IL46861A

IL46861A - Ink jet print head

Info

Publication number: IL46861A
Application number: IL46861A
Authority: IL
Original assignee: Ibm
Priority date: 1974-04-30
Filing date: 1975-03-18
Publication date: 1977-03-31
Also published as: SU644402A3; NO142972C; FR2269421A1; SE7503939L; DD119893A5; IL46861A0; JPS5735114B2; IT1031598B; ATA162775A; JPS50142120A; CH575832A5; NL7504556A; DE2509131A1; AT341796B; FI751254A; FR2269421B1; NO751488L; AU7955875A; BE827102A; NO142972B

Description

INK JET PRINT HEAD ABSTRACT OF THE DISCLOSURE An ink jet print head comp :rises a unitary construction of CL a nozzle plate having jewel orifice on one face with a passage Λ 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.

BACKGROUND OF THE INVENTION Field of the Invention This invention relates generally to ink jet printers and it has reference in particular to an ink jet print head structure.

Description of Prior Art Inkjet nozzles are known such as shown in U.S. Patent No. 3,334,351 which issued on August 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. Patent 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 print head.

More specifically it is an object of the invention to provide predetermined frequency range .

It is an object of the invention to provide an ink jet print head which has improved ink drop forming characteristic s .

It is also an object of the invention to provide an ink jet print head in which a re sonant 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 pro- ducing perturbations in an ink stream is secured between a nozzle plate and a reaction mas s 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 mas s in an ink jet print head structure for the most efficient operation thereof.

Still another obj ect of the invention is to provide for securing a nozzle plate , piezoelectric ceramic , and a reaction mas s together by cementing them together with a predetermined wire spacer in · the cement between the parts to insure consistently accurate re- lative positioning of the elements .

Yet another important object of the invention is to provide for securing a unitary structure of a reaction mas s , 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 mas s of the support means .

A further object of this invention is to provide for determining the relative proportions of a multi-element nozzle structure for operation in a predeter-mined 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 ad-vantages of the invention will be more apparent from the following more particular description of a preferred em-bodiment of the invention as illustrated in the accompany- ing drawing.

DESCRIPTION OF THE DRAWING FIG. 1 is an isometric view of an ink jet nozzle structure embodying the nvention.

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 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 Λ 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 GO 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 pass age 18 may have a diameter on the order of .030 inches by 1 piezoelectric ceramic 14, the other face of which is 2 secured to a reaction mass 16 which may comprise a 3 generally rectangular block of stainless steel for example. 4 The reaction mass is secured to a generally U-shaped 5 support 30 having a central bight portion 32 with up- 6 standing spaced apart legs 34, and 36 having projecting 7 tabs 38 and 40 at the ends to which flat springs 42 and 8 44 may be connected by means of screws 46 and 48 for 9 fastening the assembly to lugs 50 and 52 on the frame of 10 the printer and permitting movement for aiming . An arm 11 54 projecting from the central portion of the bight 32 12 may be used with suitable adjusting means (not shown) 13 for aiming the nozzle 24 and directing the ink stream, 14 As shown in FIG. 4 the nozzle plate 12 and the re- 15 action mass 16 may be cemented to the piezoelectric ceramic 16 14 by means of layers of cement 15 and XT comprising for 17 example an epoxy cement. In order to provide the best 18 bond between the nozzle plate, the reaction mass and the 19 ceramic, it is essential that the layers of cement 15 and 20 6 be accurately defined. For this purpose Molybdenum 21 wire spacers, .003 inches in diameter identified by the 22 numeral 19 may be imbedded in the epoxy. The sandwiched 23 structure is then cured while being held in a spring- 24 loaded jig, the wires 19 maintaining perfect spacing be- 25 tween the elements. 26 The reaction mass 16 is secured to the support 30 by 27 means of silicone rubber 56 having a durometer value of 28 65 for example, on the Shaw scale, and which is bonded 29 to the bight and to the two legs of the support 30, as 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 piezo-electric 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 throe media in the x directions shown.

The controlling wave equation for this physical probl omitting any damping effects. E is Young's modulus for a media and p is its density^ O t-oL IL is olisptacewiev-it*. The solution of equation 1, giving the standing waves, is: Medium I u1 = Cos tot [C1 Cos kj, χ + C2 Sin k 1 x-J Medium II u2 = Cos cot [C3 Cos k2 x2 + C4 Sin k2 x2}>(2) where: k, <= —, k = —, k = — (3) 1 V]L' 2 v2' 3 v3 and v-^, v2 and v-j, the velocities of the waves in the media are given by: There are six unknown constants in equation 2; through Cg . 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 = p Cos tot (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 eauations also yield the resonance conditio of the structure. That resonance condition is: Cos k1L1 [Sin k2L2 Cos k3L3 + H3 os k2L2 Sin k3L3^ + ^ H1 Sin k1L1 [Cos k2L2 Cos k-jLg -H3 Sin k2L2 Sin k3L3^= 0 and H3 are the acoustic impedance mismatches between media I and II and II and III respectively. They are given by : Ej A± ^ _ E3 A3 k3 Hl = E2 A2 k2 . H3 = E2 A, k, (7> where A^, A2 and A3 are the areas of the three media.

