EP0097413A1

EP0097413A1 - A fluid jet print head, and a method of stimulating the break up of a fluid stream emanating therefrom

Info

Publication number: EP0097413A1
Application number: EP19830301874
Authority: EP
Inventors: Hilarion Braun
Original assignee: Eastman Kodak Co; Mead Corp
Current assignee: Kodak Versamark Inc
Priority date: 1982-06-21
Filing date: 1983-03-31
Publication date: 1984-01-04
Also published as: JPS595071A; DE3364155D1; CA1219776A; JPH038946B2; EP0097413B1

Abstract

A fluid jet print head includes an elongated body (10) having a length substantially greater than its other dimensions and defining a fluid receiving reservoir (16) in one end. An orifice plate (12) is secured to the body (10) over the reservoir (16), and includes a row of orifices (18) from which fluid, supplied under pressure to the reservoir, emerges as a fluid filament. The print head includes a support (34) for engaging the body (10) intermediate its ends. Further, the print head includes a pair of piezoelectric transducers (36), bonded to the exterior of the print head body on opposite sides thereof, for alternately elongating and contracting in phase in the direction of elongation of the print head body. This causes mechanical vibration of the body and break up of the fluid filament into drops. Alternately, the piezoelectric transducers may be driven out of phase such that the print head body flexes.

