GB1559651A - Liquid jet modulator with hemispherical or hemicylindricaltransducer - Google Patents

Description

(54) LIQUID JET MODULATOR WITH HEMISPHERICAL OR HEMICYLINDRICAL TRANSDUCER (71) We, RECOGNITION EQUIPMENT INCORPORATED, a corporation organised under the laws of the State of Delaware, United States of America, and of 2701 E.

Grauwyler, Irving, Dallas County, Texas, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement :- This invention relates to a liquid droplet generator. The invention is particularly useful as an ink gun in a liquid jet printing system.

Ink modulators associated with ink jet printers heretofore have been electromechanically tuned to a particular drop rate.

Any deviation from the resonant frequency has caused a deterioration in the modulator efficiency, and intolerable changes in the distance between the nozzle and the droplet break-off point. Variation in the break-off point length affects the drop charging function. With variation in the deflection control, poor quality printing has resulted.

In an ink jet printer transport system, an ink jet gun projects charged ink droplets which are deflected to form an information pattern on a moving document surface. When the transport velocity changes, it is necessary to provide a corresponding change in the drop rate. In printer systems heretofore used, adjustments of the drop rate have been unavailable without a returning of the ink modulator.

A drop-on-command modulator inclues a positive displacement pump which in response to a command issues a pressure pulse to an ink chamber feeding a nozzle. If the pressure pulse is sufficient to overcome the surface tension at the nozzle tip, a drop of ink is expelled. Ink supply lines, however, represent massive pressure leaks opposing the pumping action required to push an ink drop through the nozzle.

Bowl shaped or hemispherical piezoelectric crystals long have been used in the medical and chemical industries. Hemispherical crystals hove been used as concave radiators to generate ultrasonic energy in a liquid medium. 1 Rosenberg, Sources , f f glz tsS. y Ullrus7urld, 275 and 282 (1969) discloses the use f a bowl shaped piezoelectric transducer for performing ultrasonic surgery. Further, at pages 275,286 and 287, the use of a bowl shaped piezoelectric transducer is disclosed for producing mulsions and suspensions, and for intensifying polymerization, oxidation, reduction and fine dispersion. Wells, Physical Principles of Ultrasorric Diagnosis, pages 62,63 (1969), discloses a focused bowl transducer for transferring energy to a patient immersed in water.

Other spherical radiatoring devices have been used to direct acoustical energy under water. One such device inclues a flat disc piezoelectric crystal cemented to the back side of a plano-concave lens. The crystal sends acoustical waves directly into the lens material. Waves reaching the lens-liquid interface are refracted toward the acousitical axis.

In the present invention, an ink modulator comprising a hemispherical or hemicylindrical piezoelectric transducer is provided for the generation of an ink stream comprised of uniform ink droplets throughout a drop frequency range heretofore unachievable. Further, larger changes in tem perature and ink pressure are accommodated without incurring unacceptable excursions of the drop break-off distance, or degrading the print quality. There is also provided a dropon-command modulator comprising a hemispherical piezoelectric transducer for the generation of ink droplets without a loss of drive pressure in the ink supply line.

Tn the development of some multiple nozzle configurations, complex systems having plural transducer-diaphragm combina- tions have been required. U. S. Patent 3 708 798 discloses a multi-nozzle printer having a plurality of piezoelectric transducers secured to a like plurality of dia phragms which pulsate the ink towards a set of nozzles. In U. S. Patent 3 900 162 an ink gun comprised of a diamond-shaped ink chambcrs feeds multiple orifices to form droplets at approximately the same time and a near uniform distance from the orifices.

The diamond shaped chamber is divided along its diagonal by a vibrating member and has a plurality of transducers affixed to one side of the member. According to the present invention, a liquid droplet generator comprising a piezoeletcric transducer including a piezoelectric crystal of hemispherical or hemicylindrical construction with electrodes attached to its convex and concave surfaces, means for feeding liquid to at least one exit orifice arranged on a focus of the hemisphere or hemicylinder of the crystal, the exit orifice or orifices being either acoustically coupled to the crystal by a solid filling or being capable of being acoustically coupled to the crystal by a filling of the liquid when the generator is used.

