Fluid jet deflection by modulation and coanda selection

Abstract

An ink jet recording system emits a stream of ink which is amplitude or frequency modulated to produce discrete droplets. A weir is located adjacent to the trajectory of the droplets, downstream from the jet orifice, and at a critical location near the point of drop formation where it contacts and deflects droplets of larger transverse diameter. Amplitude modulation yields ink drops of basically the same volume which break off before and after the weir, with those which break off earlier being deflected during an initial interval while they have a large transverse diameter. In frequency modulation the actual size of the drops and ultimate diameter are modulated. Such deflected droplets separate from the stream closer to the jet orifice. The deflected droplets are caught in a gutter.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ink jet recording and more particularly to means for forming a continual stream of ink droplets from a continuous stream of ink and then deflecting the droplets formed in response to an input signal.

2. Description of the Prior Art

In the prior art, deflection of an ink jet stream has been effected by control of electrostatic, and electromagnetic deflection. In addition, aerodynamic switching has been provided by variation of stimulation energy combined with provision of a transverse air flow as shown by Robertson U.S. Pat. No. 3,709,432 in which a fluid stream is deflected to a catcher, but separate drops are deflected less and reach the target.

The electromagnetic and electrostatic deflection equipment require, in addition to excitation or drop formation means, separate equipment for deflection downstream from the orifice such as magnetic coils as deflection plates in addition to a power supply. In addition magnetic deflection means provide relatively slow changes in deflection angle.

The use of variable excitation plus a transverse air current as shown in U.S. Pat. No. 3,709,432 requires a separate source of pneumatic pressure and shows a substantial chain of drops extending beyond the air slot, so that no suggestion is made that individual drops can be selected on a one for one basis. Rather, the dot stream is shown as being either on or off.

The use of a curved surface to carry drops of ink into a catcher after they have hit the surface 70 of the catcher is shown in U.S. Pat. No. 3,777,307 of Duffield. The drops hitting surface 70 are given an electrical charge during formation and then deflected by an electrical field applied between a deflection ribbon and the catchers. The deflection away from the stream is completed by the time the drop intercepts the catcher, and is independent of drop diameter.

SUMMARY OF THE INVENTION

An object of this invention is to provide a new fluid drop selection technique for switching trajectories of a fluid along alternate trajectories.

A second object of this invention is to provide a fluid drop selection technique wherein alternate drops are routed along separate trajectories without providing any additional deflection force to the system, other than drop formation excitation.

In accordance with this invention a fluid jet switching system is provided in which a high speed stream of fluid is deflected by first modulating the diameter of the stream of ink to produce discrete droplets. The droplets are sent past a deflecting surface adjacent to the stream and located down stream from the jet at a critical location where it deflects droplets separated from the stream within a predetermined range of distances from the jet in response to a predetermined range of modulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic sectional view of an ink jet ejection system made in accordance with this invention.

FIG. 2 shows a waveform of the voltage signals applied to the excitation electrode of the ink jet.

FIGS. 3A and 3B show a side view or profile of ink drops in response to various voltage levels of excitation applied at the ink jet as the ink drops form and pass the ink deflecting weir.

FIG. 4 shows an elevational view of ink ejection nozzles taken along line 4--4 in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An ink jet ejection system shown in FIG. 1 includes a pressure regulated variable output pump 10 preferably made of stainless steel supplying ink to a stainless steel manifold 11 connected to an ink jet 12 composed of a block 13 preferably made of quartz secured to the manifold 11. An orifice 14 with a diameter of about 0.002 inches is formed in block 13 by electron beam milling or the like.

The orifice 14 is about 0.050 inch long, extending through block 13. Orifice 14 communicates with manifold 11 through opening 15. Pump 10 supplies ink under pressure from reservoir 16 to manifold 11 through lines 17 at a pressure level of 25-50 psi so that a continuous stream of ink 18 is ejected from the orifice 14. Air under pressure in manifold 11 and pressure sensor 25 controlling pump 10 via line 26 regulate pressure in manifold 11.

