JPH06115069A - Droplet jet method by acoustic or electrostatic force - Google Patents

Abstract

PURPOSE: To form small droplets emitted from a mound by providing a stage forming the mound on the free surface of a liquid acoustically and a stage attracting the liquid of the mound electrostatically until the mound is torn off. CONSTITUTION: An acoustic lens 30 concentrates acoustical radiation to the small focus area in the vicinity of a free surface. Next, the action of a droplet ejector 10 depends on the acoustical formation of a mound having an apex in the vicinity of an electrode 20 and the application of voltage to the electrode. The intensity of the acoustical radiation 22 is controlled so as to be sufficient to form the mound but insufficient to emit small droplets. The quantity of motion is applied to ink accompanied by the formation of the mound and, as the apex of the mound approaches the electrode, the voltage applied to the electrode attracts ink toward the electrode and generates the larger quantity of motion. Since the mound is restricted in vol., the mound is deformed until torn off by Rayleigh's instability to form small droplets.

Description

Detailed Description of the Invention

The present invention relates generally to the ejection of acoustic droplets.

Various inkjet technologies have been and are being developed. One of such techniques is referred to below as acoustic ink printing (AIP), which produces an image on a recording medium by acoustic energy. For a more detailed description of AIP technology, see US Pat. Nos. 4,308,547 and 4,697,19.
No. 5, and No. 5,028,937, this technique essentially ejects droplets of ink onto a recording medium by the bursting of acoustic radiation concentrated near the free surface of the liquid ink.

To produce high quality images using AIP, ink droplets must be closely spaced, which requires a densely packed droplet ejector. The general method of arranging the droplet ejectors is (1) forming rows of evenly spaced droplet ejectors, (2) adjoining these rows, and the droplet ejectors of adjacent rows being offset. Rows are arranged to form columns, and (3) these ejectors are electrically connected to one another in a row and column XY plane array. To eject a droplet from a particular droplet ejector, the ejector is addressed to the row and column conductors of the droplet ejector by application of a drive signal. This approach is generally successful, but as the density of the droplet ejectors increases, the planar area available for the required electrical conductors decreases. A possible way to increase the droplet ejector would be to reduce the planar area required for electrical conductors. However, this requires a different addressing scheme for the droplet ejector. Therefore, an addressing technique that enables the increase of droplet ejectors is beneficial.

Pro by G. Taylor
c. Roy. Soc. A, 280, pp. 383-397, "Disintegration of water drops in an electric field.
inan electric field) ". Among them, it has been analytically and experimentally shown that, in a certain geometrical arrangement, a strong electrostatic electric field (E-electric field) raises a flat liquid surface into a cone having a half-angle of 49.3 degrees. . Such cones are commonly referred to as "Taylor cones. Experiments have shown that cones form on flat reservoirs of ink in the order of seconds (for a proper E-field. It has been found that once formed, they collapse within about 50 msec after removal of the E-field, while they take about 1 to 10). At the beginning of formation, it is believed to be associated with a relatively large gap between the cone forming the electrode and the liquid, so as the gap becomes smaller, the E-field begins to concentrate near the apex of the cone, The rate of cone formation increases.This hypothesis shows that this is done very quickly in E-fields with final cone formation, much less than 15 msec. It has been more support.

As observed by Tyler and further confirmed by experiments, certain E-field values cause material to be ejected from the apex of the cone in the form of a normal flow or jet. or,
It has been found that some related effects occur. If one attempts to form a cone very quickly in the liquid pool, a catastrophic jet of material will result from the rather large amount of liquid of the resulting "cones". Such cataclysmic jets should be avoided in AIP because the ink volume of such jets is much higher than the required ink drop volume. However, techniques for reducing the amount of material ejected allow the formation of cones used in AIP.