Description

The present invention relates to a fluid jet print head and, more particularly, to a print head and method for generating at least one stream of drops in which construction and operation of the print head are facilitated.
Jet drop printers and coating devices operate by generating streams of small drops of ink or coating fluid and controlling the deposit of the drops on a print receiving medium. Typically, the drops are electrically charged and then deflected by an electrical field. The dtops are formed from fluid filaments which emerge from small orifices. The orifices communicate with a fluid reservoir in which fluid is maintained under pressure. Each fluid filament tends to break apart at its tip to form a stream of drops. In order to deflect drops accurately by means of an electrical field and produce selective deposition of the drops on the print receiving medium, it is necessary for the drops to be substantially uniform in size and in interdrop spacing within each stream. _The break up of the filaments into streams of drops is facilitated by mechanical vibration of some portion or all of the print head structure in a process termed "stimulation".
One prior art stimulation technique, as shown in U.S. patent No. 3,739,393, issued June 12, 1973, to Lyon et al, is to provide the fluid orifices in a relatively thin, flexible wall of the fluid reservoir and to stimulate this wall, known as an "orifice plate", by causing a series of bending waves to travel along the plate. This technique results in substantially uniform drop size and spacing but the timing of break up of the fluid filaments varies along the length of the orifice plate.
Another approach is to vibrate the entire print head, including the ink manifold structure and the orifice plate structure, together. This is shown in U.S. patent No. 3,586,907, issued June 22, 1971, to Beam et al. Such an arrangement will necessarily fatigue the print head mounting structure, since the mounting structure experiences the same vibrations which are applied to the manifold and the orifice plate. Further, the amplitude and phase of the vibratory motion are difficult to control at the frequencies commonly used for jet drop printer operation.
A further approach to filament stimulation is disclosed in U.S. patent No. 4,095,232, issued June 13, 1978, to Cha. Using the technique disclosed in this patent, stimulators mounted in the upper portion of a fluid reservoir generate pressure waves which are transmitted downward through the fluid. Each stimulator includes a pair of piezoelectric crystals which vibrate in phase and which are mounted on opposite sides of a mounting plate which is coincident with a nodal plane. A reaction mass is positioned at the end of each stimulator opposite the stimulation member which is coupled to the fluid. The reaction mass ensures that the nodal plane is properly positioned.
In British patent specification 1422388, a print head is disclosed in which a piezoelectric crystal forms one wall of a single-jet ink jet print head. When a drop is to be emitted from the orifice, the piezoelectric transducer is electrically actuated, causing it to distort and thereby forcing a drop from the orifice.
In British patent specification 1293980, published October 25, 1972, and U.S. patent No. 4,198,643, issued April 15, 1980, to Cha et al, print heads are disclosed in which a pair of piezoelectric crystals are bonded to opposite sides of a support plate. A print head manifold structure is bonded to one of the piezoelectric crystals and a counterbalance is bonded to the other of the crystals. The weight _Qf the counterbalance is selected so as to offset the weight of the print head manifold. By this balanced arrangement,'the support plate is placed in a nodal plane when the two piezoelectric transducers are energized in synchronism. It will be appreciated,.however, that the construction of such a print head is relatively complicated and, further, that it is difficult to design such a print head to be resonant at a desired frequency. The print head must be tuned subsequent to construction, therefore, such that the resonant frequency of the print head equals the desired operating frequency.
Finally, in U.S. patent No. 3,972,474, issued August 3, 1976, to Keur, an ink drop writing system is shown in which a vibrating nozzle is used to produce a stream of drops. The length of the nozzle is selected so that its mechanical resonant frequency is much higher than the frequency at which it is driven. The nozzle, configured as a tube, is surrounded by a piezoelectric ring which, when electrically driven, provides radial contraction and expansion of the tube.
There is a need for an improved fluid jet print head in which uniform in-phase stimulation may be provided for a plurality of jet drop streams, in which mounting of the print head is facilitated, and in which construction and design of the print head are simplified.
According to one aspect of the present invention, a fluid jet print head for generating at least one stream of drops comprises an elongated print head body, the length of the body between first and second ends thereof being substantially greater than its other dimensions. The body defines a fluid receiving reservoir in its first end and at least one orifice communicating with the fluid receiving reservoir. Fluid is supplied to the reservoir under pressure by appropriate means such that it emerges from the reservoir to form a fluid stream. A transducer means is mounted on the exterior of the body and extends a substantial distance along the body in the direction of elongation from adjacent the support means toward both the first and second ends of the body. The transducer means is responsive to an electrical driving signal for changing dimension in the direction of elongation of the body, thereby causing mechanical vibration of the body and break up of the fluid stream into a stream of drops.
The transducer means comprises a pair of piezoelectric transducers bonded to opposite sides of the body and extending in the direction of elongation from points adjacent the first end to points adjacent the second end of the body. The piezoelectric transducers provide alternate lengthening and contraction of the elongated print head body in the direction of elongation of the body.
The transducer means further comprises means for electrically connecting the pair of transducers in parallel, whereby the transducers operate in phase so as to produce vibration which is in a direction substantially parallel to the direction of elongation of the elongated print head body. A support means for the print head engages the print head body intermediate and substantially equidistant from its first and second ends.
Alternatively, the transducer means may comprise means for electrically connecting the transducers so that they operate out of phase, thus producing flexure waves. The support means for the print head engages the print head body a distance from each end of the body approximately equal to .23 of the overall length of the body.
For vibration parallel to the direction of elongation,_' the support means may comprise a pair of mounting flanges, each integrally formed with the print head body, and being relatively thin. The flanges extend from the elongated print head body on opposite sides thereof and are substantially equidistant from the first and second ends of the body such that they support the body along a nodal plane. Alternatively, the support means may comprise a pair of support screws which engage the body at opposite sides thereof at points substantially equidistant from the first and second ends of the print head body.
The print head body includes means defining a slot in the first end thereof and orifice plate means, attached to the means defining a slot, and forming the fluid receiving reservoir therewith. The orifice plate means may define a plurality of orifices for production of a plurality of drop streams. The print head body may further define a fluid supply opening and a fluid outlet opening communicating with the slot. The fluid jet print head may further include fluid conduit lines connected to the fluid supply opening and the fluid outlet opening. The fluid conduit lines are formed of a material having a substantially different vibrational impedance than the print head body, whereby the conduit lines do not provide a substantial power loss. The fluid conduit lines may, for example, be made of a polymer material.
The fluid jet print head may further include means for applying an electrical driving signal of a frequency substantially equal to ^f _o = C/2L, where L is the dimension of the body in the direction of elongation, and C is the speed of sound through the body. In this case the fluid jet print head is driven at a frequency approximating its mechanical resonant frequency.
For flexure wave vibration, the transducers are driven at a frequency F_o = ac_a/L ², where a is the transverse thickness of the print head body 0 and a =1.7. In this case, two nodal mounting axes are established a distance equal to approximately .23 of the length of the print head body, centered between the transducers.
The method for stimulating the break up of a fluid stream emanating from at least one orifice communicating with the fluid reservoir in a fluid jet print head includes the steps of:

(a) providing an elongated print head which defines the reservoir and the orifice at one end thereof;
(b) applying fluid under pressure to the- reservoir so as to produce fluid flow through the orifice;
(c) supporting the print head at points in a plane substantially equidistant from the ends of the elongated print head and normal to the direction of elongation of the print head; and
(d) alternately elongating and contracting the print head substantially at the resonant frequency of the print head, whereby the print head is supported in a nodal plane and the stream is effectively stimulated to break up into drops.

The resonant frequency of the print head may be substantially equal to the resonant frequency of the fluid stream. The print head may be elongated and contracted by means of piezoelectric transducers bonded to its exterior.
The stream may also be stimulated by operating the transducers out of phase, thereby causing flexure of the print head. In this stimulation mode, the print head is mounted at points which are a distance from each end which are approximately equal to .23 times the length of the print head.
Accordingly, it is an object of the present invention to provide a fluid jet print head for generating one or more streams of drops in which the print head includes an.elongated body which is driven to elongate and contract in the direction of elongation of the body; to provide such a print head and method in which the print head is driven by means of thin piezoelectric transducers bonded to the print head exterior; and to provide such a print head in which support for the print head is provided in a nodal plane.
In order that the invention may be.more-readily understood, reference will now be made to the-accompanying drawings, in which:-

Fig. 1 is an exploded view, illustrating a first embodiment of the fluid jet print head of the present invention;
Fig. 2 is a plan view of the print head of Fig..l, with the orifice plate removed;
Fig. 3 is a side view of the print head of Fig. 1 with the electrical drive circuitry illustrated;
Fig. 4 is an enlarged partial sectional view, taken generally along line 4-4 in Fig. 2;
Fig. 5 is a graph, useful in explaining the operation of the print head of the present invention;
Fig. 6 is a graph, useful in explaining operation of the print head of the present invention.
Fig. 7 is a schematic diagram illustrating driving circuitry for the fluid print head; and
Fig. 8 is a side view of a second embodiment of the fluid jet print head of the- present invention.

