US3882508A

US3882508A - Stimulation apparatus for a jet drop recorder

Info

Publication number: US3882508A
Application number: US491154A
Authority: US
Inventors: Leonard G Stoneburner
Original assignee: Mead Corp
Current assignee: Eastman Kodak Co
Priority date: 1974-07-22
Filing date: 1974-07-22
Publication date: 1975-05-06
Anticipated expiration: 1992-05-06
Also published as: CA1008126A; IT1046086B; FR2279554A1; FR2279554B1; GB1510887A; DE2532796A1; DE2532796C2; BR7504438A; JPS5135232A; HK81479A; JPS5444455B2

Abstract

A jet drop recording head having an extended length orifice plate is stimulated by an improved travelling wave technique. In accordance with this invention the head is constructed in such a way as to taper or progressively decrease the active width of the orifice plate. This tapering counteracts or reduces the natural tendency toward attenuation of drop stimulating bending waves which are caused to travel down the length of the orifice plate. As a result of the reduction in bending wave attenuation, more uniform filament lengths are achieved and satellite drop generation is greatly reduced.

Description

United States Patent [1 1 Stoneburner STIMULATION APPARATUS FOR A JET DROP RECORDER [75] Inventor: Leonard G. Stoneburner,

Chillicothe, Ohio [73] Assignee: The Mead Corporation, Dayton,

Ohio

[22] Filed: July 22, 1974 [21] Appl. No.: 491,154

[52] US. Cl. 346/75; 118/315; 239/102 [51] Int. Cl. 601d 15/18 [58] Field of Search 346/75; 239/102, 4;

[56] References Cited UNITED STATES PATENTS Lyon et al 346/75 X [111 3,882,508 I451 May 6,1975

Primary Examiner-Joseph W. Hartary Attorney, Agent, or Firm-Biebel, French & Bugg ABSTRACT A jet drop recording head having an extended length orifice plate is stimulated by an improved travelling wave technique. In accordance with this invention the head is constructed in such a way as to taper or progressively decrease the active width of the orifice plate. This tapering counteracts or reduces the natural tendency toward attenuation of drop stimulating bending waves which are caused to travel down the length of the orifice plate. As a result of the reduction in bending wave attenuation, more uniform filament lengths are achieved and satellite drop generation is greatly reduced.

18 Claims, 12 Drawing Figures PATEN'EEBMAY 5:915

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FIG-H PP -w FIG-12 STIMULATION APPARATUS FOR A JET DROP RECORDER BACKGROUND OF THE INVENTION This invention relates generally to the field of jet drop recording and more particularly to jet drop recorders of the type shown in Sweet et al. U.S. Pat. No. 3,373,437 and in 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 one or more 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 drops which are not deposited on the moving web are caught by an appropriately positioned catcher. Drop charging, deflection, and catching are all accomplished as describedin the above mentioned Sweet et al. and Taylor et al. patents, or in Brady et al. U.S. Pat. No. 3,805,273 which illustrates a preferred embodiment of the apparatus to which this invention applies.

One of the most difficult problems encountered in the operation of jet drop recorders of the above mentioned type is that of drop stimulation. For high quality recording it is necessary that all jets be stimulated at the same frequency to cause break up of the streams into uniformly sized and regularly spaced drops. Furthermore it is necessarythat drop generation not be accompanied by generation of satellite drops and that the break up of the streams into drops occur at a predetermined location in proximity to a charging electrode. The general concept of stimulating a jet stream to break up into drops is well known in the art and is discussed in detail in the above mentioned prior art patents. Satellite drop creation which can accompany the stimulation of a jet is described in detail in Stauffer U.S. Pat. No. 3,334,351 and in Keur et al. U.S. Pat. No. 3,683,396, which deal with jet drop recorders of the single orifice type.

