
to nucleate a bubble,
divided by the nucleation time of the bubble.
Throughout the specification, references to 'self cooled' or 'self cooling'
nozzles will be understood to
be nozzles in which the energy required to eject a drop of the ejectable
liquid is less than the maximum amount of
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thermal energy that can be removed by the drop, being the energy required to
heat a volume of the ejectable fluid
equivalent to the drop volume from the temperature at which the fluid enters
the printhead to the heterogeneous
boiling point of the ejectable fluid.
The term "decap" is a reference to the phenomenon whereby evaporation from
idle nozzles reduces the
concentration of water in the vicinity of the heater (reducing bubble impulse)
and increases the viscosity of the
ink (increasing flow resistance). The term "decap time" is well known and
often used in this field. Throughout
this specification, "the decap time" is the maximum interval that a nozzle can
remain unfired before evaporation
of the volatile component of the bubble forming liquid will render the nozzle
incapable of ejecting the bubble
forming liquid.
The printhead according to the invention comprises a plurality of nozzles, as
well as a chamber and one
or more heater elements corresponding to each nozzle. Each portion of the
printhead pertaining to a single
nozzle, its chamber and its one or more elements, is referred to herein as a
"unit cell".
In this specification, where reference is made to parts being in thermal
contact with each other, this
means that they are positioned relative to each other such that, when one of
the parts is heated, it is capable of
heating the other part, even though the parts, themselves, might not be in
physical contact with each other.
Also, the term "printing fluid" is used to signify any ejectable liquid, and
is not limited to conventional
inks containing colored dyes. Examples of non-colored inks include fixatives,
infra-red absorbant inks,
functionalized chemicals, adhesives, biological fluids, water and other
solvents, and so on. The ink or ejectable
liquid also need not necessarily be a strictly a liquid, and may contain a
suspension of solid particles or be solid at
room temperature and liquid at the ejection temperature.
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 I is a sketch of a single unit cell from a thermal inkjet printhead;
Figure 2 shows the bubble formed by a heater energised by a 'printing mode'
pulse;
Figure 3 shows the bubble formed by a heater energised by a 'maintenance mode'
pulse;
Figure 4 is a voltage versus time plot of the variation of the pulse power
using amplitude modulation;
and,
Figure 5 is a voltage versus time plot of the variation of the pulse power
using pulse width modulation.
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Detailed Description of the Preferred Embodiments
Figure 1 shows the MEMS bubble generator of the present invention applied to
an inkjet printhead. A
detailed description of the fabrication and operation of some of the
Applicant's thermal printhead IC's is
provided in US Publ. No. 20060221114 and US Patent No. 7,744,195.
A single unit cell 30 is shown in Figure 1. It will be appreciated that many
unit cells are
fabricated in a close-packed array on a supporting wafer substrate 28 using
lithographic etching and deposition
techniques common within in the field semi-conductor/MEMS fabrication. The
chamber 20 holds a quantity of
ink. The heater 10 is suspended in the chamber 20 such that it is in
electrical contact with the CMOS drive
circuitry 22. Drive pulses generated by the drive circuitry 22 energize the
heater 10 to generate a vapour bubble
12 that forces a droplet of ink 24 through the nozzle 26.
The heat that diffuses into the ink and the underlying wafer prior to
nucleation has an effect on the
volume of fluid that vaporizes once nucleation has occurred and consequently
the impulse of the vapor explosion
(impulse = force integrated over time). Heaters driven with shorter, higher
voltage heater pulses have shorter ink
decap times. This is explained by the reduced impulse of the vapor explosion,
which is less able to push ink
made viscous by evaporation through the nozzle.
Using the drive circuitry 22 to shape the pulse in accordance with the present
invention gives the
designer a broader range of bubble impulses from a single heater and drive
voltage.
Figure 2 is a line drawing of a stroboscopic photograph of a bubble 12 formed
on a heater 10 during
open pool testing (the heater is immersed in water and pulsed). The heater 10
is 30 microns by 4 microns by 0.5
microns and formed from TiAl mounted on a silicon wafer substrate. The pulse
was 3.45 V for 0.4 microseconds
making the energy consumed 127 nJ. The strobe captures the bubble at it's
maximum extent, prior to condensing
and collapsing to a collapse point. It should be noted that the dual lobed
appearance is due to reflection of the
bubble image from the wafer surface.
