BRIEF DESCRIPTION OF THE DRAWINGS
In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
FIG. 1A shows a continuous ink jet print head forming a stream of ink drops;
FIG. 1B shows a continuous ink jet print head forming a stream of main ink drops and interspersed satellite droplets;
FIG. 2 is a diagram of infinite satellite droplet formation and the driving waveform;
FIG. 3 is a diagram of rearward merging satellite droplet formation and the driving waveform;
FIG. 4 is a diagram of forward merging satellite drop formation and the driving waveform;
FIG. 5 is a chart showing modulation of the relative drop characteristics;
FIG. 6 is a series of charts showing satellite droplet and main drop characteristics varying with duty cycle;
FIG. 7A illustrates another embodiment of the present invention wherein infinite satellite droplets are modulated by offset pulse pairs; and
FIG. 7B describes the offset pulse pairs of FIG. 7A.
DETAILED DESCRIPTION OF THE INVENTION
The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
The major components of a continuous ink jet printing system are well known, and have not been illustrated in this disclosure. For those who are unfamiliar with continuous ink jet printing, reference is made, for example, to U.S. Published Application US 2003/0193551 A1. Elements of such a printer that are relevant to the present invention are shown in FIG. 1 of the present disclosure. Generally, an ink supply chamber 1 directs ink toward a nozzle orifice plate 3. A stimulation device 2 is provided to create an ink jet 4 and to control break-off of ink drops 5. While a piezoelectric stimulator has been selected for the illustrated embodiment, it should be understood that one skilled in the art would understand how to apply other stimulation methods according to the present invention.
Ink jet 4 in a print head has a velocity determined, in part, by the pressure of the fluid in chamber 1 behind the nozzles, the diameter/geometry of the nozzles, and the viscosity of the fluid. According to Rayleigh (see generally, Lord Rayleigh, “On the Instability of Jets,” Proc. London Math. Soc. X (1878)) stimulation of the fluid jet by a stimulation device 2 creates perturbations on the fluid jet and if the distance between the perturbations on the fluid jet, defined as λ, is equal to or greater than πD, where D is the diameter of ink jet 4, then the fluid ink jet will produce drops 5 at certain frequencies. If the relation λ/D>π is maintained throughout the stimulation process, the ink jet will eventually break up into a series of drops. The drops produced by this process, referred to as “main drops,” have a volume, V, determined from the flow rate of the ink jet, Qv, and the stimulation frequency, Fs, such that V=Qv/Fs.
Ink jet printer design requires balancing the desire for increased resolution associated with smaller drop sizes with the disadvantage that the smaller nozzle diameters required to produce small drops are more prone to clogged nozzles and crooked jets. Furthermore, smaller nozzle diameters require higher ink pressures. Accordingly, it is an object of the present invention to provide a method for selectively creating small satellite printing droplets from a large diameter ink jet nozzle. According to a feature of the present invention, the ink jet from a larger nozzle is stimulated in a manner to produce small satellite droplets. The physical size of satellite droplets is significantly smaller than the main drops created from the same size nozzle. Thus, the invention provides a method of selectively creating and controlling satellite droplets, along with the main drop, to allow for printing with a small volume droplet from a large diameter nozzle drop generator.
One method for generating these satellite droplets in a manner that allows selection control and appropriate volumes without changing the physical dimensions of the print head or the operating pressure of the fluid is to stimulate the ink jet in a controlled way to produce an infinite satellite droplet.
FIG. 1B illustrates the creation of a satellite droplet 7 from a stranded fluid ligament 8 between the main drops and the ink jet at the drop break off point. The production of satellite droplets by stimulating continuous jets is well known. Conventionally satellite droplets create printing errors and are considered undesirable. However, the satellite droplets can be useful for ink jet printing when they are produced in a controllable manner according to the present invention. In a stable condition wherein the satellite droplets have a velocity approximating the velocity of the main drops, the satellite droplets will not merge with the main drops for a considerable distance from the nozzle. These droplets, shown in FIG. 2, are referred to as “infinite” satellite droplets 9, which means they have, in the context of a print head, an infinite lifetime.
Satellite droplets are typically produced from a continuous ink jet drop generator by adjusting the amplitude and frequency of the ink jet stimulation until a breakup profile similar to that depicted in FIG. 2 is realized. If the satellite droplets have a smaller velocity than the main drops, then these satellite droplets will merge with the main drops immediate to the rear and are referred to as rearward-merging satellite droplets 10 (see FIG. 3). Likewise, if the satellite droplets have a velocity greater than the main drops, the satellite droplets will merge with the main drop immediately preceding and are referred to as forward-merging satellite droplets 11 (see FIG. 4). In both the rearward and forward merging situations, it will be appreciated that the time required for satellite-to-main drop merger is proportional to the differences between their respective velocities.
There is no well-defined velocity difference defining a satellite droplet as infinite, rearward- or forward-merging. In one embodiment, the practical criterion for infinite satellite droplets is a velocity difference that does not produce a satellite-main drop merger before a characteristic distance has been traversed. For example, if the satellite droplets remain unmerged with the main drop through the deflection region of a continuous ink jet print head, then those satellite droplets could be considered infinite. The criterion used for assessing the production of satellite droplets in FIG. 2 was that no merger occurred within 10 λ after droplet break off point.
