ACOUSTICALLY SOFT INK JET NOZZLE ASSEMBLY

Abstract: An ink jet nozzle assembly is produced from materials, such as polyphenylene sulfide. The resulting assembly is ac­ oustically soft so that undesirable fluid and mechanical re­ sonances are substantially attenuated.

Description: This invention relates to drop marking equipment and, in particular, to nozzles used in such drop marking equipment or ink jet devices. Such devices employ inks which are supplied from a reservoir to a nozzle. The nozzle directs ink at a substrate to be marked. By use of a transducer, electrical energy is converted into mechanical energy, which is coupled to the ink in the nozzle. In one example of ink jet operation, the stream of ink ejected from an orifice at one end of the nozzle is broken up into a series of regularly spaced, discrete droplets which may be selectively given an electrical charge. In that type of drop marking device, those drops which receive a charge are deflected onto a substrate while those which are not charged are recovered and returned to the ink supply. In another type of droplet marking device, the transducer applies an impulse of energy to the fluid in the nozzle each instance that a droplet is needed. As is well known by those in the art, the complexity of such ink jet nozzles contribute to cost and speed limitations. For example, it is often desirable to group together several such nozzles to permit high speed printing on a substrate which may be, for example, magazines, envelopes, labels, beverage cans on other products moving on a conveyor. It is not uncommon for ink jet nozzles in some applications to be spaced as closely as six per inch and thus the need for a low cost, high quality, miniaturized device is apparent. A significant contributing factor to the complexity and cost of producing ink jet nozzles is the presence of both fluid and mechanical resonances in such assemblies which interfere with the nozzle's usefulness over the range of frequencies usually employed to form the ink droplets. Typical useful frequencies range from 10 KHz to 100 KHz. Such resonances vary with the type of ink employed, temperature, and the geometric dimensions of the nozzle assembly. They are also significantly affected by the type of material used to manufacture the nozzle. As a result ink jet printers have required a variety of different nozzles to permit operation at different frequencies and for different kinds of inks. Most commonly, ink jet nozzle assemblies have been manufactured from metal or glass materials and are acoustically "hard" meaning that they suffer from the presence of both fluid and mechanical resonances over the range of frequencies employed by the transducer to form the ink drops. As is well known according to acoustic principles involved in vibrating bodies, these nozzles that have fluid resonance regions also have antiresonance regions. The disturbing energy applied to the nozzle cannot be efficiently transmitted to the fluid to form droplets if the frequency selected for operation is coincidental with an antiresonance frequency region. The present invention is concerned therefore, with providing nozzles without resonance so as to eliminate the antiresonance regions in the operating frequency range and thereby extend the operating frequency range of the nozzle. To do that, acoustically soft materials were sought so that resonances would be substantially unsupported. This permits only the disturbing energy created by an electromechanical transducer, for example, a piezoelectric crystal, operating at a selected frequency to be transmitted to the fluid. The use of an acoustically soft material in nozzle production is known from US-A-4319251 which discloses a non-conductive nozzle moulded from plastic of the type sold by Dupont under the trademark DELRIN, such plastic being an acetal homopolymer which is an acoustically soft material. It is important, as aforesaid, that such nozzles be operated at a drive voltage which maintains constant drop formation over a frequency range of 10 KHz to 100 KHz. It has been found, however, that a nozzle formed of acetal homopolymer (DELRIN) encounters significant antiresonances within the ranges 10 to 20 KHz and 70 to 90 KHz which cause undesirable increases in the drive voltages and it is an object of this invention to provide an acoustically soft nozzle which obviates or mitigates such undesirable drive voltage increase. This is achieved by moulding the nozzle from polyphenylene sulfide which has a substantially flat response to the transducer disturbing frequency within the range of 10 KHz to 100 KHz. According to the present invention there is provided a nozzle for use with a transducer which provides disturbing energy to an ink stream passing through the nozzle to form ink droplets. The nozzle comprises a tubular member having an orifice at one end, with its other end being adapted for connection to a supply of ink containing solvents. The nozzle is moulded from polyphenylene sulfide, thus rendering it acoustically soft. When the transducer is coupled to the nozzle, the response of the nozzle