In FIG. 4, several orifices 14 are shown in parallel alignment with printed circuit electrodes 19 formed around them connected to control wires 20 for connection to control circuits 21, which generate a D.C. voltage of about 180 Volts and a series of pulses shown in FIG. 2 having an amplitude of about 10-20 volts yielding higher A1 pulses intended to prevent printing and lower A2 pulses intended to produce printing. The effect produced by the A1 and A2 pulses respectively upon the ink stream is to perturb the ink jet stream by modulating the waveform envelope of the ink. Relatively high voltages cause the ink jet stream to form relatively larger diameter drops transversely with respect to the axis of the ink jet orifice 14. A curved surface "weir" 22 is located to contact slightly more than tangentially at its apex 23 the path of the drops excited by the larger A1 pulses. But apex 23 is spaced away from the path of the drops excited by the smaller A2 pulses. Thus the larger drops strike the surface of weir 22 which is curved in such a way that the drops attach to the surface in accordance with the Coanda effect as shown in FIG. 3A. Portions of such drops detach from the wall but their path is deflected to a lower angle to a sufficient degree so they strike the baffle 30 and spill back into the gutter 31 flowing through drain hole 27 to drain line 28 returning to reservoir 28. Baffle 30 prevents deflected ink from striking the paper 29.

FIG. 2 shows a series of A1 and A2 pulses from control circuit 21 of 20 and 10 volts respectively on top of a D.C. bias of 180 volts applied to control wire 20. The larger A1 pulses cause greater perturbations of the ink jet 12 as shown in FIG. 3A in which case the outer amplitude of the wave is larger and the breaking off of drops from the integral stream occurs earlier than for the A2 jet stream of FIG. 2. Note in FIG. 3A that the A2 drop just above weir apex 23 is just barely clearing that surface without touching it or grazing it and like other A2 drops, it will pass over baffle 30 to strike a target 29. The A1 drops beyond weir apex 23 decline in elevation along the space defined by a line at angle .theta. with respect to the usual A2 path of drops towards the target, with portions of the drops hugging the angle .theta. line and portions attached to the curved surface of weir 22 as a function of curvature, the kinetic energy contained in the drops, and the surface tension forces within the drops.

Preferably the apex 23 is spaced within a range from 0.040 to 0.150 inch, for example, 0.080 inch away from the nozzle at the apex 23 with a radius of curvature of 0.040 inch. The angle .theta. is selected as 7.degree. to 8.degree.. The jet velocity is 700 inches/sec. However, the location of the apex 23 is a function of jet velocity, excitation and jet diameter which determine the distance at which the jet separates into drops.

It is also possible to follow the separation stage beyond baffle 30 with a raster scanning electrostatic or magnetic deflection unit.

The ink can include an electrolyte such as HCl although it is preferred that the excitation be achieved by electrostatic forces without current flow between electrodes 19 and the ink jet 18.

The curved surface can be composed of quartz as shown or brass, aluminum, TEFLON (polytetrafluoroethylene) or a porous material pumped down by pumping means into line 28 to provide filtration.

Physical Concepts Applied in Embodiment

A periodic perturbation of a cylindrical jet of fluid causes it to disintegrate into droplets of uniform size and spacing as shown in FIG. 3A. The frequency of the perturbation f, the velocity v of the jet, and the drop spacing .lambda. are in the relation

v =

The drop separation distance L depends on the amplitude a of the perturbation. From the simple theory of the drop formation process it is inferred that the perturbation grows exponentially in time with a growth rate g which depends on the surface tension of the fluid. Thus the drop separation distance is given approximately by ##EQU1## where D/2 is about one jet radius. The most unstable mode of the jet corresponds to a drop spacing .lambda. of about 4 1/2 jet diameters, or, using (1), to a frequency of perturbation ##EQU2## At this frequency, one easily infers that the ratio of the diameter of the unperturbed jet and to the diameter of the drops d is about 1/2, ##EQU3##

At a fixed amplitude of perturbation, there is a portion of the convex curve of tangency to the modulated jet which is exponentially increasing in amplitude to distance L and by varying the amplitude of the modulation the drop separation point can be shifted between the boundaries of this exponential rise. The above properties of capilliary jets are well known and easily demonstrated.

Less familiar but equally demonstrable is the fact that if a capilliary jet or drop strikes a convex solid surface 22 as depicted in FIG. 1, with an impact parameter b of less than one drop radius, then it flattens and adheres to the surface provided the radius of curvature of the target, the drop diameter, and the velocity of the drop or jet are suitably chosen. In general, an impact parameter b of about 1/6 of a drop diameter is sufficient to cause capture of a drop by a suitable convex target surface.

The phenomenon of adherence and capture of a capilliary drop or jet described above can be used to capture selectively deflected drops from a jet subjected to a perturbation of fixed frequency and amplitude as depicted in FIG. 1.

The amount of deflection necessary to capture a drop by this means is about 1/10 the amount required by usual means such as electrostatic deflection. In those cases, what corresponds to the impact parameter b must be one drop diameter plus a margin of clearance.

There are two means in accordance with this invention of capturing capilliary drops which do not require any selective deflection whatever. The first is by "frequency modulation" and the second is by "amplitude modulation" of the perturbation a.

