System for controlling droplet volume in continuous ink-jet printer

Abstract

In an ink-jet printing a succession of ink droplets are projected along a longitudinal trajectory at a target substrate. A group of droplets is selected from the succession in the trajectory, and this the group of droplets is combined by electrostatically accelerating upstream droplets of the group and/or decelerating downstream droplets of the group into a single drop.

Description

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a schematic illustration of a prior-art continuous ink-jet printer;

FIG. 2 is a schematic illustration of an embodiment according to the invention of a continuous ink-jet printer for producing variable droplet sizes;

FIG. 3 is a schematic perspective illustration of an embodiment according to the invention of the electrode assembly for velocity modulation of the deflected droplets;

FIGS. 4 a and 4b are graphs the voltage curve based on the embodiment according to the invention shown in FIG. 2 for modulating the velocity for droplet trains following in immediate succession;

FIGS. 5 a, 5b are graphs of the voltage curve based on the embodiment according to the invention shown in FIG. 2 for modulating the velocity in gaps between the successive droplet trains;

FIG. 6 is a schematic illustration of a printing apparatus having an electrode assembly for combining droplets upstream from a deflecting device; and

FIGS. 7 a and 7b are graphs of curves for the voltages between the two electrodes in an electrode assembly according to FIG. 6 for combining the droplets.

SPECIFIC DESCRIPTION

FIG. 1 shows a print head of a known, conventional continuous ink-jet printer. Ink 1 is initially pumped from a supply reservoir 2 into the pressure chamber 5 via conduits 4a by means of a pump 3. A gun or nozzle 6 is provided at one end of the pressure chamber 5. Modulating devices 7 also mounted on the pressure chamber vary the pressure in the chamber 5 such that, at a short distance after emerging, the continuous ink stream 9 emitted from the nozzle 6 breaks up into individual ink droplets 11 having essentially the same size. Shortly before such breaking-up, the individual ink droplets 11 are charged by an electrode 8.

Along their trajectory 100 the ink droplets 11 then pass into an electrical field 21 formed by plate electrodes 20a and 20b of a capacitor 20. Depending on the charge quantity and the polarity of the charges on the ink droplets 11, as well as the polarity and intensity of the electrical field 21 in the field space of the plate capacitor 20, the individual ink droplets 11 are deflected into along different paths 103 and 104 illustrated by way of example.

The total number of possible deflection angles depends solely on the energization levels of the charging electrode 8, and in principle is unlimited. The electrode plate 20a extends parallel to the trajectory 100 while the plate 20b diverges downstream from it, but they could be parallel.

Here the polarity and strength of the electrical field 21 are kept essentially constant, since a change in the field strength acts simultaneously on all the droplets 11 located in the field 21 when the strength or polarity is changes. This makes it impossible to influence a single droplet.

After the ink droplets 11 leave the field space 21 of the plate capacitor 20, electrostatic force no longer acts on the ink droplets 11 that maintain their new paths or trajectories 103 or 104. This results in a fan-shaped set of trajectories. Ink droplets 11 having little or no charge, for example, because they must be eliminated from the print image, are not deflected at all in the electrostatic field 21 of the plate capacitor 20, for example, and strike an opening 19 in a gutter or collection tube 18 for ink recycling back via conduits 4b to the ink supply 2 and is thus recycled.

FIG. 2 shows a schematic illustration of a system according to the invention for producing and deflecting ink droplets 11 having variable droplet size in a continuous ink-jet printer. The droplet themselves are produced in the manner described above with reference to FIG. 1. Of course the droplets 11 could be produced in any other way. It is instead the manner in which the droplets 11 are combined while airborne that is the invention here.

In any case, here, after the droplets 11 form, in a departure from the known design, these individual ink droplets 11 are each provided with the same electrical charge by means of the charging electrode 8.

Along their trajectory 100 the ink droplets 11 then pass through a variable electrical field 44 that is generated by an electrode assembly 40 comprised of parts 40a and 40b extending along and flanking the trajectory. The part 40a comprises a single electrode E₀ extending its full length, and the part 40b comprises a row extending parallel to the electrode E₀ of electrodes E₁ to E_n. The distances between adjacent electrodes E₁ to E_n is the same as the distance between successive ink droplets 11.