The crystal may be of a laminar construction.

The invention is particularly useful as an ink gun in a liquid jet printing system.

With a hemispherical crystal, which is used with a liquid, a ring shaped gasket may be fitted to the circula edge of the crystal and a nozzle plate placed in sealed relationship witli the gasket to form an ink chamber between the concave surface of the crystal and the nozzle plate. The plate may be secured by screws or bolts passing through the backing of acoustic damping material. The electrodes are connected, in use, to means for establishing an acoustic standing wave having a half wave length which is less than the width dimension of the crystal measured either circumferentially, or longitudinally.

For example, half the spherical circumference of the hemispherical crystal, or the length of the cylindrical axis or the hemicylindrical circumference of the hemicylindrical crystal is smaller than half the wave length of the shortest standing acoustical wave which can be established at the highest frequency of drops produced at the exit orifice. Mechanical resonance frequencies are thereby substantially separated from the operating frequency band. As a result, the break off point is stable over the operating range. Further, a substantially increased bandwidth of operating frequencies is provided within which the drop rate, temperature and liquid pressure changes may be accommodated without degradation in liquid drop uniformity.

The liquid feed tube may be supported with respect to the concave surface of a hemispherical crystal by a solid filling of, for example, epoxy material. A nozzle is then connected at an exit orifice of the feed tube. The operating frequency bandwith is substantially increased by such an arrangement and the tolerance to liquid pressure changes is improved. In addition, a greater degree of break-off distance stability is provided with typical drive voltages.

The piezoelectric crystal of hemicylin- drical construction may be fitted with a plate containing a series of exit orifices, the plate and the concave surface of the crystal defining a liquid chamber. Such an arrangement has reliable operating characteristics over a wide band of drop frequencies. The occurrence of satellite droplets is substantially eliminated and changes in temperature and drop rates are accommodated without decreasing the quality of print in an ink jet system. Operation at various frequencies permits printing on a medium which moves at a changing speed, or which starts and stops. Moreover, it is possible to print from a few drops to a full page width of drops with separate charges for each orifice and a device can be used to print bars or characters whilst using a common deflection field. The device may be quickly purged for faster turn on/turn off. More power is applied to the ink because the hemicylinder has a much greater radiating area for the same frontal area than the conventional flat crystal. The multi-orifice plate may be made of any material in which nozzles can be formed and which is hard and strong enough to support the pressure applied at the orifice area.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which :- Figure I is a cross-sectional side view of an ink modulator embodying the invention; Figure 2 is a front plan view of the modulator of Figure 1; Figure 3 is a graph of the frequency- pressure operating characteristics of the ink modulator of Figure I ; Figure 4 is a graph of the ink droplet break-off distance versus frequency-perat ing characteristics of the modulator of Figure 1 ; Figure is a cross-sectional side view of a solid fill ink modulator embodying the invention; Figure 6 is a graph of the ink droplet break-off distance versus frequency operating characteristics of the modulator of Figure 5; Figure 7 is a graph of the frequency versus ink pressure operating characteristics of the modulator of Figure 5; Figure 8 is a graph of the frequency versus drop spacing characteristics of the modulator of Figure 5; Figure 9 is a cross-sectional side view of a drop-on-command ink modulator embodying the invention; and Figure 10 is a cross-sectional side view of a second drop-on-command ink modulator embodying the invention.

Figure 11 is an exploded view of the multiorifice gun; Figure 12 is a cross sectional plan view of the assembled gun; and Figure 13 is a typical system in which the gun may be used.

Figure 14 is a graph of the piezoelectric crystal impedance plotted against frequency.

Figure 15 is the impedance plotted against frecfucncy when cystal is damped by embedding the crystal as illustrated in Figure 2.