The time taken for the bubble to nucleate is the key parameter. Higher power
(voltages) imply higher
heating rates, so the heater reaches the bubble nucleation temperature more
quickly, giving less time for heat to
conduct into the heater's surrounds, resulting in a reduction in thermal
energy stored in the ink at nucleation.
This in turn reduces the amount of water vapor produced and therefore the
bubble impulse. However, less energy
is required to form the bubble because less heat is lost from the heater prior
to nucleation. This is, therefore, how
the printer should operate during normal printing in order to be as efficient
as possible.
Figure 3 shows the bubble 12 from the same heater 10 when the pulse is 2.20 V
for 1.5 microseconds.
This has an energy requirement of 190 nJ but the bubble generated is much
larger. The bubble has a greater
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bubble impulse and so can be used for a maintenance pulse or to eject bigger
than normal drops. This permits the
printhead to have multiple modes of operation which are discussed in more
detail below.
Figure 4 shows the variation of the drive pulse using amplitude modulation.
The normal printing
mode pulse 16 has a higher power and therefore shorter duration as nucleation
is reached quickly. The large
bubble mode pulse 18 has lower power and a longer duration to match the
increased nucleation time.
Figure 5 shows the variation of the drive pulse using pulse width modulation.
The normal printing
pulse 16 is again 3.45 V for 0.4 microseconds. However, the large bubble pulse
18 is a series of short pulses 32,
all at the same voltage (3.45 V) but only 0.1 microseconds long with 0.1
microsecond breaks between. The
power during one of the short pulses 32 is the same as that of the normal
printing pulse 16, but the time averaged
power of the entire large bubble pulse is lower.
Lower power will increase the time scale for reaching the superheat limit. The
energy required to
nucleate a bubble will be higher, because there is more time for heat to leak
out of the heater prior to nucleation
(additional energy that must be supplied by the heater). Some of this
additional energy is stored in the ink and
causes more vapor to be produced by nucleation. The increased vapor provides a
bigger bubble and therefore
greater bubble impulse. Lower power thus results in increased bubble impulse,
at the cost of increased energy.
This permits the printhead to operate in multiple modes, for example:
a normal printing mode with high power delivered to each heater (low bubble
impulse, low energy
requirement);
a maintenance mode with low power delivered to each heater to recover decapped
nozzles (high bubble
impulse, high energy requirement);
a start up mode with lower power drive pulses when the ink is at a low
temperature and therefore more
viscous;
a draft mode that prints only half the dots (for greater print speeds) with
lower power drive pulses for
bigger bubbles to increase the volume of the ejected drops thereby improving
the look of the draft image; or,
a dead nozzle compensation mode where larger drops are ejected from some
nozzles to compensate for
dead nozzles within the array.
A primary objective for the printhead designer is low energy ejection,
particularly if the nozzle density
and nozzle fire rate (print speed) are high. The Applicant's US Publ. No.
20060221114 referenced above
provides a detailed discussion of the benefits of low energy ejection as well
as a comprehensive analysis of
energy consumption during the ejection process. The energy of ejection affects
the steady state temperature of
the printhead, which must be kept within a reasonable range to control the ink
viscosity and prevent the ink from
boiling in the steady state. However, there is a drawback in designing the
printhead for low energy printing: the
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low bubble impulse resulting from low energy operation makes the nozzles
particularly
sensitive to decap. Depending on the nozzle idle time and extent of decap, it
may not be possible to
eject from decapped nozzles with a normal printing pulse, because the bubble
impulse may be too low.
It is desirable, therefore, to switch to a maintenance mode with higher bubble
impulse if and when
nozzles must be cleared to recover from or prevent decap e.g. at the start of
a print job or between
pages. In this mode the printhead temperature is not as sensitive to the
energy required for each pulse,
as the total number of pulses required for maintenance is lower than for
printing and the time scale
over which the pulses can be delivered is longer.
Similarly, temperature feedback from the printhead can be used as an
indication of the ink
temperature and therefore, the ink viscosity. Modulating the drive pulses can
be used to ensure
consistent drop volumes. The printhead IC disclosed in US Publ. No.
20080084453 and US Patent No.
7,413,288 describe how 'on chip' temperature sensors can be incorporated into
the nozzle array and
drive circuitry.