The preferred, and yet more detailed and complex method of defining the infinite satellite is to consider the values of λ and D. For example, in a drop generator in which a stimulation device surrounds each nozzle for ejecting standard water-based inks, the stimulation device is operable such that it perturbs the surface of the jet of fluid when a driving pulse is received. The driving pulse is defined by shape, amplitude, and duty cycle. When successive drive pulses, having a well-defined frequency, are delivered to the stimulation device, the jet of fluid will break up into a series of equal-volume drops, as mentioned above. Additionally, by operating the stimulation device in a manner to provide a specified λ and D value, it is possible to generate satellite droplets. Particular λ and D value are thus capable of providing the infinite satellite droplet condition, as shown in FIG. 2. For the specific drop generator and stimulation device used in FIG. 2, the λ and D value required for infinite satellite droplet formation occur when λ/D is 6.1. Likewise, other λ and D values would be capable of producing either rearward- or the forward-merging satellite droplets as demonstrated in FIGS. 3 and 4.
The value of λ/D=6.1 for creation of the infinite satellite droplet condition is not arbitrarily. As shown in FIG. 5, this value corresponds to an operating point and the location of local minimum in the drop break off length as a function of λ/D. The smooth curve labeled “Rayleigh Theory” is the normalized calculated value of the growth rate associated with a periodic disturbance on the surface of a liquid stream as a function of λ/D obtained from Rayleigh's well-known analysis on liquid jet stimulation. The dashed curve labeled “Measured” in FIG. 5 was created from the normalized measured values of jet break off length as a function of λ/D (shorter break off length corresponds to larger growth rate) with the preferred continuous ink jet system.
As can be seen from FIG. 5, the simple prediction of Rayleigh reasonably describes measured data up to a λ/D or approximately 6. Above this value; however, the measured data departs significantly from the Rayleigh theoretical curve. The transition point at λ/D=6.1 in FIG. 5 indicates the point of agreement-to-disagreement between the theoretical curve and the experimentally measured data. It is the previously stated jet parameter of λ/D˜6 that provides the necessary fluid dynamics to generate the infinite satellite droplets described by the preferred embodiment.
One skilled in the art will notice that the jet disturbance growth minimum will not always be λ/D˜6 for all fluid and drop generator combinations. For example, this minimum has been observed to be as high as 7.5 for some systems, and is generally described as the local minimum in the Rayleigh normalization growth rate for λ/D greater than π. In addition, generation of infinite satellite droplets is not restricted to the value of λ/D that is exactly equal to the minimum, but reflects the preferred embodiment given the printing system and components used for the diagrammed examples. By adjusting the stimulation pulse duty cycle and amplitude, infinite satellite droplets have been generated at λ/D values of up to +/−10% of the growth rate minimum λ/D value.
In other embodiments, it becomes possible to control the volume of satellite droplets over a limited range in addition to satellite droplet lifetimes. Referring to FIG. 6, the control over droplet volume modulation is accomplished by adjusting the duty cycle and/or amplitude of the stimulation pulses for a given frequency and jet of fluid velocity. It is the disturbance of the jet of fluid at the growth rate minimum that provides the means for infinite satellite droplet generation and not the actual value of λ/D at that minimum.
The data contained in FIG. 6 demonstrate a relation between satellite droplet volumes (or diameters) and the duty cycle of the continuous ink jet system driven by pulses similar to those shown in FIG. 2. These data reveal the ability of various embodiments to produce satellite droplets of variable volumes by merely changing the duty cycle on the stimulation device. In yet other embodiments, similar results are obtained by varying the pulse amplitude and frequency.
Continuing now to FIGS. 7A and 7B, another embodiment is illustrated wherein the infinite satellite production of a particular ink jet can be modulated in a binary fashion for a single nozzle by replacing an infinite satellite generating pulse pair 12 with an offset pulse pair 13, as described in FIG. 7(B). The offset pulse pairs 13 cause the ink jet break-off dynamics to be modified in such a way that no infinite satellite droplets are produced for two pulse periods. The offset pulse pair 13 replace one infinite satellite generating pulse pair in the pulse train resulting in infinite satellite production control, as shown in FIG. 7(A) images i-vi. Images (i)-(vi) have a constant duty cycle with the periods varying in a pair-wise fashion. The waveform shaded by diagonal-dashes denotes the infinite satellite generating pulse pair 12 while the shaded waveform denotes an example of an offset pulse pair 13. Image (i) demonstrates the continuous cycle of the infinite satellite generating pulse pair 12 waveform. Images (ii)-(vi) show infinite satellite modulation by replacing successive infinite satellite generation cycles with an offset pulse pairs 13. It should be appreciated that the offset pulse pairs 13 have the same amplitude as the infinite satellite generating pulse pair, but have been altered in FIG. 7B to make slight differences apparent and for illustrative purposes only.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
PARTS LIST
1) Ink supply chamber
2) Stimulation device
3) Orifice plate
4) Jet of ink
5) Ink drop
6) Main drop
7) Satellite droplet
8) Fluid ligament
9) Infinite satellite droplet
10) Rearward merging satellite droplet
11) Forward merging satellite droplet
12) Infinite satellite generating pulse pair
13) Offset pulse pair
Patent Application
20070291058