Referring to Figure 1, a hemispherical piezoelectric crystal 10 is electroded to provide electrodes 11 and 12 on the outer convex and inner concave surfaces of the crystal, respectively. The electrodes are isolated one from the other by the non-conductive ringshaped edge of the crystal. Crystal 10 with the electrodes 11 and 12 is seated within an epoxy backing 13. A first drive lead 14 is electrically connected to electrode 11, and a second drive lead 15 is electrically connected to electrode 12. Drive lead 14 extends through backing 13 to an external voltage source. Drive lead 15 extends through an epoxy filled trough 10a cut into crystal 10 and the electrodes, and through backing 13 to the external voltage source. One surface of a polytetrafluoroethylene (PTFE) ring gasket 16 is in contact with backing 13 and to the ring-shaped edge of crystal 10. A nozzle plate 17 has a nozzle 18. Nozzle plate 17 is placed in sealed contact with the surface of gasket 16 opposite backing 13, thereby forming an ink reservoir with electrode 12.

Crystal 10, backing 13, gasket 16 and nozzle plate 17 are held in place by bolts 19 having nuts 20.

Nozzle plate 17 has formed therein a first outer transverse bore 21 leading to a first inner transverse bore 22. Bore 22 is in fluid communication with a first longitudinal bore 23 of equal diameter. Bore 23 is located just inside the inner edge of gasket 16.

Nozzle plate 17 further includes a second outer transverse bore 24, Figure 2, leading to a second inner transverse bore 25. Bore 25 is in fluid communication with a second longitudinal bore formed in the same manner as bore 23. A tube 26 is sealably press fitted into bore 21, and a tube 27 is sealably press fitted into bore 24.

The outer surface of nozzle plate 17 has formed therein an outward flaring and conicallv shaped well 17a. Centered within the well is nozzle 18. Nozzle 18 is cylindrical in shape with a conical entrance bore 18a converging to an orifice 18b.

In a preferred embodiment described herein, backing 13 has a one inch square face, and is 0.275 inches thick. The backing is formed from an epoxy resin such as that manufactured and sold by Emerson and Cuming, Inc., of Canton, Massachusetts, and identified as Model No. 2850-KT.

Crystal 10 is a piezoelectric crystal formed from a ceramic material having a low mechanical Q and a high strain per field at constant stress, generally referred to in the art as a d33 factor. It has been found that material having a mechanical Q no greater than 70, and a d33 no smaller than 550 x 10-'2 meters per volt may be used. Material manufactured by Vernitron Piezoelectric Division, of Bedford, Ohio, and available as Model PZT-5H is preferred. Such material has a mechanical Q of approximately 65 and d33 of about 593 x 10-'2 meters/volt. The thickness of the crystal wall is approximately 0.031 inches. The inner diameter of the crystal is about 0.313 inches. Electrodes 11 and 12 are made of silver, and are adhered to crystal 10 by well known means.

Ring gasket 16 is approximately 0. 005 inches thick with an inner diameter of about 0.313 inches. The ring gasket not only provides a liquid seal, but also attenuates transducer vibrations which emanate from the ring-shaped edge of crystal 10 normal to the transducer axis.

Nozzle plate 17 is made of stainless steel, and is approximately 0.090 inches in length.

The wall 17a formed in plate 17 has an opening about 0.30 inches in diameter defined by walls having an included angle of approximately 100 degrees. Bores 21 and 24 of plate 17 each are approximately 0.042 inches in diameter and 0.156 inches in length. Bores 22 and 25 each are about 0.025 inches in diameter and 0.076 inches in length. Further, bore 23 is about 0.025 inches in diameter and 0.045 inches in length.

Tubes 26 and 27 each are stainless steel gauge 19 hypodermic tubing.

Nozzle 18 is formed from a cylindrical sapphire jewel approximately 0.031 inches long and 0.046 inches in diameter. The walls of entrance bore 18a of nozzle 18 define an included angle of about 70 degrees, and converge to orifice 18b which is approximately 0.002 inches long and 0.002 inches in diameter.

