Apparatus And Method For Generation Of Drops Using Bending Waves

Abstract

Apparatus and method for generating drops in a continuous falling curtain by controlled stimulation of a set of fluid streams. The streams are formed by forcing a working fluid under pressure through a set of orifices in an orifice plate, and are stimulated to produce drops by propagating a series of bending waves down the length of the plate. It is shown that this method of stimulation provides regulation of the phase and amplitude of applied stimulation energy and accurately controls the filament length of all streams. There is also disclosed an improved jet drop recording apparatus wherein graphic printing quality is greatly improved by travelling wave stimulation of a set of digitally switched jets.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to copending patent applications TWIN ROW DROP GENERATOR, Ser. No. 189,298, now U.S. Pat. No. 3,701,998 and the present invention.

BACKGROUND OF THE INVENTION

This invention relates generally to the field of fluid drop generation and the application thereof to jet drop recorders of the type shown in Sweet et al U. S. Pat. No. 3,373,437 and Taylor et al U. S. Pat. No. 3,560,641. In recorders of this type there are one or more rows of orifices which receive an electrically conductive recording fluid, such as for instance a water base ink, from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. These recorders accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and thereafter depositing at least some of the drops on a moving web of paper or other material.

The above mentioned charging is accomplished by application of control signals to charging electrodes positioned near each of the streams. As each drop breaks off from its parent fluid filament, it carries with it a charge which is in effect a sample of the voltage present on the associated charge electrode at the instant of drop separation. Thereafter the drop passes through an electrostatic field and is deflected in the field direction a distance which is proportional to the magnitude of the drop charge. In a preferred embodiment the drops are charged binarily for print-no-print operation; some drops being uncharged and undeflected for printing, and all other drops being charged to a fixed level and deflected into a catcher.

In order to accomplish reproduction with recorders of the above described type it is necessary to control drop formation with a great deal of precision. Left to natural stimulating disturbances, the streams would break up erratically into drops of various sizes at irregular intervals to produce a recording which at best would be a poor sample of the input control voltages. Accordingly it is customary to apply a fixed frequency, constant magnitude stimulating disturbance to all of the fluid streams. This results in trains of uniformly sized and regularly spaced drops and enables reasonably good sampled data recording.

Various types of magnetostrictive and piezoelectric transducers have been proposed for fluid steam stimulation, and for multiple channel operation the transducer may be coupled to the structure of the fluid manifold as shown in the above mentioned Sweet et al. patent or to a fluid supply line as shown in Taylor et al. Unfortunately these prior art systems stimulate drop formation in a phase which varies uncontrollably and unpredictably from stream to stream. This causes a timing uncertainty which may be approximately plus or minus one half of a drop repetition period in the drop charging process and a noticeable drop positional placement error equal to the paper movement distance during that period.

There is a second and more serious difficulty with the above mentioned prior art stimulation systems. This is an acoustical cancellation and reinforcement phenomenon which causes unpredictable stream-to-stream variation in stimulation energy amplitude. Such variations do not affect the size or spacing of the drops, but they markedly vary the lengths of the continuous fluid filaments which supply liquid for the drops. This difference in length may be as much as plus or minus 3 times the drop spacing distance. In high speed photographs the filament-to-filament length difference presents itself as a sort of cusping pattern.

In order to induce a proper charge in the tip of a filament it is necessary that there be some charge electrode surface in the vicinity of the drop breakoff point. Thus it can be seen that the above mentioned filament length variations result in a requirement for a very long electrode; something which is difficult to implement in tightly packed arrays of the type here concerned. Moreover these length variations produce a relatively large drop positional placement error.

This error arises from channel to channel differences in drop flight time; that is, the elapsed time from drop break-off/charging to impact on the moving web of paper. It is somewhat analogous to a gunnery problem wherein a projectile must be aimed to hit a moving target. Here each drop is programmed to hit the paper at a precise position relative to other drops, and if it must fall a greater or lesser distance than had been anticipated, it will miss. With a web speed adjusted for slight overlap of adjacent printed dots, the above mentioned length difference of plus or minus 3 times the drop spacing distance will produce a printing error in the direction of web movement of plus or minus about three printed dot diameters. Such an error is unacceptably large for printing of graphic arts quality.