The parameters that match the head shown in FIG. 4 are : Lx= .090", L2= .250", L3= .250", v'1=v3=1.97 . 105,,sec, v2= 1.06 . 105,,/sec.

The first four resonant frequencies , as computed from equation (6), are: f2 = 238 · KHz f3 = 365.7 KHz (8) f4 = 473.2 KHz 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 Dro s.

In order to design properly an ink jet head, one must be able to relate the performance of the ink stream break-up 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. 1 initially in the shape of a cylinder in which the radius, 2 longitudinal velocity of the stream, and pressure in the 3 liquid are functions of t, time, and Z, one dimension, only. 4 Further assume that the relative coordinate system is such 5 that the DC velocity of the stream is zero. The two 6 differential equations that describe the break-up of the 7 stream into drops are : o dv . ■ dv 1 dP fo\ ■ 8 ar+ v - d p dz (9) 0 where r is the radius, v is the velocity of the stream, 1 and P is the pressure in the liquid at any point and time. 2 p is the density of the liquid. The first of these equa-3 tions is an expression of Newton's second law for fluids, 4 and the second equation is the continuity equation. The 5 pressure in the liquid is caused by the surface tension, T.

For the initial conditions, set: r = a (11) where : k = ψ (12) a and VQ are constants, λ is the wave length for the imppsed perturbation. The solution to the described problem, when approximations are made to linearize the problem, is: v = -vQ Sin kz . Cosh ut (13) vnk r = a [1 + — . Cos kz Sinh ut] where : ; u = (1-a2 k*) (14) 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.

Vq 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 , 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. Vq can then be found from equation by assuming that r = 0 ait some point at the break off time. Thus: A -utD 15 4u B Ι \ v0 = ~k * e where tfi is the break off time.

Numerous sets of data were taken to show the characteristics of the ink jet head. Figures 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: where V is the voltage amplitude and c and b arc constants. A comparison with equation 16 shows that in the range given the perturbation, VQ is directly proportional to the im-posed 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. Figures 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, VQ, 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 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 °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 .001 to .005 inches thick for the best result the Molybdenum 26 wire spacers ¥ί, .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 freouency 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 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.

What is claimed is :

Claims

1. Ink jet print head with a nozzle connected to an ink reservoir, characterized by a piezoelectric transducer (l4-)-for generating a periodic re sonance vibration, arranged between a nozzle plate -(i2") carrying the nozzle (&4ή and a reaction mas s (r6)^which in turn is secured to a support structure (32. 36) by means of a damping material 4{5£) .

2. Ink jet print head according to claim 1, characterized in that said nozzle plate -(12† carrying the nozzle (2 ·) has an opening (22) on one of its side surfaces and a hole (Jr8")~ arranged at right angles to said opening -(2¾")~and through-connected therewith to provide for ink under pres sure to be supplied to said nozzle ^ - )?

3. Ink jet print head according to claim 1, characterized in that said nozzle plate said piezoelectric transducer J(14*) and said reaction mas s (l-^ are cemented to each other in a sandwich arrangement, the thicknes s of the layers of re sin (15, 17·) being kept uniform by means of wire space rs (i^) embedded in said layers (15 , 17†.

4. Ink jet print head according to claim 1, characterized in that the damping material comprises a rubber block &6r)"cemented to the support structure (32. , . 36), said rubber block (54) at least partly surrounding said reaction mas s (1 ·.

5. Ink jet print head according to claim 1, characterized in that the relative thicknes ses of said nozzle plate J f2"), said piezoelectric transducer (½), and said reaction mas s are related by the formula: 0= cos k L (sin k L cos k L + H cos k L sin k L ) + Ηχ sin ^ (cos k2 cos k3 - sin k^ L2 sin k3 L^), where L , and are the thicknes ses of the nozzle plate .(12-), the piezoelectric transducer (14) and the reaction mas s (16), re spectively; and are the acoustic impedance mismatches between nozzle plate j£l2") and piezoelectric transducer and between the latter and the reaction mas s ^ή, respectively; and k = 0?/v , k = /v_ and k, = co/v , where v , v_ and v, are the velocities of the waves in the media of nozzle plate f, piezoelectric transducer .(14 and reaction mas s , respectively, and co is the angular frequency of the excitation signal of the piezoelectric transducer ^14)".