In operation, ink under pressure flows through tube 26 and bores 21,22 and 23 to the ink reservoir between crystal 10 and nozzle plate 17. Initially, tube 27 is opened to purge any fluid residue which may be present in the ink reservoir. Thereafter, tube 27 is closed off and the ink exits only through nozzte i8. Voltage pulses applied to leads 14 and 15 cause the crystal 10 to expand and contract in the region between electrodes 11 and 12. The ink within the ink reservoir thereby is pressure modulated. The pressure wave is transmitted through the nozzle 18 and down the ink stream. The stream breaks into droplets.

The physical dimensions of the component parts of the ink modulator are smaller than the half wavelength of the shortest standing acoustical wave that can be produced at the highest of the openable drop frequency rates in a structure of the same material as the part in question. Further, the longitudinal length of the modulator is less than the wavelength of the shortest standing longitudinal acoustic wave that can be estab lished in the assembled body of the modulator. A wide bandwidth of drop rate frequencies thereby is provided which is far enough below the mechanical resonance frequencies of the modulator to be effectively isolated from the effects of both the resonance frequencies, and the harmonics thereof which may be excited during a printing operation. As a result, the ink droplet breakoff distance is substantially uniform over the operating range, and the tolerance of the modulator to drop rate, temperature and ink pressure changes is improved. The primary ink drop path is substantially free of satellite drops. Uniform ink droplets are generated over a frequency range up to a topmost operating frequency approximately 3.5 times the lowermost operating frequency.

Crystal 10 may be characterized as a 165 degree crystal. That is, an angle of 165 degrees is defined by radial lines in the same plane and projecting from the center of curvature to opposing points on the ring shaped edge of the crystal. In Figure 3, curves 28 and 29 represent the low and high pressure limits, respectively, of an operating envelope for an ink modulator having a 165 degree crystal operating at drop frequency rates between 20 and 90 KHz.

It has been found that with a constant 30 volt crystal drive voltage, the ink modulator may be operated within the envelope defined by curves 28 and 29 without producing satellite droplets. Further, the ink droplet break-off distance from the nozzle remains substantially uniform, and sensitivity to temperature change is less acute than with prior systems.

Curves 28 and 29 further illustrate a tolerance to ink pressure changes over a band of high frequency drop rates wider than heretofore achieved.

In Figure 4 break-off distance versus frequency is plotted. Curves 30-32 are representative of the operation of an ink modulator having a 165 degree crystal driven with a constant 30 volt crystal drive voltage.

More particularly, curve 30 illustrates the change in ink drop break-off distance from the nozzle for drop rates in the 50 to 88 KHz range where ink pressure is maintained at 47 PSIG. For pressures between 50 and 60 PSIG, the break-off distance varies according to curve 30 where it has a nega tive slope of about 0. 002 inches per kilohertz. Further, the break-off distance follows curve 30 where it has a negative slope of about 0.0012 inches per kilohertz between drop rates of 60 and 73 KHz, and varies according to curve 30 where it has a negative slope of about 0.0003 inches per kilohertz between drop rates of 73 and 90 KHz.

Curve 31 illustrates the change in break o I distance as the drop rate is varied from 34 t G''ICHz. Between 34 and 40 KHz, the break-off distance follows curve 31 where it has a negative slope of about 0.0025 inches per kilohertz. Between 40 and 48 KHz, however, the break-off distance varies according to curve 31 where it has a positive slope of approximately 0.0019 inches per kilohertz. The break-off distance follows curve 31 where it has a negative slope of about 0. 00104 inches per kilohertz at drop rates between 48 and 62 KHz.

Curve 32 illustrates the change in breakoff distance as the drop rate is varied from 20 to 50 KHz at a constant ink pressure of 20 PSIG. Between 20 and 25 KHz, the break-off distance varies according to curve 32 where it has a negative slope of 0.004 inches per kilohertz. No discernible change in break-off distance is detected between 25 and 50 KHz.

From an inspection of Figure 4, it may be seen that a maximum excursion of about 0.04 inches in break-off distance is encountered between 50 and 90 KHz at 47 PSIG. Further, a maximum excursion of approximately 0.015 inches occurs between 34 and 62 KHz at 29 PSIG, and a maximum excursion of about 0.02 inches occurs between 20 and 50 KHz at 20 PSIG.