In the present invention, a technique is provided for acoustically ejecting low volume droplets by employing cone formation. Preferably, this is done so that the cone formation assists the addressing of the droplet ejector, thereby reducing the required electrical connection area and thus allowing a more dense mounting of the droplet ejector. Be seen. Ejection techniques include acoustically raising an ink mound proximate an electrode and then quickly applying an E-field to the mound. The volume of such an acoustically raised mound is limited (having a diameter on the order of the acoustic wavelength) and the applied E
-The electric field causes Rayleigh instability to tear off the mound, which causes acoustic emission (acoustic).
Due to the combined momentum imparted to the droplets by the radiation and electrode voltage, the droplets are ejected from the mound. The addressing technique acoustically creates columns (or rows) of mounds around which all electrodes of the row (or column) of droplet ejectors are located when the acoustically raised mounds are close to these vertices. Included is the selection of droplet ejectors that eject by applying a voltage. Only the droplet ejector, which has both an acoustically raised mound and an applied electrode voltage, ejects the droplet.

FIG. 1 is a simplified cross-sectional view of a droplet ejector according to the principles of the present invention.

FIG. 2 is a cross-sectional view showing another embodiment of the droplet ejector according to the essence of the present invention.

FIG. 3 is a schematic diagram in the XY direction showing the arrangement of the droplet ejectors of FIG. In the drawings, the same members are supported by the same reference numerals.

Referring to FIG. 1, a cross section of an acoustic ink droplet ejector according to the principles of the present invention is shown. Basically, the ink droplet 12 is located near the mound 14 (2 in the cross-sectional view of FIG. 1).
A voltage is applied to the electrode 20 (shown in two parts) and ejected from the acoustically raised mound 14 from the free surface 16 of the ink reservoir 18. "Nearby"
Means a physical neighborhood in which the electrode voltage, which has not yet been explained, causes the ejection of droplets. The optimal neighborhood is
It depends on factors such as the properties of the ink, the voltage applied to the electrodes, the acoustic energy that causes the mound, and the drop ejection rate.

Acoustic radiation that raises the mound 14 is generated by the transducer 24 which receives RF drive through terminal 26. Acoustic radiation is emitted by the object 28 (preferably Pyrex glass (py) until it reaches the acoustic lens 30.
rex glass)). Although the illustrated acoustic lens is a spherical lens, it may be a Fresnel lens. Anyway, acoustic lenses concentrate acoustic radiation in a small focal area near the free surface. Of course, the depth of the ink and the dimensions of the acoustic lens and object 28 are adjusted so that the focal area is close to the free surface 16.

The action of the droplet ejector depends on the acoustic formation of a mound having an apex near the electrode 20 and the application of a voltage to that electrode. Acoustic radiation 2
A strength of 2 is controlled to be sufficient to form a mound, but not to eject droplets by itself. Momentum is imparted to the ink as the mound is formed, and as its apex approaches the electrode, the voltage applied to the electrode pulls the ink toward the electrode, generating a larger momentum. Due to the limited volume of the mound, Rayleigh's instability causes the mound to deform until it is torn off, forming droplets.
The formed droplets 12 are ejected by the combined effect of momentum imparted acoustically and electrostatically. Electrode 2
0 has an opening 32 through which droplets 1 ejected
2 adheres to the recording medium 34 midway.

FIG. 2 illustrates another embodiment of a droplet ejector 40 according to the principles of the present invention. The droplet ejector 40 is almost the same as the droplet ejector 10 of FIG. 1, and the droplet ejector 40 has an electrode 20 located behind the recording medium 34, and this electrode 20 is The difference is that no opening 32 is provided. Experiments show that at least on some recording media (like paper),
It has been shown that electrostatic forces easily and sufficiently attract the ink 18 of the mound 14 to eject a droplet, regardless of the recording medium being placed therebetween.