Early attempts at stimulating array type jet drop recorders produced unsatisfactory results due to the generation of unpredictable vibrational modes within the printing head. This problem is described in some detail in Lyon et al. U.S. Pat. No. 3,739,393, which mentions a cusping pattern phenomenon that is characteristic of unsatisfactory stimulation. The cusps which comprise such a pattern are observed by looking at a row of stimulated jets and noting the locations at which the jet filaments break up into streams of drops. In accordance with the Lyon et al. invention the jets are stimulated by a travelling wave technique wherein a continuous series of bending waves are caused to propagate along the orifice plate. Each bending wave transits once past each orifice and in so doing imparts a drop stimulating disturbance to the jet associated therewith. In general these waves must be generated at a frequency near the natural frequency of the jets, and each wave, as viewed widthwise across the orifice plate, should constitute a half wave at the first order bending mode. If such waves are generated and propagated as taught by Lyon et al., then cusping patterns are eliminated, and high quality stimulation is achieved.

One problem that exists with travelling wave stimulation as taught by Lyon et al. is that the bending waves are attenuated as they travel along the length of the orifice plate. This in turn causes a progressive lengthening of the jet filaments, so that filaments at the far end of the orifice plate may be considerably longer than those which are near the source of bending wave origin. For a typical orifice plate which is 13 cm long and which has its stimulation amplitude properly adjusted, the mean filament length will increase progressively from about 0.63 mm to about 1.25 mm. Lyon et al. generally mentions such progressive lengthening and teaches a tilting of the charge ring plate in order to maintain uniform charging with relatively thin charge rings.

It has now been found that there is a critical filament length range, and that when the mean length of any filament falls outside this range, then the jet begins generating large numbers of satellite drops. It is not known precisely why an excursion of the fluid filament length outside this critical range is accompanied by generation of satellite drops, but it has been found that for a typical array type jet drop recorder having an orifice diameter of about 47 microns and a manifold pressure of about 690,000 to 835,000 dynes/cm that the maximum filament length is about 1.25 mm and that the minimum filament length is about 0.63 mm. This filament length range is the same as the above mentioned range for a typical 13 cm orifice plate stimulated in accordance with the Lyon et al. teaching. Thus the typical system considered above has been satisfactorily stimulated only by the Lyon et al. travelling wave technique, and when so stimulated the orifice plate has been practically limited to a maximum length of about 13 cm.

In order to achieve even the above mentioned 13 cm length it has been necessary to provide adjustable power supplies for the stimulation transducers. Generally speaking this permits operation of the transducer at a power level wherein all filaments are within the critical range of between about 0.63 and 1.25 mm. In the case wherein the orifice plates are excessively long, however, the filaments at the far end of the plate (i.e. the end remote from the stimulator) tend to be longer than the critical maximum and can be shortened up sufficiently only by application of fairly large stimulation power. When this is done all filaments along the plate shorten up therewith, so that the filaments at the near end become shorter than the critical minimum. As a practical matter, therefore, it has not been possible to adjust the power level of the applied stimulation so as to avoid generation of satellite drops along the entire length of an orifice plate having an active area more than about 13 cm long. This is only a very general maximum working length, and for any particular system the actual maximum will depend upon a number of factors including the width of the active area, the size and number of orifices, the thickness and stiffness of the orifice plate, the stimulation frequency, the density and viscosity of the recording liquid, and the pressure at which the recording liquid is supplied.

SUMMARY OF THE INVENTION This invention provides means for stimulating an array type jet drop recording head of increased length without generation of satellite drops. Stimulation of such a recording read is accomplished by an improved travelling wave stimulation technique wherein the active width of the orifice plate is progressively narrowed or tapered along the length thereof. Stimulation energy is applied at the wide end of the orifice plate, and the bending waves which result are guided along the length of the orifice plate as taught by Lyon et al. U.S. Pat. No. 3,739,393. However, the progressive narrowing of the orifice plate has been found to counteract the natural tendency toward attenuation of the bending waves.

In accordance with this invention the active width of the orifice plate may be progressivelynarrowed so as to maintain liquid filaments of nearly uniform length at all points along the orifice plate, or may be progressively narrowed at a somewhat reduced tapering rate for merely reducing the amount by which the liquid filaments lengthen. This latter, less severe, tapering approach is of considerable significance, because as described in detail in the specification below, there are practical maximum and minimum widths for the active area of the orifice plate. Thus an orifice plate width reduction which produces uniform filament lengths in a recording head having an orifice plate of 13 cm active length, will, if applied to a plate of 26 cm active length, so reduce filament length excursion as to produce operating conditions similar to those experienced by a head having a 13 cm plate with no taper. As discussed above, such a 13 cm recording head with a non tapered orifice plate has been the longest previously practical head. That maximum length of 13 cm may be doubled by the application of this invention.