The present invention relates to a fluid jet print head of the type which may be used for ink jet printing, coating, textile dyeing, and other purposes. As is known, such devices typically operate by electrically charging the drops in one or more jet drop streams and, thereafter, deflecting the trajectories of some of the drops by means of electrical fields. In order to produce the stream or streams of drops, fluid is typically applied to a fluid reservoir under pressure such that it then flows through one or more orifices or nozzles which communicate with the reservoir. The fluid emerges from the orifices as fluid filaments which, if left undisturbed, would break up somewhat irregularly into drops of varying size and spacing. It is not possible to charge and deflect such nonuniform drops accurately and, as a consequence, jet drop devices have typically applied mechanical stimulation in some fashion to the fluid filaments so as to cause break up of the filaments into drops of generally uniform size and spacing at a desired drop break up frequency.
A first embodiment of the print head of the present invention is shown in Figs. 1-4. The print head generally includes an elongated print head body 10, the length of which, L, is substantially greater than its other dimensions a and b. The body 10 includes an orifice plate 12 and a block of material 14. The body 10 defines a fluid receiving reservoir 16 in its first end, and at least one and preferably a number of orifices 18 which are arranged in a row across orifice plate 12. The orifice plate 12 is bonded to block 14 of material, such as stainless steel by means of a suitable adhesive. Block 14 defines a slot 20 which, in conjunction with orifice plate 12 defines the reservoir 16. The block 14 further defines a fluid supply opening 22 and a fluid outlet opening 24, both of which communicate with the slot 20.
The print head further includes means for supplying fluid to the reservoir 16 under pressure such that fluid emerges from the orifices 18 as fluid filaments which then break up into streams of drops. This includes a pump 26 which receives fluid from a tank 28 and delivers it, via fluid conduit line 30, to the reservoir 16. A conduit 32 is connected to fluid outlet 24 such that fluid may be removed from the reservoir 16 at shut down of the print head or during cross-flushing of the reservoir 16. As will become apparent, the end of the print head to which conduits 30 and 32 are attached, as well as the opposite end of the print head, is subjected to mechanical vibrations which cause the fluid filaments to break up into streams of drops of uniform size and spacing. The conduits 30 and 32 are selected from among a number of materials, such as a polymeric material, which have a vibrational impedance substantially different from that of the stainless steel block 14. As a consequence, power loss through the conduits 30 and 32 and the resulting damping of the vibrations are minimized.
The print head further includes support means, such as mounting flanges 34. Flanges 34 are relatively thin and are integrally formed with the block 14. The flanges 34 extend from opposite sides of the elongated print head body 10 and are substantially equidistant from the first and second ends of the body. As a result, the flanges may be used to support the body 10 in a nodal plane. The flanges 34 are therefore not subjected to substantial vibration.
The print head further comprises a transducer means, including thin piezoelectric transducers 36 and 38. The transducers are bonded to the exterior of the body of block 14 and extend a substantial distance along the body in the direction of elongation thereof, from adjacent the support means toward both the first and second ends of the body. The transducers 36 and 38 respond to an electrical driving signal, provided by power supply 40 on line 42, by changing dimension, thereby causing mechanical vibration of the body and break up of the fluid streams into streams of drops.
The piezoelectric transducers 36 and 38 have electrically conductive coatings on their outer surfaces, that is the surfaces away from the print head block 14, which define a first electrode for each such transducer. The metallic print head block 14 typically grounded, provides the second electrode for each of the transducers. The piezoelectric transducers are selected such that when driven by an a.c. drive signal, they alternately expand and contract in the direction of elongation of the print head. As may be seen in Fig. 3, transducers 36 and 38 are electrically connected in parallel. The transducers are oriented such that a driving signal on line 42 causes them to elongate and contract in unison. Since the transducers 36 and 38 are bonded to the block 14, they cause the block to elongate and contract, as well.
If desired, an additional piezoelectric transducer 44 may be bonded to one of the narrower sides of the print head to provide an electrical output potential on line 46 which fluctuates in correspondence with the elongation and contraction of the print head block 14. The amplitude of the signal on line 46 is proportional to the amplitude of the mechanical vibration of the block 14.
The mechanism by which the first embodiment of the print head of the present invention functions may be described as follows. The elongated print head body is somewhat analogous to an ordinary helical spring. If such a spring is compressed and then quickly released, it will oscillate about its center at a frequency f_o, called its fundamental longitudinal resonant frequency. In this condition, both ends of the spring move toward and away from the center of the spring, while the center remains at rest. Therefore, if one fixes the center of the spring and repeats the above described operation, the spring will oscillate in the same manner at the frequency ^F _o.
The steel block 14 which forms a part of the print head body can be considered to be a very stiff spring. If properly mechanically stimulated, it may therefore be held at its center, as by flanges 34, while both ends of the block 14 alternately move toward and away from the center. Since the center of the block lies in a nodal plane, the flanges 34 are not subjected to substantial vibration and the support for the print head does not interfere with its operation. As the end of the print head body 10 which defines the fluid receiving reservoir 16 is vibrated, the vibrations are transmitted to the fluid filaments which emerge from the orifices 16, thus causing substantially simultaneous uniform drop break up. Note that the reservoir 16 is small in relation to the overall size of the block 14 and is centered in the end of the block. As a consequence, the reservoir 16 does not interfere significantly with the vibration of the block 14, nor affect the resonant frequency of the print head substantially.
