Impulse fluid or ink jets are designed and driven so as to eject a droplet of fluid such as ink from the chamber through an orifice of the ink jet device. In many applications, it is not necessary to operate the ink jet device at high performance levels, i.e., at high velocities and long throw distances. However, many applications including industrial applications require high performance ink jet devices.
For example, in various industrial ink jet applications, it is very important to eject droplets at high velocities with long throw distances so as to reach targets some distance from the ink jet orifice while maintaining a relatively small droplet size to create a high resolution dot on the target itself. In order to achieve this result, it is important that the head of the droplet as well as its tail remain attached to each other and travel at the same relatively high velocity.
In the prior art, it has been difficult to achieve high velocity and long throw distances. For example, with expanding piezoelectric transducers in ink jet print heads of the type disclosed in U.S. Pat. No.4,646,106, high performance is achieved in terms of frequency response using fluidic Helmholtz frequencies of from 25 to 50 kHz and comparable piezoelectric length mode resonant frequencies. However, the droplets formed have long tails which tend to lower the droplet velocity and the throw distance thus precluding optimum performance.
U.S. Pat. Nos. 4,523,201 and 4,523,200 disclose similar print heads driven by voltage waveforms having a first pulse of longer duration and a second pulse of shorter duration designed to achieve early break off of the droplet tail. However, the devices disclosed therein are designed to operate at Helmholtz frequencies of less than 50 kHz and there is no disclosure of the effect of exciting higher harmonic frequencies to achieve break off of the tail for producing higher velocity droplets with improved throw distance. Rather the second pulse merely improves aiming.
Reference is now made to FIGS. 1A through 1F which schematically depict the drive waveform in FIG. 1A and the ink jet device itself at various points in time in FIGS. 1B through 1F. Referring to FIG. 1A, the drive waveforms depicted with voltage on the ordinate and time on the abscissa. At time A, the ink jet device as depicted in FIG. 1B is maintained in the quiescent state with the transducer 10 unenergized and a predetermined volume of ink 12 contained within the chamber 14 behind an orifice 16. At time B as shown in FIG. 1A, the transducer 10 is driven by the voltage pulse 17 as shown so as to contract the length of the transducer 10 thereby increasing the volume of ink 12 within the chamber 14 and pulling back the meniscus 18 in the orifice 16 to the position shown.
As shown in FIG. 1C, which corresponds to time C of FIG. 1A, the transducer 10 begins to expand as the voltage is reduced as applied to the transducer 10. As a result, the volume of ink 12 within the chamber 14 begins to contract while advancing the meniscus 18 through the orifice 16 as shown in FIG. 1D. At a slightly later time than C but before D as shown in FIG. 1E, the transducer 10 has nearly returned to the quiescent state as shown in FIG. 1E and a droplet 20 with a ligament 21 has begun to form at the orifice 16. In FIG. 1F which corresponds to time D in FIG. 1A, the droplet 20 has traveled some distance from the orifice 16 with a slowly moving tail 22 attached. As depicted in FIG. 1F, the tail 22 has just broken off from the meniscus 18 at the orifice 16 before the volume of ink within the chamber 14 returns to the condition shown in FIG. 1B. As can be readily seen from FIG. 1F, the tail 22 is elongated in a manner so as to create a "lay over" condition on a target assuming the tail 22 and the head 20 remain attached throughout their flight to the target. Tail 22 which is relatively slow moving as compared to the head velocity which makes the tail grow in length and break up thereby decreasing the overall throw distance to the target.
In the devices of the prior art of the type disclosed in U.S. Pat. Nos. 4,459,601, 4,509,059, 4,646,106 and 4,697,193, ink jet devices have been characterized by Helmholtz resonant frequencies of approximately 40 kHz and piezo length mode resonant frequencies of 45 kHz. The tail which was formed at the meniscus saw pressure disturbances of approximately 45 kHz. As a consequence, the tail would be broken off as shown in FIG. 1F in response to this disturbance during the negative velocity part of the cycle so as to provide a very low acceleration component thereby producing a drop with a high head velocity and low tail velocity and a tail that grows longer over long print gaps producing poor print quality.