Method and system for controlling drop collisions in a drop on demand printing apparatus

A method of producing a three-dimensional object by combining drops in flight, the method comprising:

• (a) providing a drop-on-demand printhead having at least two nozzles each dispensing a respective liquid;
• (b) selectively discharging a plurality of first drops from a first nozzle and a plurality of second drops from a second nozzle such that at least some of the first and second drops coalesce in flight and form a plurality of combined drops;
• (c) directing the combined drops onto a build surface to form layers of a three-dimensional object;
• and
• (d) during the build process, monitoring a collision outcome of at least one pair of said first and second drops via a measurement system and automatically modifying one or more nozzle control parameters to maintain a desired collision outcome for subsequent drops.

A drop-on-demand printing system configured to coalesce drops in flight, comprising:

• (a) at least a first dispenser and a second dispenser, each configured to discharge drops of respective liquids along intersecting paths;
• (b) a substrate positioned to receive combined drops formed by coalescence of the drops discharged from the first and second dispensers;
• (c) at least one sensor arranged to observe a flight parameter of at least one combined drop while in flight;
• and
• (d) a control unit operatively coupled with the dispensers and the at least one sensor, wherein the control unit is configured to:
• compare the observed flight parameter to a target criterion;
• and
• automatically modify one or more discharge settings of at least one of the first dispenser or the second dispenser if the observed flight parameter deviates from the target criterion,
• whereby the system operates in a feedback loop to regulate collisions of the drops for printing purposes.