SUMMARY OF THE INVENTION

It is an object of this invention to improve the recording quality of the above mentioned prior art fluid drop recorders. This object is accomplished by combining a laminated print head of the type generally disclosed in Beam et al. U. S. Pat. No. 3,586,907 with an ultrasonic transducer in a manner whereby drop stimulating vibrations are generated in a continuing series of travelling waves. More particularly the jet forming fluid is caused to flow through a row of orifices in a plate sealed against a fluid supply manifold and concomitantly to be subjected to the drop stimulating action of bending waves travelling longitudinally down the length of the plate. At the same time extraneous drop stimulating disturbances are surpressed by terminating the orifice plate in a manner precluding reflection and repropagation of the waves.

In accordance with the practice of this invention it will be seen that each jet will be excited by a drop stimulating disturbance each time a bending wave passes the jet forming orifice. The time interval (phase delay) between successive drop releases in adjacent jets will be seen to depend upon the orifice spacing and the bending wave propagation speed. Bending waves for this purpose may be generated by placing the ultrasonic transducer in direct contact with the orifice plate; preferably at one end thereof. Bending wave reflection is preferably suppressed by use of absorbing elements at each end of the orifice plate.

A second object of the invention is to simplify charge electrode design problems in multiple channel jet drop coating devices by controlling the mean length of the fluid filaments and minimizing jet-to-jet variation thereof.

Another object of the invention is to improve drop stimulation in a row of cooperating fluid streams by regulating the phase and amplitude of the stimulating energy applied to each stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.. 1 is an exploded perspective view of a recording head assembly;

FIG. 1A is a perspective view of a supply manifold with portions broken away;

FIG. 2 is a cross sectional through the assembly of FIG. 1;

FIG. 3 is a perspective of an orifice plate and attached dampers;

FIGS. 4A and 4B illustrate graphically bending waves which may be induced in the orifice plate;

FIG. 5A is a diagrammatic representation of drop generation in accordance with the prior art; and

FIG. 5B illustrates drop generation in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of this invention is illustrated in exploded pictorial form in FIG. 1 together with other elements comprising a complete multiple channel recording head assembly 10. As shown in the figure, the various elements of the head are assembled for support by a support bar 12. Assembly thereto is accomplished by attaching the elements by means of machine screws (not shown) to a clamp bar 14 which is in turn connected to the support bar 12 by means of clamp rods 16.

The recording head comprises an orifice plate 18 soldered, welded or otherwise bonded to fluid supply manifold 20 with a pair of wedge-shaped acoustical dampers 22 therebetween. Orifice plate 18 is preferably formed of a relatively stiff material such as stainless steel or nickel coated beryllium-copper but is relatively thin to provide the required flexibility. Preferably dampers 22 are cast in place by pouring polyurethane rubber or other suitable damping material through openings 24 while tilting manifold 20 (orifice plate 18 being attached) at an appropriate angle from the from the vertical. This is a two step operation as dampers 22 require tilting in opposite directions (See FIG. 1A).

Orifice plate 18 preferably contains two rows of orifices 26 and is simulated by stimulator 28 which is threaded into clamp bar 14 to carry stimulation probe 30 through manifold 20 and into direct contact with plate 18. Orifice plate 18, manifold 20, and clamp bar 14 together with a filter plate 32 and O rings 34, 36 and 38 (see also FIG. 2) comprise a clean package which may be preassembled and kept closed to prevent dirt or foreign material from reaching and clogging orifices 26. Conduit 40 may be provided for flushing of the clean package. Service connections for the recording head include a coating fluid supply tube 42, air exhaust and inlet tubes 44 and 46, and a tube 48 for connection to a pressure transducer (not shown).

Other major elements comprising the recording head are a charge ring plate 50, an electrically conductive deflection ribbon 52, and a pair of catchers 54. Catchers 54 are supported by holders 56 which are fastened directly to fluid supply manifold 20. Spacers 58 and 60 reach through apertures 62 and 64, respectively, in charge ring plate 50 to support holders 56 without stressing or constraining charge ring plate 50. Deflection ribbon 52 is also supported by holders 56 and is stretched tightly therebetween by means of tightening block 66. Ribbon 52 extends between catchers 54 as best shown in FIG. 2.

Catchers 54 are laterally adjustable relative to ribbon 52. This adjustability is accomplished by assembling the head with catchers 54 resting in slots 68 of holders 21, and urging them mutually inward with a pair of elastic bands 70. Adjusting blocks 72 are inserted upwardly through recesses 74 and 76 to bear against faces 78 of catchers 54, and adjusting screws 80 are provided to drive adjusting blocks 72 and catchers 54 outwardly against elastic bands 70. Holders 56 are made of insulative material which may be any available reinforced plastic board.