It will therefore be seen that this invention applies to jet drop recorders of the type which employ travelling wave stimulation for drop generation and that it is a principal object of this invention to reduce the attenuation of the travelling bending waves which characterize such recorders.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional drawing of a print head configured for use of the invention herein disclosed and claimed;

FIG. 2 is an enlarged view taken along lines 22 of FIG. 1;

FIG. 3 is a view looking downwardly on an orifice plate holder with an orifice plate attached thereto;

third order widthwise resonance modes for an orifice DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the invention may be incorporated into a recording head 20 as illustrated in FIG. 1. With the exception of a tapered wall configuration to be described in detail below, recording head 20 is structurally configured in the same manner as the recording head which is described in detail in Brady et al. U.S. Pat. No. 3,805,273. Accordingly recording head 20 will be described herein only in brief detail, the disclosure of the Brady et al. patent being herein incorporated by reference thereto.

For an understanding of the invention herein disclosed, it may be noted that recording head 20 includes a stimulator 21, a recording liquid supply tube 28 and a recording liquid return tube 29. While within recording head 20, the recording liquid is maintained within plate holder 22 to close out the bottom of the liquid pool, but recording liquid escapes therefrom via a series of orifices 31 in orifice plate 23. After passage through orifices 31 the recording liquid forms a series of liquid filaments 32, as illustrated in FIGS. 6 and 10, and these filaments in turn break up into a series of drops 33, under the stimulating control of a series of bending waves which are initiated by stimulator 21.

Filaments 32 extend downwardly through a series of charge rings 34 in a charge ring plate 24 for selective charging as described in detail in Mathis U.S. Pat. No. 3,701,998 and in Taylor et al. U.S. Pat. No. 3,560,641. Other important elements of recording head 20 are a deflection strip 26 and a pair of catchers 25, which provide a twin row operation as described in detail in the Mathis patent. Very briefly, certain of drops 33 are selectively charged by charge rings 34 for delfection by electrostatic deflection fields set up by the deflection strip 26 and the faces of catchers 25. The drops which are so charged and deflected are caught by catchers 25, while uncharged drops deposit upon a continuously moving web 27.

Stimulation of filaments 32 is produced as taught by Lyon et al. U.S. Pat. No. 3,739,393, which stimulation method is hereinafter referred to simply as travelling wave stimulation. For effecting travelling wave stimulation, transducer 21 has a tip 35 which contacts orifice plate 23 to set up a series of bending waves which travel along a channel 36 (FIG. 4). Channel 36 is merely the portion of orifice plate 23 which is not bonded to orifice plate holder 22 and may be thought of as an acoustical wave guide. A pair of acoustical dampers 37 prevent reflection of the bending waves at either end of the orifice plate. Reference may be made to the Lyon et al patent for a description of the operation of travelling wave stimulation. FIG. 5 generally illustrates the stimulation geometry, with the stippled region 40 of orifice plate 23 representing the area of bonding to the lower surface of orifice plate holder 22.

As discribed in the Lyon et al. patent, the bending waves travel along orifice plate 23 in the direction of arrow 38. In the course of their travel, the waves tend to become attenuated, thereby causing a lengthening of filaments 32. This lengthening of filaments 32 causes to reach down to positions which may roughly reach the slanted line A-C of FIG. 6. In accordance with this invention, the inner walls 39 are tapered from a width as shown by the arrows F of FIG. 3 down to a somewhat narrower width as shown by the arrows G. This narrowing or tapering of walls 39 produces a tapering of channel 36, which has been found to counteract the naturally occurring attenuation of the bending waves. As a result thereof, filaments 32 may be caused to have a relatively uniform mean length, thus all reaching down to positions along the line A--B of FIG. 6.

The amount of wave guide tapering which is required in any case for production of filaments of uniform mean length is a function of many variables and may be established by simple experimental procedures. As as example a series of experiments were performed at a stimulation frequency of 50 KHz upon a recording head having an orifice plate active width (i.e. wave guide width) of about 5 mm. The orifice plate contained two rows of 47 micron diameter orifices having a row spacing of 1.52 mm and a center to center spacing of 0.51 mm between orifices in a row.

