Ink jet nozzle

Abstract

An ink jet nozzle comprises a unitary construction of a nozzle plate having a jewel orifice on one face with a passage through the plate at right angles thereto for supplying ink to the nozzle under pressure, a reaction mass and a piezoelectric ceramic connecting the nozzle plate and the reaction mass. The assembly is secured to a support by a silicone rubber isolation damper which permits the nozzle plate, piezoelectric ceramic, and the reaction mass to vibrate as a unit at a selected frequency to effect uniform break off of ink drops from the ink stream issuing from the nozzle.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to ink jet printers and it has reference in particular to an ink jet nozzle structure and a method of making it.

2. Description of the Prior Art

Ink jet nozzles are known such as shown in U.S. Pat. No. 3,334,351 which issued on Aug. 1, 1967 to N. L. Stauffer, showing schematically a right angle nozzle with a magnetostrictive transducer driving element for producing vibration of the nozzle.

U.S. Pat. No. 3,596,275 which issued on July 27, 1971 to R. G. Sweet also discloses schematically a similar right angle nozzle structure.

SUMMARY OF THE INVENTION

Generally stated it is an object of this invention to provide an improved ink jet nozzle.

More specifically it is an object of the invention to provide an ink jet nozzle which is most efficient and effective within a predetermined frequency range.

It is an object of the invention to provide an ink jet nozzle which has improved ink drop forming characteristics.

It is also an object of the invention to provide an ink jet nozzle in which a resonant perturbation condition is attained, so as to enhance the formation of uniform ink drops from an ink stream.

Yet another object of the invention is to provide a sandwich type nozzle structure wherein a piezoelectric ceramic for producing perturbations in an ink stream is secured between a nozzle plate and a reaction mass to vibrate as a unit at a selected frequency.

It is also an object of the invention to provide for defining the relative proportions of a nozzle plate, a piezoelectric ceramic and a reaction mass in an ink jet nozzle structure for the most efficient operation thereof.

Still another object of the invention is to provide for securing a nozzle plate, piezoelectric ceramic, and a reaction mass together by cementing them together with a predetermined wire spacer in the cement between the parts to insure consistently accurate relative positioning of the elements.

Yet another important object of the invention is to provide for securing a unitary structure of a reaction mass, a piezoelectric ceramic, and a nozzle plate, to support means by means of a silicone rubber composition which provides a damped connection, permitting the unitary structure to float and vibrate as a unit, independently of the mass of the support means.

A further object of this invention is to provide for determining the relative proportions of a multielement nozzle structure for operation in a predetermined frequency range.

A still further important object of the invention is to provide an integrated ink jet nozzle structure wherein the perturbance velocity amplitude is proportional to the current of the exciting signal.

The foregoing and other objects, features and advantages of the invention will be more apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawing.

DESCRIPTION OF THE DRAWING

FIG. 1 is an isometric view of an ink jet nozzle structure embodying the invention.

FIG. 2 is a partial cross-sectional view of the nozzle plate and ceramic of FIG. 1.

FIG. 3 is an isometric view of the nozzle plate piezoelectric ceramic and reaction mass.

FIG. 4 is an enlarged partial cross-sectional view of the assembly in FIG. 3 showing how the elements are cemented together.

FIG. 5 is a schematic showing of the equivalent electric circuit of the assembly shown in FIG. 3.

FIG. 6 is a curve showing the relationship between the admittance and the frequency of oscillation for the nozzle structure of the invention.

FIG. 7 is a curve showing the relationship between the phase and frequency for the equivalent circuit and the actual nozzle structure of the invention.

FIG. 8 is a curve showing the relationship between break off time versus voltage at 60 KHz.

FIG. 9 is a curve showing the relationship between break off time and voltage at 100 KHz.

FIG. 10 is a curve showing the reciprocal of the break off time versus the wave length for 60 KHz.

FIG. 11 is a curve showing the reciprocal of the break off time versus wave length for 100 KHz.

FIG. 12 is a curve showing the relationship between the velocity perturbation per volt versus the frequency.

FIG. 13 is a curve showing the relationship between the velocity perturbation per volt versus the admittance.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1 the reference numeral 10 denotes generally an ink jet nozzle structure comprising a rectangular nozzle plate 12 cemented to a piezoelectric ceramic 14, which is in turn cemented to a reaction mass 16. The nozzle plate 12 comprises a generally rectangular plate of stainless steel having a passage 18 entering from one side by means of a nipple 20 to which a flexible hose may be fastened for supplying ink to the passage 18 under pressure. Pressures in the range of 30 to 60 pounds may be used. The plate is provided with an opening 22 at the center connecting with the passage 18, and over which a jewel nozzle 24 may be secured in any suitable manner such as by cementing, the nozzle having a central orifice 26 which is perpendicular to the passage 18 and through which a stream of ink may be projected under pressure. The passage 18 may have a diameter on the order of 0.030 inches by way of example.