The fully assembled recording head is shown in cross section in FIG. 2. As therein illustrated coating fluid 82 flows downwardly through orifices 26 forming two rows of streams which break up into drops 84. Drops 84 then pass through two rows of charge rings 86 in charge ring plate 50 and thence into one of the catchers 54 or onto the moving web of paper 88. Switching of drops between "catch" and "deposit" trajectories is accomplished by electrostatic charging and deflection as hereinafter described. Coordinated printing capability is achieved by staggering the two rows of streams in accordance with the teachings of Taylor et al. U. S. Pat. No. 3,560,640. As taught in that patent, the drops in the forward row of streams (i.e. the row most advanced in the direction of web movement) are switched in a time reference frame delayed from that of the rear row by a time d/V where d is the row spacing and V is the web speed. This produces a coherence such that the two rows of streams function as a single row with an effective stream spacing equal to half the actual spacing in either of the real rows.

Formation of drops 84 is closely controlled by application of a constant frequency, controlled amplitude, stimulating disturbance to each of the fluid streams emanating from orifice plate 18. Disturbances for this purpose are set up by operating transducer 28 to vibrate probe 30 at constant amplitude and frequency against plate 18. This causes a continuing series of bending waves to travel the length of plate 18; each wave producing a drop stimulating disturbance each time it passes one of the orifices 26. Dampers 22 prevent reflection and repropagation of these waves. Accordingly each stream comprises an unbroken fluid filament and a series of uniformly sized and regularly spaced drops all in accordance with the well known Rayleigh jet break-up phenomenon.

As each drop 84 is formed it is exposed to the charging influence of one of the charge rings 86. If the drop is to be deflected and caught, an electrical charge is applied to the associated charge ring 86 during the instant of drop formation. This causes an electrical charge to be induced in the tip of the fluid filament and carried away by the drop. A static electrical field is set up between deflection ribbon 52 and the faces of each of the catchers 54 (by opposite polarity electrical charging thereof), and when the drop traverses this field it is deflected to strike the face of the appropriate catcher. Thereafter the drop runs down the face of the catcher, is ingested, and carried off. Drop ingestion may be promoted by application of a suitable vacuum to the ends 90 of catchers 54. For drops which are to deposit on the web 88, no electrical charge is applied to the associated charge ring.

Appropriate charges for accomplishment of the above mentioned drop charging are induced by setting up an electrical potential difference between orifice plate 18 (or any other conductive structure in electrical contact with the coating fluid supply) and each appropriate charge ring 86. These potential differences are created by grounding plate 18 and applying appropriately timed voltage pulses to wires 92 in connectors 94 (only one connector illustrated). Connectors 94 are plugged into receptacles 96 at the edge of charge ring plate 50 and deliver the mentioned voltage pulses over printed circuit lines 98 to charge rings 86. Charge ring plate 50 is fabricated from insulative material and charge rings 86 are merely coatings of conductive material lining the surfaces of orifices in the charge ring plate. Voltage pulses for the above purpose may be generated by circuits of the type disclosed in Taylor et al, and wires 92 receiving these pulses may be matched with charge rings 86 on a one-to-one basis. Alternatively the voltage pulses may be multiplexed to decrease the number of wires and connectors. For such an alternative embodiment solid state demultiplexing circuits may be employed to demultiplex the signals and route the pulses to the proper charge rings. Such solid state circuits may be manufactured by known methods as a permanent part of charge ring plate 50.

The printing head as above described is adapted to be employed in combination with another such head further in accordance with the teachings of Taylor et al. Such a combination will produce solid printing coverage with the streams in each row on 16 mil centers, which is within the state of the art for current orifice plate and charge ring plate manufacturing techniques. The effective stream spacing for the equivalent single row is 4 mils, and this will produce solid printing coverage if each drop makes a printed dot in the order of about 5 mils. Suitable drops for such printed dots may be produced with 1.5 mil orifices, an fluid pressure of about 11 p.s.i. and a stimulation frequency of about 60 KHz. To achieve similar solid coverage in the direction of web travel the speed of web 88 should be set at about 1200 ft. per sec.