to the disturbing energy of the transducer over a frequency in the range of 10 KHz to 100 KHz is substantially flat, the disturbing energy being transmitted to the ink within the nozzle without substantial amplification, attenuation or the creation of harmonic resonances of a frequency characterizing the disturbing energy. Brief Description of the Drawings Figure 1 is a cross sectional view of a nozzle assembly according to a preferred embodiment of the present invention. Figure 2 is an enlarge sectional view of the nozzle and tail piece according to the preferred embodiment. Figure 3 through 9 are similar curves illustrating the response characteristics for a number of different material tested as to their suitability for use in the present invention. Detailed Description As indicated in the background portion of this specification, the present invention relates to a nozzle assembly for ink jet printing which has significant advantages over presently known assemblies which are typically machined from metal, glass or other acoustically "hard" materials, or formed of an unsatisfactory acoustically soft material (see US-A-4319251). For a nozzle to be useful over a range of frequencies it should operate, as has been previously stated, at a substantially constant drive voltage level at all frequencies in the range required regardless of ink characteristics. Typical useful frequencies range from 10 KHz to 100 KHz. Typical inks suitable for use in ink jet printers have the following range of characteristics: Surface Tension 2.2 to 7.2 Nm⁻¹ (22 to 72 dyne/cm) Viscosity 0.0015 to 0.01 Nsm⁻² (1.5 to 10 centipoise) Density 850 to 1100 kg m⁻³ (0.85 to 1.1 gm/cm³) Velocity of Sound 1,000 to 1,650 ms⁻¹ The last characteristic, the velocity of sound in the ink, is of significant concern in the design of nozzles. The velocity of sound in such a fluid varies with the temperature of the fluid and, therefore, the fluid resonances (related to the velocity of sound) change frequency as a function of temperature changes in the nozzle. Thus, the resonances may be different during initial operation, when the nozzle is cool, than after the nozzle has been in use for a period of time. Also, the velocity of sound is affected by changes in the composition of the ink due mainly to evaporation of solvents. According to the present invention these problems are overcome by the use of a nozzle assembly which is acoustically soft. Although there are many materials which might meet this criteria, it is necessary to consider the severe operating environment. The nozzle may need to be extremely small to work in some applications, subjected to continual temperature changes and vibration and, most importantly, is in contact with different inks containing water or various alcohols, ketones and other solvents. It is necessary, therefore,to select materials which can stand up to this environment in addition to being acoustically soft. Through materials testing a number of materials were identified as being potentially suited for the application. These include acetal homopolymers such as Delrin, acetal copolymers, polypropylene, polyphenylene sulfide, polyphenylene oxide. These materials were selected for testing because they are moldable, have long term stability in contact with the solvents contained in typical inks and they were expected to be acoustically soft. It was believed that at least some of these materials would eliminate or attenuate resonances in the body of the nozzle (mechanical resonance) and in the ink (fluid resonance). In order to determine which, if any, of these materials were suitable, nozzle bodies were designed, molded and tested. Figure 2 illustrates the nozzle assembly molded from the various materials for purposes of testing. A nozzle 30 is an elongated, hollow cylindrical member. At one end thereof is a female coupling 32 adapted to receive a tail piece 34 having a male coupling member 36. The tail piece 34, in turn, can be coupled to a conduit member for providing an ink supply to the nozzle 30. The distal end of the nozzle 30 has a recessed portion 37 adapted to receive and retain an orifice jewel 38 therein. Retention is accomplished by dimensioning the recess to provide an interference fit which firmly seats the jewel and prevents leakage. It was found that an interference fit of approximately 0.0038cm (0.0015 inch) was adequate to retain the jewel in place with a recess depth of approximately two times the thickness of the jewel. With such dimensions the nozzle material closes around the jewel to retain it securely in place. Prior to testing the nozzle 30 of Figure 2, a piezoelectric transducer was coupled by adhesive bonding. The bonding agent was selected to insure a good coupling between the piezoelectric device and the nozzle for transmission of energy to the fluid. Epoxies are preferred and, in particular, a one part binder which is not too viscous is best. This permits the binder to flow well in the space between the nozzle and the piezo electric device to avoid gaps which can cause undesirable variations in the applied energy, require higher