Deflection voltages U₀, U₁ to U_n are applied to the respective electrodes E₀ and E₁ by a control circuit 41. If different applied voltages are shifted downstream synchronously with the movement of the droplets 11, that is at the same velocity along the path 100, it is possible to control the lateral deflection of a single droplet, creating an effect similar to that in the prior-art system of FIG. 1. Thus to deflect a single droplet a certain amount a voltage differential is moved at the droplet-travel speed from electrode E₁ to electrode E₂ to electrode E₃ synchronously to pick a single droplet 11 out of the path to the recycle gutter 18. By varying the intensity and duration of the electrical field acting on each ink droplet, different deflection angles may be produced for the ink droplets 11. By discontinuing the concurrent electrical field, no deflection, for example, is imparted to the ink droplets 11 to be eliminated from the print image and recycled.

According to the invention it is also possible to deflect a plurality or group 12 of ink droplets 11. Thus, the same electrical field preferably acts on each droplet of a droplet group 12 in the deflection direction. The number of different drop sizes can be determined by a system controller and is thus not depending of the number of electrodes E₀-E_n of the deflection unit 40. If for example a number of 8 droplets 11 per group 12 chosen, a total number of 8 different drop sizes can be realized which corresponds to a total number of 9 greyscale levels or intensities, including the case of non-printing. Corresponding to a specific grayscale a defined number of droplets of each group leaves the deflection electrode assembly 40 into one specific deflected direction, whereby the number of deflected droplets 11 in each group 12 can be different as described.

The droplet groups 12 thus produced then pass into an electrode assembly 50 comprised of electrodes 50a and 50b, i.e. Ek₁ and Ek₂, having respective openings 51a and 51b. The electrodes 50a and 50b are configured in such a way that the electrical field generated by application of an electrical voltage is directed essentially in the direction of travel of the ink droplets 11. The electrodes 50a and 50b are also designed and configured so that the ink droplets 11 pass through the openings 51a and 51b in the electrodes 50a and 50b, no matter whether they are deflected laterally only slightly, or to a maximum.

Furthermore, the distance between the electrodes 50a and 50b is such that at any time only one individual ink droplet 11 is located in the space between the electrodes 50a and 50b. If an electrical voltage Uk is then applied to the electrodes EK₁ and EK₂, an electrical field develops in this space between the electrodes EK₁ and EK₂ that, depending on its intensity and polarity, either accelerates or decelerates ink droplets 11 present in this field space.

Because only one individual ink droplet 11 is present in the space at any time, the force thus produced acts only on this ink droplet 11. By changing the intensity and/or the polarity of the applied voltage Uk, successive ink droplets 11 may thus be accelerated or decelerated to different degrees. To combine the individual ink droplets 11 in a droplet group 12 into a single ink drop 101, according to the invention the leading droplets 11 in a droplet group 12 are decelerated and the lagging droplets 11 are accelerated, such that after a short distance downstream from the electrode assembly 50 all the ink droplets 11 in the group 12 combine while airborne in a common center of gravity of the droplet group 12. This ensures that the differently sized ink drops 101 thus produced are essentially the same distance from one another, and after the ink drops 101 strike a substrate 200 a print image is obtained that contains different sizes of print dots 201 and also has a uniform distance between print dots. Normally the printing assembly and the target or substrate 200 are relatively moved in a direction perpendicular to the view plane of FIG. 2, that is perpendicular to the transverse direction in which the droplets 11 are deflected from the trajectory 100 by the electrode assembly 40.

FIG. 3 schematically shows in a perspective illustration the deflecting device 40, the downstream electrode assembly 50, and a number of deflection paths 103, 104, and 105 for the ink droplets 11 and schematically illustrated droplet groups 12. The shapes of the electrodes 50a and 50b may be different, and may be, for example, rectangular, circular, oval, or of another shape adapted to the particular system. The same is true for the openings 51a and 51b that preferably may be designed such that the most homogeneous electrical field distribution possible is obtained in the space between the electrodes 50a and 50b traversed by the ink droplets 11. The electrode assembly 50 in the direction of travel of the ink droplets 11 may also have a cylindrical, cup-shaped, or a generally concave design, so that, regardless of the deflection angle of the respective ink droplets 11, the ink droplets 11 consistently traverse the space between the electrodes 50a and 50b in a precise path along the electrical field lines.

FIGS. 4 a, 4b and 5a, 5b schematically show the relationship of the voltage U_k to the respective ink droplets 11 in the respective droplet group 12. FIG. 4a shows by way of example a saw-tooth curve of the voltage U_k from a positive voltage +U_k to a negative voltage −U_k, each segment 13a, 13b, 13c of a saw-tooth voltage interval 13 acting only on the ink droplets 11, illustrated in the drawing above the curve that at that time have traversed the electrode assembly 50. Thus, each drop is acted on only by the field intensity intended for the drop, thereby more or less strongly decelerating or accelerating the droplet.