Unexpectedly it has been found that solid printing coverage may be obtained by operating a single printing head as above described but at a reduced web travel speed. In particular, a web speed of about 450 ft. per sec. has been found to be satisfactory. This reduction in web speed results in a decreased longitudinal (i.e. web movement direction) spacing of drop deposit points. In fact when two consecutive drops in one stream are both deposited they tend to pile up and spread in all directions. They behave much like one drop of larger volume, and they fill the laterally adjacent marking cell left empty by omission of the second recording head. This, of course, degrades the resolution of the resulting "print", but a recording head has been saved. For operation in such a mode it is necessary to slow down the rate of the input drop switching data for avoidance of dimensional scaling distortion in the longitudinal direction. Thus a signal which would cause catching of (or permit deposition of) one drop in the faster two head system is stretched to catch on the average about 2.7 drops in the single head system. Catching or deposition of a single drop is not meaningful for the above mentioned single head recorder unless it is desired to make gray scale reproductions as taught for instance in Sweet it al. U.S. Pat. No. 3,373,437.

The travelling waves which are a central feature of this invention are illustrated pictorially in FIG. 3. As shown therein the waves 100 originate in an area 102 of orifice plate 18, passing orifices 26 as they travel. Area 102 receives vibrations for generation of the waves by physical contact with the probe 30 of stimulation transducer 28. Orifice plate 18 is bonded to fluid supply manifold 20 in the region of the shaded area 104. Reflection and repropagation of waves 100 are prevented as mentioned above by use of acoustical dampers 22.

It will be appreciated that this invention reduces the stimulation problem to a matter of acoustical waveguide design. Accordingly the dimensions for orifice plate 18 and acoustical dampers 22 may be established by solution of classical wave equations. In this regard it should be observed that the most significant calculation involved is that of the resonance frequencies in the orifice plate widthwise direction. Application of this technique to surface wave generation is discussed in Seidel et at U. S. Pat. No. 3,488,662. For a rigorous treatment of the mathematics and the general structural mechanics involved, reference may be made to Vol. 3 Chapter 11 of Electromechanical Dynamics by Herbert H. Woodson and James R. Melcher, John Wiley & Sons, 1968.

FIGS. 4A and 4B illustrate the two lowest widthwise resonance modes for orifice plates which are constrained respectively by pinning and clamping at their edges. The figures represent cross sections across the width of the orifice plates. In each case the first order resonance mode is represented by the Roman numeral I, and the second order resonance mode is represented by the Roman numeral II. For a given unsupported or resonating width there is a minimum stimulation frequency below which each of the illustrated resonance modes may not be excited. For satisfactory stimulation in accordance with the practice of this invention it is desirable to excite mode I for propogation down the length of the plate, but to avoid excitation of mode II. Orifice plate 18 as illustrated in FIG. 1 has clamped edges, and has been satisfactorily operated at 60 KHz stimulation with a thickness of 10 mils. a length of 5 inches, and an effective resonating width of 0.25 inches.

FIG. 5B illustrates typical observed results of tests conducted upon clamped orifice plates stimulated as above described at 60 KHz. The acoustical dampers used for these tests were about 1 inch long. Corresponding results for stimulation by prior art methods are illustrated in FIG. 5A. In each case the illustrations show a section of the orifice plate as it may be observed under magnification and stroboscopic illumination. Each stream of FIGS. 5A and 5B comprises a series of spaced drops 84 and an unbroken fluid filament 106. Each filament 106 has some nominal length and varies cyclically about that length each time a drop is formed. During drop formation that filament lenghtens, pinches off to form a drop, and then shortens a distance about equal to the spacing between downstream drops.

For stimulation by prior art methods, it will be observed that different streams exhibit a marked difference in nominal filament length. This results in a cusping pattern as illustrated in FIG. 5A with the shortest filament being about 6 drop separation distances shorter than the longest filaments. In contrast thereto the streams stimulated in accordance with the practice of this invention have a nearly uniform nominal filament length; the only significant variation being a slightly longer length for filaments displaced along the plate in the direction of bending wave travel. This lengthening is the predictable result of bending wave attenuation during propagation, and the relatively small printing errors associated therewith may be corrected by introducing fixed time delays in the charge ring circuits. The charge ring plate may also be inclined slightly to maintain the charge rings in positional alignment with the fluid filament tips. Spacers 58 and 60 facilitate inclination of the charge ring plate without distorting the plate or disturbing other head elements.

The significance of uniform filament lengths is best understood by reference again to the prior art stimulation of FIG. 5A wherein two charging signals 108a and 108b are simultaneously fed to lines 92a and 92b. The charge rings thereupon charge filaments 106a and 106b for charging of drops then in the act of breaking off. This results in a sizeable vertical separation between drops which are correlated in time. Thus drops 84a and 84b are correlated in time, but drop 84a will strike the moving web 88 six drop repetition periods later than drop 84b. Movement of web 88 during that period of time will necessarily produce a large printing error. Additionally, as shown, charge rings 86 must be quite long to accommodate the large prior art variation in filament length. These difficulties are all avoided by the practice of this invention.