The resonant frequency of the block 14 can generally be said to be given by
where C is the speed of sound through the print head block 14 material, L is the length of the print head body in the direction of elongation, E is the modulus of elasticity of the material forming block 14 and is the density of the material forming the block 14. Preferably the print head is designed to operate at or near its resonant frequency, and this frequency, in turn, is selected within an appropriate fluid jet stimulation frequency range, e.g., 50KHz to 100KHz.
By providing a pair of piezoelectric transducers 36 and 38 on opposite sides of the block 14, the block 14 is elongated and contracted without the flexure oscillations which would otherwise result if only one such piezoelectric transducer were utilized. Additionally, the use of two piezoelectric transducers allows for a higher power input into the print head for a given voltage and, consequently, for a higher maximum power input into the print head, since only a limited voltage differential may be placed across a piezoelectric transducer without break down of the transducer.
As is well known, E, p and L are temperature dependent and, as a consequence, the resonant frequency of the print head varies with changes in temperature. The variation Δf in f for a temperature change of AT, at or near room temperature, is given by Δf = Δf_okΔT/2, where k is approximately 4 x 10^-4/C° for stainless steel.
When the dimensions a and b are small as compared to L, the print head can be driven at a frequency off resonance. Fig. 5 illustrates the changes in the driving voltage applied to the transducers which are required in order to drive a single jet print head for a constant nominal filament length of 16.5 x 10^-3". In general, the nominal filament length is a function of both the driving voltage and the driving frequency. At any given driving frequency the nominal filament length decreaes with increases in the driving voltage.
From Fig. 5, it is clear that at resonance, 83 KHz, the print head requires a drive voltage of approximately 20 volts peak-to-peak. When driven by an oscillator at a frequency to either side of the resonant frequency, the driving voltage must be increased substantially in order to maintain the filament length at 16.5 x 10^-3". On either side of the resonant frequency, the voltage required rises approximately linearly with frequency. There is, however, a maximum voltage which may be applied to the piezoelectric transducers and, so long as the maximum voltage is not exceeded, the transducers may be driven on the positive slope portion of the curve of Fig. 5, or the negative slope portion of the curve. Assuming that the resonant frequency remains constant, the driving frequency may be varied in synchronization with fluctuations in speed of the print receiving medium upon which drops from the print head are to be deposited, thereby compensating for such fluctuations. In such an instance, the frequency of the drive signal is monitored, however, and the voltage of the drive signal adjusted accordingly in order to compensate for the frequency shift and thereby maintain the desired fluid filament length.
If desired, the additional piezoelectric transducer 44 may be utilized to monitor the frequency of the drive signal and amplitude of vibration of the print_.head. In Fig. 6, the voltage output on line 46 is plotted against the frequency of the driving signal for the maintenance of a single jet print head nominal fluid filament of a length equal to 16.5 x 10^-3", and a diameter of approximately 1 x 10-3". Assuming no change in the resonant frequency of the print head or the-jet, a fluid filament of a desired length can be maintained by monitoring the output voltage and frequency on line 46 and adjusting the level of the driving signal as needed to maintain the output voltage on line 46 at a reference voltage level specified by the curve of Fig. 6.
It will be appreciated that numerous variations may be made in the disclosed print head within the scope of the present invention. For example, flanges 34 may be deleted. Another arrangement, such as support screws may be provided for attaching the print head body to appropriate support structure, as long as the point or points of attachment lie substantially in the nodal plane intermediate the ends of print head body 10.
Reference is made to Fig. 7 which illustrates a circuit which provides a means for supplying an electrical driving signal. The output of a fixed frequency oscillator 48 is supplied to transducers 36 and 38 via a voltage controlled attenuator circuit 50, a power amplifier 52 and a step-up transformer 54. The output from transducer 44 on line 46 is used to control the amount of attenuation provided by circuit 50. The signal on line 46 is amplified by amplifier 56, converted to a d.c. signal by converter 58, and then compared to a selected reference signal by summing circuit 60 to produce a signal on line 62 which controls the attenuation provided by circuit 50. By this feedback arrangement, the amplitude of the driving signal on line 42 and the amplitude of the mechanical vibration of the print head are precisely controlled.
Fig. 8 is a side view illustrating a second embodiment of the present invention, with elements corresponding to the print head of Fig. 1 being labeled with identical reference numerals. In this embodiment the transducers 36 and 38 are oriented on the print head body such that a positive driving signal on line 42 causes one of the transducers to elongate and the other transducer to contract, while a negative driving signal has the opposite effect. As a consequence, as an a.c. driving signal is supplied to line 42, the print head is caused to vibrate in its first flexure mode. This vibrational mode is illustrated in Fig. 8 by medial lines 64 which, although greatly exaggerated in flexure for purposes of clarity, indicate the extent of movement of the center of the print head body 14. It should be noted that lines 64 cross at points which are approximately .23L inward from each end of the print head body, thus indicating nodal points. Mounting holes 66 are drilled into body 14 at the nodal points and a second corresponding pair of mounting holes are drilled into the opposite side of the print head body. By providing mounting pins which extend into holes 66, pivot supports are provided which do not interfere with flexure of the print head.
This flexure mode may be excited by driving the transducers at a frequency
where a is approximately 1.76. This is a simplification of the resonant frequency equation
where K is the radius of gyration, which for the print head illustrated equals a/2.
It will be further appreciated that the present invention is not limited to the precise method and form of apparatus disclosed, and that changes may be made in either without departing from the scope of the invention as defined in the appended claims.