drive voltages, contribute to mechanical resonance and lead to premature failure of the device. Preferably the bonding material is relatively stiff to maintain drive efficiency. One suitable adhesive bonding agent is an anaerobic adhesive sold under the trade name Permalok by Permabond International Corporation, Englewood, New Jersey. Completed test nozzles molded from the materials believed to be suitable were then subjected to testing. The results of these tests are illustrated in Figures 3 through 6. In each case the drive voltage, RMS or peak-to-peak as noted on the plots, necessary to maintain constant drop formation was plotted over a frequency range of 10KHz to 100KHz. Referring to Figure 3, the test results for the acetal homopolymer (see US-A-4319251), are shown. As can be seen, the drive voltage in the frequency range 20KHz to 70KHz is reasonably flat and less than approximately 15 volts. However, in the ranges of 10 to 20KHz and 70 to 90KHz significant antiresonances are encountered causing undesirable increases in the drive voltages. Figure 4 shows the test data for polypropylene. It has a variety of antiresonance throughout the frequency range of interest and is therefore not suitable for present purposes. Figure 5 illustrates the test data for the acetal copolymer which has undesirable antiresonances at 10 to 20 KHz and above 90 KHz. Figure 6 illustrates that data for polyphenylene sulfide (two tests are shown, one in which the nozzle is potted in a block, the other unpotted). As can be seen, the material is much better than the prior art metal nozzles and significantly better than any of the other acoustically soft materials tested. Its response characteristic is essentially flat from 10KHz to 100KHz. This indicates, particularly in view of the low drive voltage required to maintain constant droplet production, that the material very efficiently couples the piezoelectric device and the fluid while at the same time being acoustically soft to not support fluid resonance. Because it is a molded part and is directly coupled to the driving device by an adhesive, there is little mechanical resonance created. This material was designated as the preferred material for the production of a new, highly efficient nozzle assembly for ink jet printing. Such a nozzle can be driven at a substantially uniform voltage over the desired operating range of frequencies. To verify the remarkable properties of this compound, additional tests were run using inks having different properties and, in particular, different velocity of sound values. The curves for this testing are illustrated in Figures 7 through 9. In each case the response curve for the polyphenylene sulfide was essentially flat over the frequency range of interest. Referring to Figure 1, there is shown a preferred embodiment of the nozzle assembly employing the preferred materials of the present invention. A nozzle 50 formed of polyphenylene sulfide is coupled to a tail piece 52 preferably formed of the same materials. In turn, the tail piece is coupled to a fitting 54 for connection to an ink supply conduit. A jewel 56 is provided in the forward portion of the nozzle and captured therein by virtue of the dimensions of the nozzle recess as previously described. Concentrically mounted over the nozzle 50 is a piezoelectric transducer 58 adhesively bonded in place. The devices are electrically driven by means of a cable 61, the conductors contained therein being soldered to the outside of the transducers as indicated. The nozzle assembly is preferably potted and disposed within a nozzle head assembly or block 60. The completed assembly is small enough to permit spacing in the order of six separate print heads per 2.54 cm (per inch). The nozzles according to the present invention have good, long term resistance to ink solvents, are relatively temperature insensitive, and can be driven at substantially uniform drive voltages over a wide range of operating frequencies. At the same time, because they are acoustically soft, the fluid does not "experience" a rigid confining wall and does not form standing waves which generate fluid resonances within the nozzle body. By eliminating fluid resonances, the antiresonances representing sharp increases in the acoustic impedance of the ink are also eliminated. Thus, droplet formation is accomplished across a broad frequency range by a substantially uniform driving voltage. If desired, because of the electrical isolation of the ink within the nozzle body, an independently controlled potential may be applied to the ink permitting, for example, increased deflection by the techniques taught in US-A- 4,319,251. In addition, phasing of drop formation and drop charging is facilitated by permitting charging currents in the ink to be reliably detected. While the invention has been described with reference to a preferred embodiment of a nozzle assembly having a single orifice through which ink is ejected, it is within the teachings of the present invention to provide a plurality of orifices in the nozzle assembly configured in an array. Either a separate chamber