Figure 5 illustrates a solid fill ink modulator which provides improved operating characteristics over the embodiment of Figures 1 and 2.

A fluid feed tube 35 passes through a central bore 36 of a hemispherical piezoelectric crystal 37. An annular air pocket is provided between the tube 35 and the walls of the bore 36 to isolate tube 35 from the radial vibrations of the crystal. Tube 35 is held in place by an epoxy backing 38 which is adhered to the convex surface of crystal 37. The concavity of crystal 37 is filled with epoxy in which the nozzle 39 is mounted and capture. A bore 40 is formed from the end of tube 35 to provide a fluid bath between tube 35 and nozzle 39. Bore 40 is aligned with tube 35 and nozzle 39 on a longitudinal axis passing through the center of curvature of the crystal.

The crystal 37 is electroded to provide electrodes 41 and 42 on the outer convex and inner concave surfaces of the crystal, respectively. The electrodes are isolated one from the other by the non-conductive ringshaped edge of the crystal. A drive lead 43 is electrically connected to electrode 41, and a drive lead 44 is electrically connected to electrode 42. The ring-shaped crystal edge is exposed to air to isolate nozzle 39 from radial crystal vibrations.

The epoxy backing 38 comprising the body of the modulator is of a type such as that manufactured and sold by the HYSOL Division of the Dexter Corporation of Olean, New York, and identified as MOdel No. 1C.

The epoxy fill completely encloses all modulator parts except the extreme ring-shaped end of crystal 37, the orifice of nozzle 39 and that part of tube 35 exposed for connection to the ink supply.

It has been found that when backing 38 has a shape other than a hemispherical shape concentric to crystal 37, the likelihood of a standing acoustical waveform occurring in the modulator body is substantially reduced.

The fluid feed tube 35 is made of nylon, and has an outer diameter of about 0.042 inches. Bore 36 of crystal 37 is approximately 0.062 inches in diameter. Tube 35 extends past the concave crystal face a distance of about 0.030 inches. Annular air pocket 36 is approximately 0.010 inches wide between the tube 35 and the walls of bore 26.

Nozzle 39 is formed from a cylindrical ruby having a length of approximately 0.031 inches and a diameter of approximately 0.046 inches.

In operation, ink under pressure is admitted into tube 35 and bore 40 leading to nozzle 39. When crystal 37 is excited by a voltage pulse applied to drive leads 43 and 44, the ink in bore 40 is pressure modulated.

The pressure wave is transmitted through nozzle 39 and down the ink stream to the droplet break-on'point.

As with the ink modulator of Figure 1, each part of the solid fill modulator has physical dimensions smaller than the half wavelength at the shortest standing acoustical wave that may be produced at the highest of the operable drop frequency rates in a structure of the sarne material as the part in question. A wider bandwidth of drop rates is provided thereby within which larger variations in ink pressure may be tolerated than with the liquid fill modulator of Figure 1.

Figure 6 illustrates the break-off distance verusus frequency operating characteristics of the solid fill modulator of Figure 5.

Curves 50-53 are representative of the operation of the solid fill modulator at a constant 120 volt crystal drive voltage.

More particularly, Curve 50 illustrates the variation in ink drop break-off distance from the nozzle as the operation frequency is varied between 55 and 127 Kz at 64 PSIG.

No. discernible change in break-off distance is detected below 106 KHz Between 106 KHz and 127 KHz, however, the break-off distance varies according to curve 50 where it has a negative slope of about 0.0005 inches per kilohertz.

Curve 51 illustrates changes in the breakoff distance as the operating frequency is varied between 47 KHz and 96 KHz at an ink pressure of 47 PSIG. Between 47 KHz and 59 KHz, the break-off distance varies according to curve 51 where is has a negative slope of 0.00038 inches per kilohertz.

Between 59 and 96 KHz, the break-off distance varies according to curve 51 where it has a positive slope of 0.0028 inches per kilohertz.