The orifice plate was a laminated metal structure of approximately 0.178 mm thickness. The effective ratio of density to Youngs modulus for the plate is approximately 4.5 X sec m. This ratio is higher than that of a solid metal due to the weakening effect of the orifice holes. It was found that for such an arrangement a taper of about 0.025 cm per cm of orifice plate length produced nearly uniform filaments of about 1.0 mm mean length, when the recording liquid was a water base ink under a pressure of about 690,000 to 835,000 dynes/cm and stimulation was induced at a suitable power level. For lesser amounts of taper the variation in mean length of the filaments could be decreased but could not be held uniform. Usage of such lesser amounts of taper may be important in some cases for reasons which presently will become apparent.

As taught by the Lyon et al. patent, it is desirable that the bending waves which travel down the length of orifice plate 23 be waves of the first order widthwise resonance mode as shown by the dotted line I of FIG. 7. If orifice plate 23 is sufficiently wide, then it is also possible to generate a series of waves of the second order widthwise resonance mode as shown by the dotted line II of FIG. 7. A still wider orifice plate 23 would permit generation of bending waves of the third order widthwise resonance mode as shown by dotted line III. The bending wave modes as illustrated in FIG. 7 are for a clamped edge condition. For a typical orifice plate as described and stimulated in accordance with the above example, the minimum width of channel 36 for supporting the first order widthwise resonance mode is about 0.320 cm. The corresponding minimum widths for supporting the second and first order widthwise resonance modes are respectively about 0.533 cm and 0.747 cm.

In an ideal system the spacing between walls 39 would be such as to define a wave guide channel 36 at all points wider than the minimum width required for supporting the first order widthwise resonance mode, but narrow enough to prevent the second order widthwise resonance mode. As a practical matter, however, it has been found that a channel 36 which is wide enough to support the second order widthwise resonance mode will not bend in this mode, if steps are taken either to prevent excitation of the orifice plate in this mode or to prevent any second order bending waves from reaching the active area of channel 36. If second order waves (or any higher order waves) are permitted to travel along channel 36, then filaments 32 will exhibit a cusping pattern as shown in FIG. 10. Such a cusping pattern is described in some detail in the Lyon et al. patent.

At this point it should be clear that while filament length uniformity may be obtained by progressive tapering of channel 36, there are limits to the amount of taper which may be tolerated. At one end channel 36 may not be so narrow so as to prevent excitation of orifice plate 23 in the first order widthwise resonance mode, and at the other end channel 36 may not be so wide as to enable vibration of orifice plate 23 in mode III or other even higher order modes.

Now referring to FIG. 7, there will be noted an arrow 42. Arrow 42 points to the geometrical center of orifice plate 23, which is also the node for the second order widthwise resonance mode. It has been found that the first order mode may be excited in a relatively wide orifice plate without excitation of the second order mode, if stimulation energy is applied to orifice plate 23 precisely at the center point as indicated by the arrow 42. The precise center point may be found by a tuning procedure utilizing a stimulator having an off-center point on its tip 35. In this procedure the stimulator is energized and then the tip 35 is rotated until the desired stimulation condition is achieved. The locus of contact points achieved by rotation of tip 35 is indicated by the dotted line 43 in FIGS. 4 and 5. I-Iauser U.S. Pat. No. 3,701,476 may be referred to for disclosure of a suitable stimulator having a rotatable tip with an off-center contact point.

Alternative methods of avoiding excitation of channel 36 in the second order widthwise resonance mode are illustrated in FIGS. 8 and 9. As illustrated in FIG. 8, the orifice plate may be clamped to a necked-down support as at points 44 to create a passage too narrow for the second order mode. With such an arrangement the orifice plate active area may be wider than the above mentioned 0.533 cm and the plate may be stimulated at a convenient point such as point 45. This will produce mode 11 resonance at point 45, but the necked down regions 44, act as an acoustical filter, permitting only first order bending waves to reach the active area of the orifice plate. In general for operation of this embodiment orifice plate must be wide enough in the necked-down area to support a first order mode and narrow enough in the active area to suppress excitation of mode II. Under such conditions a first order mode, as illustrated by FIG. 7 will travel down the length of the orifice plate, without creation of the second order mode, again avoiding the undesirable cusping pattern of FIG. 10.