The nozzel plate 12 is secured to one face of a piezoelectric ceramic 14, the other face of which is secured to a reaction mass 16 which may comprise a generally rectangular block of stainless steel for example. The reaction mass is secured to a generally U-shaped support 30 having a central bight portion 32 with upstanding spaced apart legs 34, and 36 having projecting tabs 38 and 40 at the ends to which flat springs 42 and 44 may be connected by means of screws 46 and 48 for fastening the assembly to lugs 50 and 52 on the frame of the printer and permitting movement for aiming. An arm 54 projecting from the central portion of the bight 32 may be used with suitable adjusting means (not shown) for aiming the nozzle 24 and directing the ink stream.

As shown in FIG. 4 the nozzle plate 12 and the reaction mass 16 may be cemented to the piezoelectric ceramic 14 by means of layers of cement 15 and 16 comprising for example an epoxy cement. In order to provide the best bond between the nozzel plate, the reaction mass and the ceramic, it is essential that the layers of cement 15 and 16 be accurately defined. For this purpose Molybdenum wire spacers, 0.003 inches in diameter identified by the numeral 19 may be imbedded in the epoxy. The sandwiched structure is then cured while being held in a springloaded jig, the wires 19 maintaining perfect spacing between the elements.

The reaction mass 16 is secured to the support 30 by means of silicone rubber 56 having a durometer value of 65 for example, on the Shaw scale, and which is bonded to the bight and to the two legs of the support 30, as well as to the reaction mass 16. The thickness of the rubber 56 is not critical and any value from a few mils to about 1/4 inch may be used. This permits the assembly of the nozzle plate 12 ceramic 14 and reaction mass 16 to float and vibrate independently of the mass of the support 30. Connectors 58 and 60 may be connected to the reaction mass 16 and the nozzle plate 16 for applying a voltage therebetween to effect vibration of the piezoelectric ceramic thus causing the nozzle plate and the nozzle to vibrate longitudinally relative to the ink stream thus imposing the velocity perturbation thereon.

The frequency of response of an ink jet head can be obtained approximately by considering the response of three semi-infinite slabs adjacent to one another such as shown in FIGS. 3 and 4. Media I and III represent the nozzle plate 12 and the reaction mass 16, respectively and Medium II represents the piezoelectric ceramic 14. Thickness mode vibrations of the ceramic slab 14 cause longitudinal standing waves to be set up in all three media in the x directions shown.

The controlling wave equation for this physical problem is ##EQU1## omitting any damping effects. E is Young's modulus for a media and .rho. is its density.

The solution of equation 1, giving the standing waves, is:

Medium I

u.sub.1 = Cos .omega.t [C.sub.1 Cos k.sub.1 x.sub.1 + C.sub.2 Sin k.sub.1 x.sub.1 ]

Medium II

u.sub.2 = Cos .omega.t [C.sub.3 Cos k.sub.2 x.sub.2 + C.sub.4 Sin k.sub.2 x.sub.2 ]

Medium III

u.sub.3 = Cos .omega.t [C.sub.5 Cos k.sub.3 x.sub.3 + C.sub.6 Sin k.sub.3 x.sub.3 ] (2)

where: ##EQU2## and v.sub.1, v.sub.2 and v.sub.3, the velocities of the waves in the media are given by: ##EQU3##

There are six unknown constants in equation 2; C.sub.1 through C.sub.6. These constants will be determined by boundary conditions. At the boundaries A and D in FIG. 3 the force transmitted per unit area equals zero. At boundaries B and C, u, the displacement is continuous. Also at boundaries B and C, the force per unit area which is transmitted is discontinuous; the discontinuity at the two boundaries being equal in magnitude but opposite in direction. This discontinuity is given by:

Pressure discontinuity = .rho. Cos .omega.t (5)

and is the driving force per unit area given by the piezoelectric action.

The six boundary conditions yield six equations which determine the six unknown constants in equations 2 uniquely. These six equations also yield the resonance condition of the structure. That resonance condition is:

Cos k.sub.1 L.sub.1 [Sin k.sub.2 L.sub.2 Cos k.sub.3 L.sub.3 + H.sub.3 Cos k.sub.2 L.sub.2 Sin k.sub.3 L.sub.3 ] + H.sub.1 Sin k.sub.1 L.sub.1 [Cos k.sub.2 L.sub.2 Cos k.sub.3 L.sub.3 -H.sub.3 Sin k.sub.2 L.sub.2 Sin k.sub.3 L.sub.3 ]= 0 (6)

H.sub.1 and H.sub.3 are the acoustic impedance mismatches between media I and II and II and III respectively. They are given by: ##EQU4## where A.sub.1, A.sub.2 and A.sub.3 are the areas of the three media.