A. frequency Modulation

If the frequency of the perturbation is changed by a factor of 2 and the velocity is held constant, the diameters of the resulting drops are in the ratio ##EQU4##

Thus, if the target is disposed relative to the nozzle at some distance larger than the drop separation length L, and such that the smaller (high freq.) drops graze the target and are not captured, the larger (low freq.) drops will have an impact parameter b of about .125d.sub.1. By this method, two or more drops in sequence may be abstracted from a uniform stream of drops of the smaller size. Printing in this scheme is achieved by blanks corresponding to removal of an even number of drops.

B. amplitude Modulation

The preferred method of capturing an arbitrary subsequence of a uniform drop stream is by modulation of the amplitude of the perturbation of the jet. This scheme of capturing drops without any selective deflection is as follows. Two levels of the amplitude of the perturbation are chosen. To each level there corresponds a drop separation distance, say L.sub.1 and L.sub.2. At a distance L.sub.1 < L < L.sub.2 from the nozzle a convex target is placed such that, at the smaller amplitude, the continuous portion of the jet just grazes the target, or has a slightly negative impact parameter, as in FIG. 3B. At the larger amplitude of perturbation the drop detachment point lies between the nozzle and the target as in FIG. 3A. Since the ratio of drop to jet diameters is about 2 at the optimal frequency, a difference in impact parameters of about one drop radius can be achieved by suitable location of the target.

Advantages of this method of drop shuttering are

a. No electrostatic fields, electrodes or other deflecting means are necessary.

b. The throw distance from nozzle to paper can be as small as 1/4 inch, thus practically eliminating aerodynamic errors in placement accuracy.

c. The only electronic circuits needed are for the drop formation generator.

d. The only material property of the fluid relevant to the process is its surface tension and even this does not have to be controlled too closely.

A multiple nozzle printing element operating under this principle must have separately addressable drop generators so that the amplitude of each perturbation can be separately controlled. Several schemes for achieving this seem possible.

Claims

What is claimed is:

1. A fluid jet switching system including jet means for producing a high speed stream of fluid, means for modulating the diameter of said stream of fluid to produce discrete droplets, a droplet deflecting surface adjacent to said stream of fluid located downstream from said jet means at a critical location adapted to contact with an impact parameter sufficient to cause capture of a drop by said convex surface and thereby deflect droplets separated from said stream within a predetermined range of distances from said jet means in response to a predetermined range of modulation.

2. A fluid jet switching system including jet means for producing a high speed stream of fluid, means for modulating the diameter of said stream of fluid to produce discrete droplets, a droplet deflecting surface adjacent to said stream of fluid located downstream from said jet means at a critical location adapted to contact and thereby deflect droplets separated from said stream within a predetermined range of distances from said jet means in response to a predetermined range of modulation, said droplet deflecting surface comprising a curved surface having an apex parallel to the direction of said jet adjacent to the drop detachment point of said jet.

3. Apparatus in accordance with claim 1 wherein said means for modulating comprises a source of varying electrical potential with an electrode adjacent the end of said jet means nearest said deflecting surface.

4. A fluid jet switching system including jet means for producing a high speed stream of fluid, means for frequency modulating to vary the size and diameter of said stream of fluid to produce discrete droplets with varying diameters, a convex droplet deflecting surface positioned immediately adjacent to said stream of fluid located downstream from said jet means at a critical location adapted to contact selected droplets with an impact parameter sufficient to cause capture of a drop by said convex surface having a lateral diameter greater when passing said deflecting surface than a predetermined diameter thereby providing a transverse deflection force to said selected droplets, said selected droplets being generated by a predetermined amount of modulation.

5. A fluid jet switching system including jet means for producing a high speed stream of fluid, means for modulating the diameter of said stream of fluid to produce discrete droplets, a droplet selection deflecting surface adjacent to said stream of fluid located downstream from said jet means at a critical location adapted to contact and thereby deflect droplets larger than a predetermined diameter from hitting a target, when said droplets are within a predetermined range of distances from said jet means in response to a predetermined range of modulation, and said contact of droplets being with an impact parameter sufficient to cause capture of said larger diameter droplets by said selection surface.

6. Apparatus in accordance with claim 5 wherein said droplet selection deflecting surface comprises a curved surface having an apex parallel to the direction of said jet adjacent to the drop detachment point of said jet.

7. Apparatus in accordance with claim 5 wherein said means for modulating comprises a source of varying electrical potential with an electrode adjacent the end of said jet means nearest said deflecting surface.