Note: Descriptions are shown in the official language in which they were submitted.
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BRIEF SUMMARY OF THE _INVENTION
The invention can be utilized in any pressure
pulse drop ejector system; however, the greatest benefits
are realized when the nozzles made in accordance with
the present invention are used in an ink jet recording
system. Accordingly, the present invention will be
described in connection with an ink jet reco~ding system.
When an ink droplet is expressed from an outlet
orifice, the new meniscus formed by the remaining fluid
in the orifice vibrates until it reaches a stable condition.
Since the meniscus must be stabilized in order to express
controlled droplets, the duration of the vibration affects
the frequency at which controlled volume droplets can
be expressed from the orifice. The longer the duration
of vibration, the lower the frequency of acceptable
operation. The problem is discussed in detail in commonly
owned U.S. Patent 4,024,544.
Further, it has been found in ink jet recording
systems that even relatively minor defects, such as
chips or cracks in drop outlet orifice rims, can cause
relatively large trajector~ errors. q'rajectory errors
in an ink jet recording system can cause poor quality
reproductions and can even render the system unusable.
Nozzles designed in accordance with the present invention
provide meniscus vibration damping and are relatively
not affected by orifice rim defects.
An aspect of the invention is as follows:
A nozzle for a pressure pulse drop ejector
system, which comprises a throat section and an enlarged
meniscus section in which a meniscus is formed in a
fluid in said meniscus section, the relationship between
the meniscus section and the throat section being such
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that drop formation shear occurs as fluid-to-fluid
shear.
BRIEF DESCRIPTION OF THE DRAWINGS
_
FIG. lA and lB are side sectional schematic
representations of the operation of nozzles in accordance
with prior art.
FIG. 2A and 2B are side sectional schematic
- 2a-
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representations of the operation of nozzles in accordance with
the present invention.
Referring now to FIG. lA, there is shown housing 10
having nozzle 12 formed therein. Nozzle 12 is filled with fluid
14 which, in its rest condition, forms meniscus 16.
FIG. lB illustrates the typical shape of the meniscus
16 when fluid 14 has been acted upon by a pressure pulse but
before drop separation has occurred. The purpose of FIG. lB is
to illustrate that in conventional nozzles, part of the fluid
which forms the drop is in contact with solid material. The
drag between the fluid and the solid material influences drop
formation. Any defect, such as a chip or crack in the orifice
material since it is in contact with the fast moving fluid
material, can cause the drop to be expelled at an angle rather
than, for example, straight; i.e., cause trajectory errors.
Referring now to FIG. 2A, there is shown housing 20
and a nozzle 22 formed therein in accordance with the present
invention. Nozzle 22 has been widened at the orifice outlet to
form an enlar~ed meniscus area. For purposes o discussion, the
nozzle diameter before the enlargement will be referred to as the
throat diameter shown as Dt in FIG. 2. The enlarged outlet
orifice diameter will be referred to as the meniscus diameter
shown as Dm in FIG. 2. FIG. 2B illustrates the key feature of
the instant invention. The drop which is being formed is formed
of fast moving fluid, which is not in contact with solid material
at the time of drop formation and drop separation.
There is an area represented by a in FIG. 2A of
relatively slow or stagnant fluid formed between the fast moving
fluid and the orifice rim. The drop formation shear accordingly
occurs as fluid-to-fluid shear rathe, than a fluid-to-solid shear.
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The separation of the fluid, which forms the drop from the rim
of the outlet orifice, minimizes or eliminates drop trajectory
error caused by rim imperfections. It has also been found that
increasing the meniscus area reduces the amplitude of meniscus
vibration,and decreasing the throat area increases meniscus
vibration damping. When a drop is expressed, a new meniscus is
formed. This new meniscus vibrates for a time before coming to
rest. It is only when the meniscus is at rest that an accurately
sized drop can be expressed. Accordingly, the amount of time
that it takes the meniscus to come to rest determines how fast
the ink jet system can function. The improvement in meniscus
damping occurs because the meniscus area is made independent of
the throat area. Meniscus vibration decays in a manner dependent
upon the compliance of the meniscus and the inertance and
resistance of the nozzle. The compliance is a function of:
(meniscus cross-sectional area)2
(fluid surface tension)
Inertance is a function of:
(fluid density)(nozzle len th)
(throat cross-sectlonal area)~~
The nozzle resistance is a function of:
(fluid viscosity)(nozzle length)
(throat cross-sectional area)~~
There is one relationship among design variables that achieves
critical damping or the shortest time for meniscus relaxation.
For a given meniscus disturbance to be relaxed in the shortest
time, the fluid "circuit" representing the pressure pulse drop
ejector must be critically damped. ~ince the throat cross-
sectional area, which determines resistance and inertance, can,
according to the invention, be independently chosen from the
meniscus cross-sectional area, which controls compliance,
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selection of these cross-sectional areas can be made to substan-
tially achieve critical damping of the meniscus. As an example,
a 20 ~um diameter throat having a 20 ~m deep, 40 ,um diameter ou let
has a damping time one fourth that of a 40Jum straight nozzle.
The degree of damping can be measured by observing the meniscus
through a microscope during either sinusoidal or pulsed jet
excitation. It is thus relatively simple to compare meniscus
damping in different nozzle designs. Such measurements have
confirmed the teaching of this invention.
A further advantage of the improved nozzle is that the
surface of the material from which the nozzle is formed need not
be of a special wetting or non-wetting character since the rim of
the meniscus forming area is remote from the fast-moving fluid
which forms the drops shown in FIG. 2B.
Referring again to prior art FIG. lB, it is seen that
the meniscus forms almost a 90 angle between the jet of ink and
the outer nozzle surface. FIG. 2B conversely shows that the
meniscus at the point of contact with the nozzle surface maintains
an approximately 180 angle even during ejection. Hence, there
is much greater resistance to wetting of the outer jet surface.
A typical nozzle in accordance with the present
invention would have a throat diameter of about 20 ~m to about
80 ~um. The diameter of the meniscus would be from about 12 ~m
to about 25 ~m larger than the throat diameter. The ratio of
the area of the enlarged area to the area of the throat area is
accordingly from about 1.5:1 to about 5 1. The depth of the
enlarged area would be typically between about 25 ,um and about
50~um deep. Although the above measurements were given as
diameters inferring circular cross-sectional areas, the throat
cross-section and the meniscus cross-section can he triangles,
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squares, rectangles or any conveniently formed shape and need
not be the same. Further, the enlarged space may be cylindrical,
conical or any conveniently formed shape and need not even be
coaxial or concentric with the nozzle throat.