Still another means for avoiding generation of the second order widthwise bending mode is illustrated in FIG. 9. This technique employs an area of localized thickening 46 for the orifice plate in the area of application of stimulation energy. It is believed that this 10- calized thickening causes a rather considerable increase in the width required for mode II generation. For orifice plates of the type described in the above example, a thickening of about 20 microns which runs across the orifice plate and along its length for about 1 cm will prevent excitation of the second order mode. Then, so long as the orifice plate is narrow enough to suppress the third order mode, only the mode I will propagate therealong.

If steps are taken to suppress the second order widthwise resonance mode, then for a recording head of the above described type (operated in accordance with the above set forth example), the distance F of FIGS. 3 and 4 may be as great as about 0.747 cm and the distance G may be as small as about 0.320 cm. If distances F and G are set about these values, and if the length of the orifice plate between cross sections F and G is about 26 cm, then for an applied stimulation signal having an appropriate amplitude and a frequency of about 50 KHZ, the lengths of filaments 32 will be found to range from about 0.63 mm at cross section F to about 1.25 mm at cross section G. As discussed above in the Background of the Invention, these filament lengths represent maximum and minimum conditions for satisfactory stimulation without generation of satellite drops. Thus by tapering walls 39 of orifice plate holder 22 in accordance with this invention, it has been possible to obtain satisfactory travelling wave stimulation of a recording head having an active length of about 26 cm. This is double the maximum length of 13 cm obtainable by a similarly constructed recording head employing the teachings of the prior art, and the doubling is accomplished with a taper ratio of about 0.016 cm per cm of orifice plate length.

It is not known exactly why the above mentioned tapering counteracts the natural tendency toward attenuation of the travelling waves, but it has been observed that the attenuation can be partially offset, entirely offset, or even reversed by proper selection of the taper ratio. Thus as described above, a taper ratio of 0.025

cm per cm of orifice plate length for some recording heads entirely eliminates wave guide attenuation, while I a taper ratio greater than this amount will actually increase the amplitude of the bending waves as they travel along the length of the orifice plate.

An alternative embodiment of the invention is illustrated in FIG. 11, and a recording head employing this embodiment of the invention may have an active orifice area as long as 52 cm. In this embodiment the orifice plate holder 122 is fitted with an orifice plate 123 and acoustical dampers 137. It is stimulated at a centrally located point 145 and has a double taper. For this embodiment, the active width of the orifice plate at the central cross section between arrows P-P may be about the same as the width across section F for the embodiment of FIG. 3. From cross section P the inner walls 139 of orifice plate holder 122 may be tapered down to produce a wave guide channel of a width as indicated by the arrows Q--Q at one end and by the arrows R-R at the opposite end. Each of these end widths may approximate the width at cross section G of FIG. 3.

For the alternative embodiment of FIG. 11, it is necessary to stimulate the orifice plate at an offset position I away from the two rows of orifices. If the width of the active orifice plate area at cross section P is sufficiently great to support the second order widthwise resonance mode, then the mode should be suppressed by local thickening of the orifice plate as shown at 146. This local thickening corresponds to the thickened area 46 of FIG. 9. The wave guide channel which is provided by the embodiment of FIG. 12, is illustrated in exaggerated form in FIG. 12.

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

5 jets to break up into streams of regularly formed drops,

and means for charging and deflecting said drops to create intelligible patterns therefrom; the improvement wherein the marginal attachment of said orifice plate to said manifold defines a wave guide of progressively decreasing width to counteract the naturally occurring attenuation of said waves.

2. The improvement of claim 1 wherein the interior of said manifold defines a progressively narrowing slot lengthwise of said head and said orifice plate is rigidly secured flush with the lower surface of said manifold.

3. The improvement of claim 1 wherein the wave guide so defined has a width at least wide enough at all orifice locations for supporting a first order widthwise resonance mode.

4. The improvement of claim 3 wherein said wave guide has a taper in the order of about 0.025 cm per cm of wave guide length.

5. The improvement of claim 3 wherein the wide end of said wave guide is wide enough for naturally supporting a second order widthwise resonance mode and is provided with means for preventing transmission thereto of said second order mode.

6. The improvement of claim 5 wherein the wide end of said wave guide is too narrow for naturally supporting a third order widthwise resonance mode.