Curve 52 illustrates the break-off distance variation as the operating frequency is varied between 29 and 70 KHz at an ink pressure of 29 PSIG. Between 29 and 46 KHz, the hrv ak-off distance varies according to curve 52 where it has a positive slope of 0.0006 inches per kilohertz. The break-off distance follows curve 52 where it has a negative s'ope of 0.004 inches per kilohertz between 46 and 54 KHz. Further, the break-off distance varies according to curve 52 where it has a positive slope of 0.0012 inches per kilohertz between 54 and 70 KHz.

Curve 53 illustrates the changes in breakoff distance which occur when the operating frequency is varied between 23 and 51 KHz at 20 PSIG. Between 23 and 29 KHz. the break-off distance follows curve 53 where it has a negative slope of 0.003 inches per kilohertz. Further, the break-off distance varies according to curve 53 where it has a positive slope of 0.0006 inches per kilohertz between 29 and 46 KHz. Between 46 and 51 KHz, however, the break-off distance follows curve 53 where i has a negative slope of 0.002 inches per kilohertz.

The maximum excursion of the break-off distance at an ink pressure of 64 PSIG is 0.01 inches between 55 and 127 KHz. The maximum excursion at 47 PSIG is 0.030 inches between 47 and 96 KHz. Between 29 and 70 KHz, the maximum excursion is 0.035 inches at 29 PSIG. Further. the maximum excursion at 20 PSTG is 0.02 inches between 23 and 51 KHz.

The solid fill modulator thus accommodates a larger excursion in ink pressure over a larger bandwidth of drop rates than the liquid fill modulator of Figure 1.

Where the ink pressure and crystal drive voltage of the solid fill modulator may be dynamically adjusted as the frequency is varied between 25 and 125 KHz. a variation in break-off distance less than + 0.020 inches from a substantially linear norm has been observed over the entire operating range.

A comparison of the droplet break-off distances over the respective operating ranges of the liquid fill and solid fill modulators, as described herein, reveals a shorter break-off distance for the solid fill modulator at each operable drive voltage. Further, the average displacement of break-off distances from substantially linear norms is twice as great for the liquid fill modulator than for the solid fill modulator.

Figure 7 illustrates the frequency-pressure operating characteristics of the solid fill modulator of Figure 5 at a constant 120 volt crystal drive voltage.

Between a low pressure limit curve 60 and a high pressure limit curve 61, the ink pressure may be varied at any frequency between 20 and 700 KHz without the generation of satellite droplets. The low limit curve 60 occurs when either the ink droplets are almost tangent in the ink stream or a satellite condition occurs. At the high pressure limit as illustrated by curve 61, satellite drops begin to form.

Figure 8 illustrates the drop spacing versus frequency characteristics of the solid fill modulator of Figure 5.

Within the operating frequency-pressure envelope illustrated in Figure 7 and with a constant 120 volt crystal drive voltage, the operating frequency of the solid fill modulator may be varied between 20 and 130 KHz to determine the drop spacing boundaries illustrated as a curve 70 and a line 71 in Figure 8. Curve 70 defines the maximum drop spacing boundarv beyond which a satellite condition occurs. The vertical line 71 indicates the minimum drop spacing that may be acquired. Since a drop diameter is approximately 0.004 inches, the minimum drop center spacing with 0.001 inches of drop separation is approximately 0.005 inches.

If the crystal drive voltage and ink pressure are dynamically adiusted with a change in frequency, the modulator produces satellite free droplets from 5 KHz to above 160 KHz.

Figure 9 illustrates a positive displacement drop-on-command ink modulator embodying the invention.

A fluid feed tube 80 having a flow restrictor 80a is sealably press fitted into a bore 81 of an epoxy backing 82. In fluid communication with bore 81 is a central bore 83 of a 150 to 165 degree hemispherical piezoelectric crystal 84. The concave and convex surfaces of the cryst tively. Bonded to backing 108 and the rire- shaped edge of crystal 112 is a nozzle plate 115 having a nozzle 116. An ink reservoir 117 is formed thereby between the concave surface of electrode 114 and the nozzle plate 115. Bore 102, tube 110, bores 109 and 111, chamber 117 and nozzle 116 are in fluid communication.