The parameters that match the head shown in FIG. 4 are:

L.sub.1 = 0.090 inch, L.sub.2 = 0.250 inch, L.sub.3 = 0.250 inch, v.sub.1 =v.sub.3 =1.97 . 10.sup.5 inches sec. v.sub.2 = 1.06 . 10.sup.5 inches/sec.

The first four resonant frequencies, as computed from equation (6), are:

f.sub.1 = 101.6 KHz

f.sub.2 = 238.4 KHz

f.sub.3 = 365.7 KHz

f.sub.4 = 473.2 KHz (8)

The frequency characteristics of the head shown in FIG. 1 were measured. FIG. 6 shows the admittance curve for the head while FIG. 7 shows the phase difference versus frequency. In both figures the points represent experimental data and the solid line is the result of the fitted equivalent circuit shown in FIG. 5. Of course, the equivalent circuit would only fit in the given frequency range 30 KHz to 180 KHz. The measured first mechanical resonance is 103.3KHz and the corresponding parallel resonance point is 120.5KHz.

Break-up of Ink Stream Into Drops.

In order to design properly an ink jet heat, one must be able to relate the performance of the ink stream breakup properties back to the initial perturbation imposed on the stream by the head. Lord Rayleigh first presented an analysis that would provide a relationship required. However, the initial conditions used by him are not suitable to the present problem. A variation of the analysis will therefore be considered.

It is possible to show the variation of the analysis using a one dimensional model. Assume a stream of liquid initially in the shape of a cylinder in which the radius, longitudinal velocity of the stream, and pressure in the liquid are functions of t, time, and Z, one dimension, only. Further assume that the relative coordinate system is such that the DC velocity of the stream is zero. The two differential equations that describe the break-up of the stream into drops are: ##EQU5## where r is the radius, v is the velocity of the stream, and P is the pressure in the liquid at any point and time. .rho. is the density of the liquid. The first of these equations is an expression of Newton's second law for fluids, and the second equation is the continuity equation. The pressure in the liquid is caused by the surface tension, T. Approximately; ##EQU6## For the initial conditions, set: r = a

v = -v.sub.0 Sin kz (11)

where: ##EQU7## a and v.sub.0 are constants, .lambda. is the wave length for the imposed perturbation. The solution to the described problem, when approximations are made to linearize the problem, is:

v = -v.sub.0 Sin kz . Cosh ut (13) ##EQU8## where: ##EQU9##

These equations show Lord Rayleigh's conclusion. If ak is less than one, u is real and the radial disturbance grows until drop break-off occurs. If, however, the wave length of the disturbance is less than the original circumference of the stream (a . k greater than one), the hyperbolic functions become circular functions and an equilibrium condition would occur.

v.sub.0 is the amplitude of the perturbation imposed on the stream by the ink jet head. However, the break-up properties of the stream are dependent on all of the other parameters of the analysis as well. The analysis is used, therefore, to relate the observed properties of stream break-up to v.sub.0, thus eliminating the influence of the other parameters.

The break-off time is measured by measuring the break-off distance and dividing by the velocity of the stream. v.sub.0 can then be found from equation 14 by assuming that r = 0 at some point at the break off time. Thus: ##EQU10## where t.sub.B is the break off time.

Numerous sets of data were taken to show the characteristics of the ink jet head. FIGS. 8 and 9 show the break-off time as a function of the logarithm of the voltage amplitude of the impressed signal at constant frequency and wave length. From the straight line relationship, one can say: ##EQU11## or:

V = be.sup..sup.-ct.sbsp.b

where V is the voltage amplitude and c and b are constants. A comparison with equation 16 shows that in the range given the perturbation, v.sub.0 is directly proportional to the imposed electric signal. Furthermore, c in equation 17 equals u in equation 16. The ink jet head should always be operated in the linear region where the perturbation is proportional to the signal for satisfactory operation.

Sets of data have been taken for the break-up time as a function of wave length; frequency and head excitation being kept constant. The wave length was controlled by changing the ink pressure which caused the stream velocity to change. FIGS. 10 and 11 show two of the sets of data taken. The points represent the experimental data, while the solid line is the theoretical curve from equations 13 and 15. The initial perturbation amplitude, v.sub.0, is chosen so as to provide the best fit.