7. The improvement of claim 6 wherein said wave guide has a taper in the order of at least about 0.016 cm per cm of wave guide length.

8. The improvement of claim 3 wherein said wave guide is widest at its central point and is tapered inwardly toward each of its ends.

9. The improvement of claim 3 wherein said wave guide is progressively tapered in an amount for causing all of said liquid filaments to have substantially the same mean length.

10. The improvement of claim 1 wherein the maxi- 'mum width of said wave guide at any orifice location is great enough for naturally supporting a second order widthwise resonance mode buttoo narrow for support-' fold having an exit opening for escape of a recording liquid therefrom, an orifice plate provided with a plurality of orifices arranged along a line and sealed against said manifold with said orifices communicating with said exit opening, means for delivering a supply of recording liquid to said manifold under sufficient pressure to force said liquid to flow continuously through said orifices and form a plurality of free standing jets, means for generating a series of drop generating disturbances and causing said disturbances to travel along said orifice plate in the form of bending waves, and means for selective charging and deflection of drops generated by said jets under the stimulation-of said bending waves, the improvement wherein said exit has a narrowing taper for reinforcing said waves and comopening has a width in the order of about 3.97 mm at its narrowest position of communication with any of said orifices.

16. The improvement of claim 15 wherein said exit opening has a width in the order of about 7.15 mm at its widest position of communication with any of said orifices.

17. The improvement of claim 13 wherein said exit opening has a width in the order of about 3.97 mm at its narrowest position of communication with any of said orifices and a width in the order of about 7.15 mm at its widest position of communication with any of said orifices.

18. The improvement of claim 13 wherein said exit opening is wider than a full wave length of said bending waves at its widest position of communication with any of said orifices and wherein said recording head comprises means to prevent generation of any mode ll bending waves on the active area of the orifice plate.

Claims

1. In a jet drop recording head comprising an orifice plate provided with a plurality of orifices arranged along a line, a common manifold connecting with said orifices, means marginally securing said orifice plate to said manifold for definition of an acoustical wave guide, means for supplying a recording liquid to said manifold at sufficient pressure for forcing the liquid through said orifices and creating a line of liquid jets, means for causing a series of bending waves to travel lengthwise along said orifice plate and stimulate said jets to break up into streams of regularly formed drops, and means for charging and deflecting said drops to create intelligible patterns therefrom; the improvement wherein the marginal attachment of said orifice plate to said manifold defines a wave guide of progressively decreasing width to counteract the naturally occurring attenuation of said waves.

10. The improvement of claim 1 wherein the maximum width of said wave guide at any orifice location is great enough for naturally supporting a second order widthwise resonance mode but too narrow for supporting a third order resonance mode, and further wherein the minimum width of said wave guide at any orifice location is great enough for supporting a first order resonance mode.

11. The improvement of claim 10 further comprising means for preventing generation of bending waves of the second order widthwise resonance mode.

12. In a jet drop recording head comprising a manifold having an exit opening for escape of a recording liquid therefrom, an orifice plate provided with a plurality of orifices arranged along a line and sealed against said manifold with said orifices communicating with said exit opening, means for delivering a supply of recording liquid to said manifold under sufficient pressure to force said liquid to flow continuously through said orifices and form a plurality of free standing jets, means for generating a series of drop generating disturbances and causing said disturbances to travel along said orifice plate in the form of bending waves, and means for selective charging and deflection of drops generated by said jets under the stimulation of said bending waves, the improvement wherein said exit has a narrowing taper for reinforcing said waves and compensating against the naturally occurring attenuation thereof.

13. The improvement of claim 12 wherein said exit opening is wider than one-half the wave length of said bending waves but narrower than one and one-half times said wave length at all positions of communication with any of said orifices.

14. The improvement of claim 13 wherein said exit opening has a taper in the order of about 0.025 cm per cm of exit opening length.

15. The improvement of claim 14 wherein said exit opening has a width in the order of about 3.97 mm at its narrowest position of communication with any of said orifices.

18. The improvement of claim 13 wherein said exit opening is wider than a full wave length of said bending waves at its widest position of communication with any of said orifices and wherein said recording head comprises means to prevent generation of any mode II bending waves on the active area of the orifice plate.