In operation, ink under pressure is admitted into tube 101 and throughbore 102 to ink chamber 117. When crystal 112 is excited, the pressure of ink chamber 117 incrcascs to generate an ink drop at nozzle 116. Fluid channels 104-107 are shaped to provide a restriction to fluid now from tube 110 to tube 101. Thus, the increase in pressure in chamber 117 is not dissipated through the ink supply line.

Referring to Figure 11, a hemicylindrical piezoelectric crystal 212 is embedded in an epoxy material 213 in a carrier base 211.

The epoxy and tenon provide provide damping and attenuating as hereinafter explained.

The base 211 may be for example, a Teflon (Registered Trade Mark) material having a recess therein to receive the electrical crystal. The crystal then is imbedded and held within the base 211 by the epoxy 213.

The crystal has contact areas on both sides thereof (not illustrated) and contact is made thereto by the contact wires 222 and 223.

These wires may be potted in the epoxy and brought through a portion thereof to make contact to the imbedded crystal. As gasket 21 fits down over the imbedded crystal and seals the ink chamber when the orifice plate 16 is positioned on top of a gasket. Front plate 217, orifice place and gasket are held in position against base 211 by assembly screws 218.

The orifice plate illustrated has eight orifices 219 therein, however, depending on the desired purpose and utilization of the printer, quantities other than eight may bc used.

In one embodiment, one half of a cylindrical crystal was used. The crystal was, for example, the type manufactured by Bernitron, Piezoelectric Division of Bedford, Ohio and designated as PZT-5H type material. Such material has a mechanical Q of approximately 65 and a high"strain per field at constant stress factor" (D. ll) of meters per volt. One half of a cylinder one half inch in diameter and one half long N as used. The wall thickness was one-thirty second inch thick.

The orifice plate has eight orifices therein.

A plate with as many orifices as desired may be used and the'holes may be placed close together so long as the ink droplets in flight do not interfere one with the other. The plate may be, for example, of stainless steel.

In Figure 12 is a plan view of the assembled gun showing the base 211 having the crystal 212 imbedded therein and held in place by the epoxy 213. Tubes 214 and 215 are used to supply ink to the gun and for purging the cavity. Gasket 221 resides on the top of the base 211 and across the ends of cylinder 212 sealing the ink chamber along the edges thereof.

In operation, ink under pressure flows through the tubes 214 and the orifice 219.

Initially tube 215 is opened to purge any solvent residue which may be present in the ink cavity. Thereafter, tube 215 is closed off and ink exists only through the nozzle orifice 219. Voltage pulses are applied to the leads 222 and 223 (not shown in Figure 12) to cause crystal 212 to expand and contract between the electrodes. The ink within the reservoir thereby is pressure modulated.

The pressure wave is transmitted through the nozzles 219 down the ink stream, causing the ink stream to break into droplets at the rate modulated.

The physical dimensions of the crystal are smaller than one-half wavelength of the shortest standing acoustical wave that is produced at the highest drop rates. The epoxy potting and the teflon carrier damp and attenuate unwanted resonances and reflections which cause changes in efficiency with changes in frequency. Impedance of the crystal plotted against frequency is illu- strated in Figure 14 for the crystal when tested alone. Figure 15 is a plot of the crystal impedance when the crystal is damped and attenuated by plotting it in epoxy in the tenon base. A wide bandwith of drop rate frequencies is thereby provided which is far enough below and undamped mechanical resonance frequency of the modulator to be effectively isolated from the effects of both the resonant frequency and the harmonic thereof which may be excited during a printing operation. As a result, the ink droplet breakoff distance is substantially uniform over the operating range and the tolerance of the modulator to drop rate and temperature, is improved.

To illustrate the use of the gun, a typical system is illustrated in Figure 13. In this system the crystal drive, which may be variable over the frequency operating range, drives the piezoelectric crystal. Ink intro duced through 14 exits through the orifice and is directed through a charge assembly which causes each droplet to take on an electric charge. The droplets may be individually charged by charge amplifiers, there being one charge amplifier per stream. Alter natively it will be possible to charge the droplets in all streams simultaneously with a single charge assembly. After passing through the charge assembly, the droplets go between deflection plates and to a catcher.