FIG. 4 b shows by way of example another type of energization of the electrodes 50 and 50b by means of a stepwise voltage curve, so that a different but constant field intensity, corresponding to the voltage acting at this time in the respective segment 13a, 13b, 13c of the voltage interval 13, is imparted to each ink droplet upon passing through the field space. In each case it is advantageous to keep the sum of the accelerating and decelerating voltages constant, particularly preferably equal to zero. The accelerating or decelerating voltages may also have different magnitudes that in particular for the combination of odd numbers of ink droplets 11 may advantageously result in a single ink drop.

It may also be advantageous to superimpose upon each variable accelerating or decelerating voltage for each droplet group 12 a correcting voltage in such a way that deviations in position, which may occur between odd-number and even-number drop volumes, may be compensated for in the labeling plane.

Between the respective droplet groups 12, it may be practical to deflect one or more ink droplets 11 into the gutter in order to technically facilitate the voltage jump between successive droplet groups 12 from −U_k to +U_k illustrated in FIGS. 4a and 4b, for example. The ink droplets 11 missing at these locations are denoted by reference numeral 11b in FIGS. 5a and 5b. FIGS. 4a, 4b, 5a, 5b also show that, depending on the desired size of the resulting ink drop 101, it is not necessary for all ink droplets 11 to be present within a droplet group 12. These missing ink droplets 11 are denoted by reference numeral 11a. It is further noted that each of the illustrated droplet groups 12 may have a different deflection angle.

In another embodiment shown in FIG. 6, the individual ink droplets 11 in a droplet group 12 are combined upstream from the deflecting device formed from the electrode assembly 40 by means of the electrode assembly 50, so that in the deflecting device 40 different sizes of ink droplets 11 may be united corresponding to the desired impact position on a substrate to be printed. In this case, the droplets 11 for the drops to be combined are accelerated or decelerated in the described manner, whereas the voltage U_k for the drops to be masked but is set to zero, as shown in FIGS. 7a and 7b.

Claims

1. An ink-jet printing method comprising the steps of: projecting a succession of ink droplets along a longitudinal trajectory at a target substrate;selecting a group of droplets from the succession in the trajectory; andcombining the group of droplets into a single drop whereby the drop then strikes the target substrate.

2. The method defined in claim 1 wherein the droplets are emitted at a starting end of the trajectory from a nozzle and all have the same size.

3. The method defined in claim 1, further comprising the step of imparting to all of the droplets at a charge location along the trajectory a charge, the droplets all being identically charged at the charge location.

4. The method defined in claim 1 wherein the droplets of the group are combined by decelerating or accelerating droplets in the group.

5. The method defined in claim 4 wherein the droplets are decelerated or accelerated by passing them through an electrostatic field extending along the trajectory.

6. The method defined in claim 5 wherein the electrostatic field is created by a pair of electrodes spaced apart along the trajectory.

7. The method defined in claim 6 wherein a strength of the electrostatic field is varied as the droplets of the group pass the electrodes.

8. The method defined in claim 7 further comprising the step of spacing the electrodes along the trajectory by a distance equal generally to a spacing between succeeding droplets of the succession.

9. The method defined in claim 1, further comprising the step of laterally deflecting the group from the trajectory.

10. An ink-jet printer comprising: nozzle means for projecting a succession of ink droplets along a longitudinal trajectory at a target substrate;means for selecting a group of droplets from the succession in the trajectory; andmeans for combining the group of droplets into a single drop, whereby the drop then strikes the target substrate.

11. The ink-jet printer defined in claim 10 wherein the means for combining includes a pair of charged electrodes spaced apart along the trajectory of the droplets and control means for applying a varying potential to the electrodes such that droplets of the group are accelerated or decelerated.

12. The ink-jet printer defined in claim 11 wherein the electrodes are spaced apart by a distance equal at most to a spacing between succeeding droplets of the succession, whereby an individual droplet can be accelerated or decelerated.

13. The ink-jet printer defined in claim 12 wherein the electrodes are annular and form a passage through which the trajectory passes.

14. The ink-jet printer defined in claim 10 wherein the means for selecting can vary the number of droplets in the group.

15. The ink-jet printer defined in claim 14, wherein the means of selecting includes a control unit and a software to define the number of droplets in the group.

16. The ink-jet printer defined in claim 11, further comprising means for identically charging all the droplets at a charging location along the trajectory, the nozzle means creating droplets all of the same size.