Claims

1. A fluid jet print head for generating at least one stream of drops, said print head having a print head body (10) defining a fluid receiving reservoir (16) and at least one orifice (18) communicating with the fluid receiving reservoir, means (26, 28) for supplying fluid to the reservoir under pressure such that fluid emerges from the orifice to form a fluid stream support means (34) for engaging said print head body, and transducer means (36, 38), responsive to an electrical driving signal, for causing mechanical vibration of said body and break up of said fluid stream into a stream of drops, characterized in that:

said print head body (10) is elongated, the length of said body between first and second ends thereof being substantially greater than its other dimensions, said body defining said reservoir (16) in said first end of said body,

said transducer means (36, 38) is mounted on the exterior of said body and extends a substantial distance along said body in the direction of elongation of said body, and

said support means (34) engages said print head body intermediate said first and second ends.

2. A fluid jet print head as claimed in claim 1, further characterized in that said transducer means comprises a pair of piezoelectric transducers (36, 38) bonded to opposite sides of said body and extending in the direction of elongation from points adjacent said first end to points adjacent said second end.

3. A fluid jet print head as claimed in claim 2,further characterized in that said piezoelectric transducers (36, 38) provide alternate lengthening and contraction of said elongated print head.body in the direction of elongation thereof.

4. A fluid jet print head as claimed in claim 3,further characterized in that said transducer means further comprises means (42) for electrically connecting said pair of piezoelectric transducers in parallel.

5. A fluid jet print head as claimed in claim 4,further characterized in that said piezoelectric transducers are connected to elongate and contract in phase.

6. A fluid jet print head as claimed in claim 5,further characterized in that said support means (34) engages said print head body substantially intermediate and equidistant from said first and second ends thereof.

7. A fluid jet print head as claimed in claim 4,further characterized in that said piezoelectric transducers (36, 38) are connected to elongate and contract out of phase, thereby producing flexure of said print head body.

8. A fluid jet print head as claimed in claim 7,further characterized in that said support means (66) pivotally engages said print head body at flexure nodes.

9. A fluid jet print head as claimed in claim l,further characterized in that said support means comprises a pair of mounting flanges (34), each integrally formed with said print head body, and being relatively thin, said flanges extending from said elongated print head body on opposite sides thereof and substantially equidistant from said first and second ends of said body such that said flanges support said body along a nodal plane.