A practical acoustic ink printer includes a large number (perhaps thousands) of individual droplet ejectors 10 physically and operatively arranged in rows and columns. The row is the print page width (eg about 8.
5 inches) and is intended to have about 75 droplet ejectors per inch. The rows are stacked such that the columns of droplet ejectors are formed such that they are offset by a small angle (see FIG. 3 and the description below). This allows an effective linear density of droplet ejectors that is much larger than 75 droplet ejectors per inch. For example, such eight offset rows move the recording medium 34 in a controlled manner to eject droplets from the individual droplet ejectors at the proper time to produce a 600 pixel pitch (75 pixels).
8 times) can be achieved. As previously mentioned, electrically connecting all droplet ejectors in a single plane tends to limit the density of droplet ejectors.

Electrostatic attraction
Adopting acoustic radiation with traction enables an increase in droplet ejector density by reducing the planar area required for droplet ejector coupling. Addressing is performed by driving all the transducers in a row to acoustically form a mound in a row, while applying an electrode voltage to all electrodes of the droplet ejector in one row. Only droplet ejectors that respond to both the acoustically raised mound and the applied electrode voltage will eject droplets.

FIG. 3 assists in the description of addressing for the droplet ejector invention.
Multiple droplet ejectors 50 (13 of them)
The individual pieces are shown) are arranged in a number of rows 52 (of which only four are shown). These rows are arranged adjacent to each other such that the corresponding droplet ejectors of adjacent rows are slightly offset. This slight offset creates a droplet ejector column 54 having a small angle. The transducers of each column of droplet ejectors are electrically connected. Similarly, the electrodes of the droplet ejectors in each column are also electrically connected. A column of mounds is formed by individually driving the transducers of the droplet ejectors in each column. Applying an electrode voltage to each electrode of a row of droplet ejectors will result in a droplet drop that corresponds to both the acoustically raised mound and the applied electrode voltage.
The ejector ejects a small droplet. Of course, those skilled in the art will appreciate that the row and column configurations described above can be converted to each other.

From the above description, numerous variations and modifications of the essence of the invention will be apparent to those skilled in the art. Therefore, the scope of the invention is limited only by the claims.

[Brief description of drawings]

FIG. 1 is a simplified cross-sectional view of a droplet ejector according to the principles of the present invention.

FIG. 2 is a cross-sectional view showing another embodiment of the droplet ejector according to the essence of the present invention.

FIG. 3 is a schematic view in the XY direction showing the arrangement of the droplet ejector of FIG.

[Explanation of symbols]

10 Droplet ejector, 12 Droplets, 14
Mound, 16 free surface, 18 ink pool, 2
0 electrode, 22 acoustic emission, 24 transducer

Claims (4)

[Claims] 1. A method of acoustically forming a mound on a free surface of a liquid, and electrostatically attracting the liquid of the mound electrostatically until the mound is ruptured by Rayleigh instability. How to eject small droplets. 2. A droplet ejector for ejecting a droplet from a free surface of a liquid onto a recording medium, the ink reservoir holding the liquid so that the liquid has a free surface, and the free surface of the liquid. A droplet ejector having means for acoustically forming a mound thereon and means for electrostatically attracting the liquid of the mound until the mound is torn off by Rayleigh instability. 3. A method for addressing a plurality of droplet ejectors each having an acoustic transducer and an electrode, the method comprising: acoustically forming a column of mounds on a free surface of a liquid; Electrostatically radiating an attractive force from a row of electrodes so that a droplet is ejected from one of the mounds. 4. A printhead having an ink reservoir for holding the ink so that the ink has a free surface, the printhead further ejecting a plurality of ink droplets retained in the ink reservoir. A droplet ejector, each of which includes a transducer that radiates acoustic energy toward the free surface such that a mound is formed on the free surface, and each droplet ejector has a corresponding A droplet ejector of the plurality of droplet ejectors such that a first mound is formed on the free surface. And a means to drive the Rayleigh instability to tear the droplet from the first mound. So as to attract the first mound by electrostatic attraction.
A printer comprising means for applying a voltage to the electrodes of the ejector and means for positioning the recording medium such that the droplets are deposited on the recording medium.