FIG. 12 is a graph showing the perturbing velocity amplitude per volt of exciting signal as a function of the frequency. The solid line is just a smooth curve drawn through the points. FIG. 13 shows the same dependent variable plotted against the admittance of the ink jet head, with a straight line being drawn through the data. The slope of the straight line on this log-log plot is one, showing a direct proportionality between the current through the head and the resulting perturbation.

The result that the perturbing velocity amplitude is proportional to the exciting signal is most important. This proportionality depends on the relatively short length of the liquid cavity parallel to the axis of the head. Others have shown that when the cavity is one quarter wave length long when acting as an open ended tube, or a half wave length when acting as a closed tube, (wave length measured in liquid) a resonance that enhances drop break off occurs. Such a resonance would probably not manifest itself as an admittance change back at the piezoelectric ceramic.

A commercial, one compound epoxy is used to bond the piezoelectric sandwich. The curing cycle for the epoxy is heating at 250.degree.F for 2 hours. While the epoxy is warming up it passes through a stage where it flows quite freely. Since the epoxy layer should be from 0.001 to 0.005 inches thick for the best result the Molybdenum wire spacers 19, 0.003 inches in diameter, are imbedded in the epoxy to insure uniform thickness since the sandwiches are cured while being held in spring-loaded jigs.

An ink jet head structure has been disclosed that uses a piezoelectric sandwich concept with the nozzle in the front plate. The vibration of the front nozzle plate and the nozzle longitudinally with the stream causes the stream break-up action. The head is relatively flat and can be used separately or can be packaged in densities of up to six heads per inch. Enough room exists between heads to permit individual aiming of the heads.

For different frequency applications the resonant point may be shifted by an appropriate choice of longitudinal dimensions. An equation has been presented, equation 6, so that required dimensions may be calculated.

A variation of theory has been presented which relates the drop break-off characteristics with the perturbing velocity amplitude at the nozzle. Thus the action of the head on the jet can be observed independently of the action of the jet after it leaves the nozzle.

The perturbing velocity amplitude has been shown to be directly proportional to the current amplitude of the exciting signal. Therefore, with the use of a constant current source, a network in series with the head to provide constant current or a feedback loop to control current, the response of the head can be made level.

While the invention has been shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. In an ink jet printer apparatus,

a support member,

a substantially flat ink jet nozzle structure disposed for connection to a source of ink,

a reaction mass,

damping means resiliently connecting said reaction mass to said support member, and

transducer means operating in a thickness mode connecting said nozzle structure to said reaction mass for producing resonant periodic bodily vibration of said nozzle and reaction mass independently of said support member to produce periodic drops of ink from said nozzle.

2. The invention as defined in claim 1 characterized by said ink jet nozzle structure comprising a plate having a nozzle opening on one surface and an opening through said plate at right angles to said nozzle opening for supplying ink thereto under pressure.

3. The invention as defined in claim 2 characterized by said transducer means comprising a piezoelectric crystal plate sandwiched between said nozzle plate and one surface of said reaction mass.

4. The invention as defined in claim 3 characterized by said nozzle plate, piezoelectric crystal, and reaction mass being cemented to each other in a sandwich arrangement by layers of epoxy resin with wire spacers included in said layers to maintain a predetermined uniform thickness of said layers.

5. The invention as defined in claim 3 characterized by said damping means comprising a body of silicone rubber secured to said support member and to and partially surrounding said reaction mass on two opposite sides and the intervening surface away from the surface secured to said piezoelectric crystal.

6. The invention as defined in claim 3 characterized by the relative thicknesses of the nozzle plate, piezoelectric crystal, and reaction mass being related by the formula Cos k.sub.1 L.sub.1 [Sin k.sub.2 L.sub.2 Cos k.sub.3 L.sub.3 + H.sub.3 Cos k.sub.2 L.sub.2 Sin k.sub.3 L.sub.3 ] + H.sub.1 Sin k.sub.1 L.sub.1 [Cos k.sub.2 L.sub.2 Cos k.sub.3 L.sub.3 -H.sub.3 Sin k.sub.2 L.sub.2 Sin k.sub.3 L.sub.3 ]= 0 where L.sub.1 L.sub.2 and L.sub.3 are the thicknesses of the nozzle plate, piezoelectric crystal and reaction mass, respectively; H.sub.1 and H.sub.3 are the acoustic impedance mismatches between the nozzle plate and piezoelectric crystal, and the piezoelectric crystal and reaction mass respectively; and k.sub.1 k.sub.2 and k.sub.3 = equal .omega./v.sub.1 .omega./v.sub.2 and .omega./v.sub.3, where v.sub.1, v.sub.2 and v.sub.3 are the velocities of the waves in the media of the nozzle plate, piezoelectric crystal and reaction mass, respectively.