for each orifice or a common chamber for a plurality of orifices may be used dependent upon which droplet formation technique is desirable in the particular ink jet device in which the nozzle is employed. There is ink confined to the chamber in either instance, and forming the wall or walls of the nozzle ink chamber of acoustically soft material in accordance with the teachings of the present invention assures that the disturbing energy coupled to the chamber is transmitted to the ink within the chamber without substantial amplification, attenuation or the creation of harmonic resonances of any frequency characterizing the disturbing energy. The present invention is useful also in ink jet printers that employ a pulsed nozzle to form droplets. Zolton U.S. Patent 3,683,212 discloses one example of that type of nozzle. The impulses of electrical energy used to drive such a nozzle commonly have a duration of 10 microseconds to 100 microseconds. A Fourier analysis of those energy pulses manifests that reliable droplet formation necessitates that the nozzle respond consistently to frequencies in the range of 10KHz to 100KHz. It is desirable that the nozzle chamber not support fluid resonances in that frequency range. A nozzle which has a fluid chamber with walls made of acoustically soft material as taught by the present invention will not support resonances in that region, and thus will have a substantially flat response to energy impulses characterized by frequencies that are within the operating frequency range. As a result, droplet formation is more nearly proportional to the characteristics of the energy pulse applied to the fluid to improve control and enhance the marking results. In addition, spurious oscillations in the impulse nozzle ink chamber that occur after a pulse has directed formation of a droplet are absorbed if the walls are made of acoustically soft material. Those spurious oscillations can distort the energy applied to the fluid when a succeeding command pulse is transmitted to the fluid. Clearly, an impulse or pulse driven nozzle can be operated more advantageously by following the teachings of the present invention.

Claims: 1. A nozzle (50) for use with a transducer (58) which provides disturbing energy to an ink stream passing through the nozzle to form ink droplets, the nozzle (50) comprising a tubular member (52) having an orifice at one end, with its other end being adapted for connection to a supply of ink containing solvents, the nozzle (50) being characterized in that it is molded from polyphenylene sulfide, thus rendering it acoustically soft so that when the transducer (58) is coupled to the nozzle (50), the response of the latter to the disturbing energy of the transducer (58) over a frequency in the range of 10 KHz to 100 KHz is substantially flat, the disturbing energy being transmitted to the ink within the nozzle (50) without substantial amplification, attenuation or the creation of harmonic resonances of a frequency characterizing the disturbing energy. 2. A nozzle (50) as claimed in claim 1 characterized in that the nozzle (50) is molded as a single piece. 3. A nozzle assembly for an ink jet printer comprising a nozzle (50), which comprises a tubular member (52) having an orifice at one end, with its other end being adapted for connection to a supply of ink containing solvents, and a transducer (58) coupled to the nozzle (50) for transmission of a disturbing energy through the tubular member (52) to cause the ink to form droplets as it leaves the orifice characterized in that the nozzle (50) is molded from polyphenylene sulfide, thus rendering it acoustically soft so that the response of the nozzle (50) to the disturbing energy of the transducer (58) over a frequency in the range of 10 KHz to 100 KHz is substantially flat, the disturbing energy being transmitted to the ink within the nozzle (50) without substantial amplification, attenuation or creation of harmonic resonances of a frequency characterizing the disturbing energy. 4. A method of forming ink droplets from a supply of ink comprising the steps of: supplying the ink to a chamber (50) the walls of which have at least one outlet therefrom through which ink may pass; creating a disturbing energy having one or more predetermined frequencies; transmitting the energy to the ink through the walls of the chamber (50) to form droplets as the ink passes out of the chamber (50), characterized in that the walls of the chamber (50) are molded from polyphenylene sulfide, thus rendering the chamber acoustically soft so that the response of the walls of the chamber (50) to the disturbing energy, over a frequency in the range of 10 KHz to 100 KHz is substantially flat, the disturbing energy being transmitted to the ink without substantial amplification, attenuation or the creation of harmonic resonances of the said one or more frequencies of the disturbing energy.

Inventors: KEUR, ROBERT I., SLISZ, ROGER T., SOURLIS, GEORGE, ZYZNIEUSKI, NIKODEM
Applicants: VIDEOJET SYSTEMS INTERNATIONAL, INC.
Application year: 1987
Main classification: B41J2
IPC codes: B41J29, B41J2