While the method and form of apparatus herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to this precise method and form of apparatus, and that changes may be made therein without departing from the scope of the invention.

Claims

What is claimed is:

1. Method of generating drops comprising the steps of producing a set of fluid filaments by forcing a fluid simultaneously through a set of orifices spaced along a flexible plate, and breaking said filaments up into drops by generating a series of drop stimulating bending waves and guiding said waves unidirectionally along said plate on a path joining said orifices.

2. Method according to claim 1, said bending waves being generated at constant frequency.

3. Method according to claim 2, said bending waves all being of the same initial amplitude.

4. Apparatus for generating a curtain of falling drops comprising:

1. a flexible orifice plate provided with a plurality of spaced orifices,

2. a fluid supply manifold sealed against said orifice plate and communicating with said orifices,

3. means for pressurized delivery of a working fluid to said orifices, the pressure so applied being of sufficient magnitude to force said fluid to flow through said orifices and form a corresponding number of parallel streams at the exit sides thereof,

4. means for generating a series of drop generating disturbances at a point on said plate, and

5. confining means for causing said disturbances to propagate as travelling waves from orifice to orifice along said plate without reflection and repropagation thereof.

5. Apparatus according to claim 4, said confining means comprising means for sealing the fluid supply manifold rigidly against the orifice plate and creating a clamped joint therebetween.

6. Apparatus according to claim 5 said orifices being uniformly sized and uniformly spaced along a straight line.

7. Apparatus according to claim 6 said confining means further comprising acoustical damping means at both ends of the orifice plate.

8. Apparatus according to claim 7 said acoustical damping means being of wedge-shaped configuration, and said fluid supply manifold being provided with a pair of apertures for casting said damping means in place.

9. In a jet drop recording apparatus comprising drop forming means for forming a plurality of streams of uniformly sized and regularly spaced drops, a common pressurized fluid supply manifold for supplying coating fluid to all of said streams, and means for selectively deflecting drops within each of the streams; the improvement wherein said drop forming means comprises:

1. an orifice plate sealed against the supply manifold and provided with a plurality of orifices arranged at regular intervals along a straight line,

2. means for launching regularly timed bending waves along the orifice plate and following said line, and

3. damping means secured to the orifice plate at the end of said line for absorbing said bending waves and suppressing backwardly directed reflection thereof.

10. Jet drop recording apparatus comprising:

1. a flexible orifice plate provided with a plurality of uniformly sized and spaced orifices arranged along a straight line,

2. a fluid supply manifold rigidly sealed against said orifice plate and communicating with said orifices,

3. means for pressurized delivery of a working fluid to said orifices, the pressure so supplied being of sufficient magnitude to force said fluid to flow through said orifices and form a corresponding number of parallel streams at the exit sides thereof,

4. means for inducing periodic localized bending in said orifice plate and propagating bending waves down the length of said plate to stimulate streams to break up into drops,

5. an electrically nonconductive charge ring plate positioned across the path of said streams and provided with a set of charge ring apertures for passage of the streams therethrough; said charge ring apertures being coated with electrically conductive material for selective charging of drops created as aforesaid,

6. charge control means in separate electrical communication with the coatings in said charge ring apertures,

7. means for maintaining a steady electrical field for deflection of drops created and charged as aforesaid,

8. means for catching of drops created, charged, and deflected as aforesaid, and

9. means for suppressing backwardly directed reflection of said bending waves.

11. Apparatus for generating a curtain of falling drops comprising:

1. a flexible orifice plate provided with a plurality of uniformly sized orifices spaced uniformly along a straight line,

2. a fluid supply manifold communicating with said orifices and rigidly sealed against said orifice plate for creation of a clamped joint therebetween,

3. means for pressurized delivery of a working fluid to said orifices, the pressure so applied being of sufficient magnitude to force said fluid to flow through said orifices and form a corresponding number of parallel streams at the exit sides thereof,

4. means for propagating bending waves down the length of said plate, whereby said streams may be stimulated to break up into drops by passage of bending waves successively past said orifices, and

5. acoustical damping means at both ends of the orifice plate for absorbing said bending waves and suppressing back-wardly directed reflection thereof.

12. Apparatus according to claim 11 said acoustical damping means being of wedge-shaped configuration, and said fluid supply manifold being provided with a pair of apertures for casting said damping means in place.