When no document is in front of the ink gun, the drops are normally directed into the catcher where the ink drains back into an ink reservoir and pump assembly so it can be recycled through the system. Whenever a document is to be printed, the ink droplets are electrically charged and the deflection plates cause the ink droplets to deflect out of the catcher and onto the document. To compensate for variations in fluid parameters a phase control is provided which detects the charge on the catcher due to the charged ink droplets thereon.

A phase control in conjunction with control electronics controls the crystal drive and the charge which is applied to each of the droplets. A control system such as illustrated here is disclosed into the catcher, the phase control system would have to sequentially detect the phase of each ink stream in order to provide compensation for each ink stream independent of the other streams.

Claims (19)

WHAT WE CLAIM IS :-

1. A liquid droplet generator comprising a piezoelectric transducer including a piezoelectric crystal of hemispherical or hemicylindrical construction with electrodes attached to its convex and concave surfaces, means for feeding liquid to at least one exit orifice arranged on a focus of the hemisphere or hemicylinder of the crystal, the exit orifice or orifices being either acoustically coupled to the crystal by a solid filling or being capable of being acoustically coupled to the crystal by a filling of the liquid when the generator is used.

2. A generator according to claim 1 including a plate extending across the circular or longitudinal edges of the respective hemispherical or hemicylindrical crystal so as to enclose a cavity bounded by the concave surface of the crystal whereby, when a liquid filling is used, the liquid occupies said cavity for acoustically coupling the crystal to the orifice or orifices.

3. A generator according to claim 2 in which the crystal, which is hemispherical, is provided with a nozzle as a single orifice at its focus.

4. A generator according to claim 2 or 3 wherein said plate incorporates a liquid feed bore for feeding said liquid to said cavity.

5. A generator according to claim 2 in which the crystal, which is hemicylindrical, is provided with a plurality of orifices arranged on a central and longitudinal focal axis.

6. A generator according to claim 1 in which the liquid feeding means comprises a tube passing through an aperture in the crystal, said filling being solid and supporting the tube with respect to the concave surface of the crystal.

7. A generator according to claim 6 wherein said filling is made of epoxy material.

8. A generator according to claim 4 including a second piezoelectric transducer of a similar construction to the first-mentioned piezoelectric transducer and arranged serially therewith for feeding liquid to said first transducer.

9. A generator according to claim 8 wherein the hemispherical crystals of the first and second transducers have aligned aperturfs therethrough, the aperture in the crystal of the second transducer being connected to a liquid feed tube having a flow restrictor which co-operates with the crystal of the second transducer to provide a liquid diode action for preventing a pressure drop within the crystal of the first transducer to enable the production of single drops of liquid at the exit orifice of the first transducer.

10. A generator according to claim 3 in which the liquid feeding means comprises a tube connected, via a valvular arrangement for providing a series of feedback channels to an aperture passing through the crystal.

11. A generator according to any one of the preceding claims in which the crystal or crystals have a mechanical Q less than 70, and a strain per field at constant stress greater than 550 x 10-'2 metres per volt.

12. A liquid droplet generator substantially as herein described with reference to Figs. I and 2 of the accompanying drawings

13. A liquid droplet generator substantially as herein described with reference to Fig. 5 of the accompanying drawings.

14. A liquid droplet generator substantially as herein described with reference to Fig. 9 of the accompanying drawings.

15. A liquid droplet generator substantially as herein described with reference to Fig. 10 of the accompanying drawings.

I6. A multi-orifice gun substantially as herein described with reference to Fig. 11 of the accompanying drawings.

17. An ink jet printing system including the liquid droplet generator according to any one of the preceding claims wherein said liquid is ink.

18. A system according to claim 17 in cluding means for establishing an acoustic standing wave having a half wave length which is less than the width of the dimension of the crystal measured either circum Ferentially or longitudinally.

19. A system according to claim 17 or 18 and substantially as herein described with reference to Fig. 13 of the accompanying drawings.