System for controlling droplet volume in continuous ink-jet printer

1. An ink-jet printing method comprising the steps of:

projecting a succession of ink droplets along a longitudinal trajectory at a target substrate;

imparting to all of the droplets at a charge location along the trajectory a substantially identical charge;

selecting a group of droplets from the succession in the trajectory; and

combining the group of droplets into a single drop.

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 wherein the droplets are decelerated or accelerated by passing them through an electrostatic field extending along the trajectory to combine the droplets into a single drop.

4. The method defined in claim 3 wherein the electrostatic field is created by a pair of electrodes spaced apart parallel to the trajectory.

5. The method defined in claim 4 wherein a strength of the electrostatic field is varied as the droplets of the group pass the electrodes.

6. An ink-jet printing method comprising the steps of:

projecting at a target substrate a succession of ink droplets along a longitudinal trajectory between a pair of electrodes;

charging the electrodes to create an electrostatic field through which the electrodes pass;

spacing the electrodes parallel to the trajectory by a distance equal generally to a spacing between succeeding droplets of the succession;

selecting a group of droplets from the succession in the trajectory; and

varying a strength of the electrostatic field and thereby decelerating or accelerating droplets of the group to combine the group of droplets into a single drop.

7. An ink-jet printer comprising:

nozzle means for projecting a succession of ink droplets along a longitudinal trajectory at a target substrate;

a pair of charged electrodes flanking the path and spaced apart by a distance equal at most to a spacing between succeeding droplets of the succession;

means for selecting a group of droplets from the succession in the trajectory; and

means for applying a varying potential to the electrodes such that individual droplets passing between the electrodes are accelerated or decelerated and the droplets of the group are combined into a single drop, whereby the drop then strikes the target substrate.

8. The ink-jet printer defined in claim 7 wherein the electrodes are annular and form a passage through which the trajectory passes.

9. The ink-jet printer defined in claim 7 wherein the means for selecting can vary the number of droplets in the group.

10. The ink-jet printer defined in claim 9 , wherein the means for selecting includes a control unit and a software to define the number of droplets in the group.

11. An ink-jet printer comprising:

nozzle means for projecting a succession of ink droplets of the same size along a longitudinal trajectory at a target substrate;

means for identically charging all the droplets at a charging location along the trajectory;

a pair of electrodes flanking the path;

means for selecting a group of droplets from the succession in the trajectory; and

control means for applying to the electrodes a varying potential such that the droplets of the group are accelerated or decelerated and combined into a single drop, whereby the drop then strikes the target substrate.

12. The ink-jet printer defined in claim 11 wherein the charging means is upstream of the means for selecting and of the control means.

13. An ink-jet printer comprising:

nozzle means for projecting a succession of ink droplets along a longitudinal trajectory at a target substrate;

an electrode assembly having to one side of the trajectory a main electrode of predetermined length measured parallel to the trajectory equal at least to a length of a biggest possible group to be selected and to an opposite side of the trajectory a row of secondary electrodes spaced apart by a distance equal generally to a spacing between succeeding droplets of the succession;

means for selecting a group of droplets from the succession in the trajectory; and

control means for applying varying voltages to the secondary electrodes and thereby combining the group of droplets into a single drop, whereby the drop then strikes the target substrate.

14. The ink-jet printer defined in claim 13 wherein the control means applies to the secondary electrodes voltages that vary sawtooth fashion synchronously with movement of the droplets along the trajectory.