10. A fluid jet print head as claimed in claim 1, further characterized in that said support means comprises a pair of support screws which engage said body at opposite sides thereof at points substantially equidistant from said first and second ends of said print head body.

ll. A fluid jet print head as claimed in claim 1, further characterized in that said print head body includes means defining a slot (20) in the first end thereof and a fluid supply opening (22) communicating with said slot, and orifice plate means (12), attached to said means defining a slot, and forming said fluid receiving reservoir therewith.

12. A fluid jet print head as claimed in claim llfurther characterized in that said orifice plate means (12) defines a plurality of orifices (18) for production of a plurality of drop streams.

13. A fluid jet print head as claimed in claim 11, further characterized in that said print head further defines a fluid outlet opening (24) communicating with said slot.

14. A fluid jet print head as claimed in claim 13, further characterized by fluid conduit lines (30, 32) connected to said fluid supply opening (22) and said fluid outlet opening (24), said fluid conduit lines being formed of a material having a substantially.different vibrational impedance than said print head body, whereby said conduit lines do not provide a substantial power loss.

15. A fluid jet print head as claimed in claim 14,further characterized in that said fluid conduit lines (30, 32) are made of a polymer material.

16. A fluid jet print head as claimed in claim 1, further characterized by monitor transducer means (44), mounted on the exterior of said body and providing an electrical monitor signal in response to dimensional changes of said body.

17. A fluid jet print head as claimed in claim 5, further characterized by means (40) for applying an electrical driving signal of a frequency substantially equal to ^f _o, where

L is the dimension of said body in the direction of elongation, and C is the speed of sound through said body, whereby said fluid jet print head may be driven at a frequency approximately its mechanical resonant frequency.

18. A fluid jet print head as claimed in claim 1 further characterized by

monitor transducer means (44), mounted on the exterior of said body and providing an electrical monitor signal in response to dimensional changes of said body, and

means (50, 60), responsive to said monitor transducer means, for applying an electrical driving signal to said transducer means of an amplitude dependent upon said electrical monitor signal.

19. A fluid jet print head as claimed in claim 7, further characterized by means (40) for applying an electrical driving signal of a frequency substantially equal to F where

L is the dimension of said body in the direction of elongation, C is the speed of sound through said body, and K is the radius of gyration of said body.

20. A method for stimulating the break up of a fluid stream emanating from at least one orifice (18) communicating with a fluid reservoir (16) in a fluid jet print head, comprising:

(a) providing an elongated print head (10) which defines the reservoir (16) and the orifice (18) at one end thereof,

(b) applying fluid under pressure to said reservoir (16) so as to produce fluid flow through the orifice (18),

(c) supporting said print head at points in a plane substantially equidistant from the ends of the elongated print head and normal to the direction of elongation of the print head, and

(d) alternately elongating and contracting said print head substantially at the resonant frequency of said print head, whereby said print head is supported in a nodal plane and said stream is stimulated to break up into drops.

21. A method as claimed in claim 20,further characterized in that the resonant frequency of the print head is substantially equal to the resonant frequency of the fluid stream.

22. A method as claimed in claim 20 further characterized in that said print head is elongated and contracted by means of piezoelectric transducers (36, 38) bonded to its exterior. -

23. A method for stimulating the break up of a fluid stream emanating from at least one orifice

(18) communicating with a fluid reservoir (16) in a fluid jet print head, comprising:

(b) applying fluid under pressure to'said reservoir so as to produce fluid flow through the orifice, and

(c) vibrating said print head in its first flexure mode substantially at the resonant flexure frequency of said print head, while supporting said print head at nodal points such that said stream is stimulated to break up into drops.

24. A method as claimed in claim 23, further characterized in that said print head is vibrated in its first flexure mode by means of piezoelectric transducers (36, 38) bonded to its exterior.