Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-07-23
2001-09-04
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06283580
ABSTRACT:
BACKGROUND
This invention relates generally to droplet emitters and more particularly concerns an acoustically actuated droplet emitter which is provided with a continuous, high velocity, laminar flow of liquid.
FIG. 1
shows a cross-sectional view of a standard droplet emitter
10
for an acoustically actuated printer such as is shown in U.S. Pat. No. 5,565,113 by Hadimioglu et al. titled “Lithographically Defined Ejection Units” and incorporated by reference hereinabove. The droplet emitter
10
has a base substrate
12
with transducers
16
on one surface and acoustic lenses
14
on an opposite surface. Attached to the same side of the base substrate
12
as the acoustic lenses is a top support
18
with channels, defined by sidewalls
20
, which hold a flowing liquid
22
. Supported by the top support
18
is a capping structure
26
with arrays
24
of apertures
30
. The transducers
16
, acoustic lenses
14
, and apertures
30
are all axially aligned such that an acoustic wave produced by a single transducer
16
will be focussed by its aligned acoustic lens
14
at approximately a free surface
28
of the liquid
22
in its aligned aperture. When sufficient power is obtained, a droplet is emitted.
FIG. 2
shows a perspective view of the droplet emitter
10
shown in FIG.
1
. The arrays
24
of apertures
30
can be clearly seen on the capping structure
26
. Each array
24
has a width W and a length L where the length L of the array
24
is the larger of the two dimensions. Arrow Lf shows the flow direction of the flowing liquid
22
through the top support
18
, which is in the direction of the length L and orthogonal to the width W of the channels formed by sidewalls
20
and is along a length L of the arrays
24
. This is due to the channels formed by sidewalls
20
being constructed such that the flowing liquid
22
flows in the direction of the length L of the each array. This configuration has many advantages. It is compact and allows the precisely aligned production of multiple arrays
24
of apertures
30
where each array is associated with a liquid having different properties. For instance, to enable a color printing application each array might be associated with a different colored ink. Furthermore, this configuration is easy to set up and attach to an ink pumping system. However, the pressure loss of the liquid
22
along the channel length L is dependent on the cross sectional area defined by sidewalls
20
and the channel length L. As the channel length L increases, the pressure loss along the flow direction increases. The portion of the pressure loss due to flow frictional losses is largely dependent upon and limited by the height h of the channel.
This pressure loss along the flow direction can become large and results in a limited flow rate. The pressure loss and the limited flow rate impacts the performance of the droplet emitter
10
by limiting the droplet emission rates possible in three ways. Firstly, the pressure loss will change the level of the free surface
28
of the flowing liquid in the apertures along the length L. At the very least, different liquid levels will contribute to focussing errors of the acoustic energy focussed by the acoustic lenses
14
and result in emitted droplets not landing in their target spots. For example, using a configuration of the type shown in
FIGS. 1 and 2
, with a length L of 1.7 inches and a flowing liquid having a viscosity of less than 1.3 centepoise, a flow rate which exceeds 10 ml per minute will exceed the focussing level tolerance of the acoustic lenses because the difference in meniscus position between the first and last emitter will exceed 5 microns. If the flow rate exceeds 35 ml per minute, the system can not sustain the free surface
28
of the flowing liquid
22
in the apertures
30
. At these flow rates both simultaneous spilling and air bubble ingestion occurs.
Secondly, the slow flow rate will also mean that the flowing liquid
22
and the substrate
12
will heat up from the portion of the acoustic energy that is absorbed in the flowing liquid
22
and the substrate
12
which is not transferred to the kinetic energy of the ejected drops. The liquid can sustain temperature increases by only a few degrees centigrade before emitted droplets show drop misplacement on the receiving media. In a worst case scenario, the flowing liquid
22
can absorb enough energy to cause it to boil. The practical consequences of this are that either the array length L, and hence the droplet emitter length must be very short to allow for faster flow rates or that the emission speed must be kept very slow to prevent the liquid from absorbing too much excess energy and heating up to unacceptable levels.
Using the example given above, with a configuration as shown in
FIGS. 1 and 2
and a length L of 1.7 inches running under a maximum emission rate with all emitters emitting at approximately 30 watts, the temperature difference between the first and last emitter is approximately between 39 degrees centigrade and 75 degrees centigrade. This temperature differential is clearly above the preferred range of just a few degrees centigrade and affects the accuracy of droplet placement quality greatly. To correct this issue either the flow rate of the flowing liquid must be increased or the emission rate must be greatly reduced so that less heat energy is generated in the base substrate
12
and the flowing liquid
22
. However, using the design shown in
FIGS. 1 and 2
, increasing the flow rate of the flowing liquid
22
results in an unacceptable pressure loss and meniscus position variance as discussed above. Therefore, using the design shown in
FIGS. 1 and 2
, emission rates must be kept low to prevent excess heating of the flowing liquid
22
to achieve acceptable drop placement accuracy.
Thirdly, if the droplet emitter is emitting droplets at high emission rates, a greater volume of fluid will be lost to droplet emission than can be replaced by the slow flow rates. Again the practical consequences of this are that either the array length L, and hence the droplet emitter length must be very short to allow for faster flow rates or that the emission speed must be kept slow to allow sufficient replenishment times.
Therefore, it would be highly desirable if a droplet emitter
10
could be designed to maintain a substantially constant pressure along the emission portion of the liquid flow path and which also has a faster flow rate for a droplet emitter array of any arbitrary length L with a minimal rise of the liquid flow temperature at high emission speeds and has sufficient liquid replenishment rates.
Further advantages of the invention will become apparent as the following description proceeds.
SUMMARY OF THE INVENTION
Briefly stated and in accordance with the present invention, there is provided a method of operating an acoustic droplet emitter which utilizes high liquid flow rates. The droplet emitter has a first substrate which has been constructed to provide an array of focussed acoustic waves. The droplet emitter also has a second substrate which is spaced from the first substrate. The second substrate has an array of apertures which are so arranged such that each aperture may receive focussed acoustic waves. Further, there is a liquid flow chamber at least partially interposed between the first and second substrates. The liquid flow chamber has an inlet and an outlet and is constructed and arranged to receive a laminar flow of a liquid where a free surface of the liquid is formed by each of the apertures in the second substrate. The focussed acoustic waves received by each aperture are focussed substantially at the free surface of the liquid formed in the aperture. The laminar flow of liquid flows in through the inlet, out through the outlet at liquid flow rates of at least 35 ml per minute.
REFERENCES:
patent: 5087931 (1992-02-01), Rawson
patent: 6134291 (2000-10-01), Roy et al.
patent: 0 931 650 A1 (1993-07-01), None
patent: 1 013 432 A2 (2000-06-01), None
patent: WO 98/57809 (1998-12-01), None
Elkin Jerry
Hadimioglu Babur B.
Roy Joy
Weisberg Michael C.
Barlow John
Brooke Michael S.
McBain Nola Mae
Xerox Corporation
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