Droplet ejection apparatus alignment

1. An assembly for depositing droplets on a substrate during relative motion of the assembly and the substrate along a process direction, the assembly comprising:

a first printhead module and a second printhead module contacting the first printhead module, each of the printhead modules including an array of nozzles through which fluid droplets can be ejected,

wherein each nozzle in the first printhead module's nozzle array is offset with respect to a corresponding nozzle in the second printhead module's nozzle array in a direction orthogonal to the process direction, and

wherein the first printhead module comprises at least one alignment datum that registers with a corresponding alignment datum on the second printhead module, the alignment datum of the first printhead module comprising a precision surface offset from an adjacent region of the first printhead module, and the precision surface being smoother than a surface of the adjacent region of the first printhead module.

2. The assembly of claim 1 , wherein each of at least some of the nozzles in the first printhead module's nozzle array is offset by an amount less than the spacing of adjacent nozzles in the nozzle array.

3. The assembly of claim 1 , wherein the precision surface has an arithmetical mean roughness of less than about 10 micron.

4. The assembly of claim 1 , wherein the arrays of nozzles of the first and second printhead modules each comprise a row of regularly spaced nozzles.

5. The assembly of claim 1 , further comprising one or more additional printhead modules, each additional printhead module being coupled to the first and second printhead modules to form a 2D printhead array.

6. The assembly of claim 5 , wherein each additional printhead module contacts at least one other printhead module.

7. The assembly of claim 1 , further comprising a fluid supply configured to supply the first and second printhead modules with a fluid.

8. The assembly of claim 1 , further comprising a frame having an opening extending through the frame and configured to expose the nozzles of the first and second printhead modules when the printhead modules are mounted in the frame.

9. The assembly of claim 8 , wherein the frame comprises a spacer.

10. The assembly of claim 8 , wherein the frame comprises a registration plate, the alignment data of the first and second printhead modules being on the registration plate.

11. The assembly of claim 10 , wherein the registration plate comprises a rigid material.

12. The assembly of claim 10 , wherein the registration plate comprises a material having a similar thermo-mechanical property to a material from which printheads in at least one of the first and second printhead modules are formed.

13. The assembly of claim 1 , further comprising a clamp securing the first printhead module to the second printhead module to form a 2D printhead array.

14. The assembly of claim 1 , wherein at least one alignment datum of the first printhead module or the second printhead module comprises multiple precision surfaces that register the first and second printhead modules relative to each other in multiple directions.

15. The assembly of claim 1 , wherein each alignment datum of the first printhead module and the second printhead module comprises a planar surface, a protruding surface, or a recessing surface.

16. The assembly of claim 1 , wherein at least one of the first printhead module and the second printhead module comprises a drop-on-demand ink-jet printhead module.

17. The assembly of claim 16 , wherein the drop-on-demand ink-jet printhead module comprises a piezoelectric drop-on-demand ink-jet printhead module.

18. The assembly of claim 1 , wherein the assembly is incorporated in an ink-jet printing device.

19. The assembly of claim 18 , wherein the ink-jet printing device has a maximum resolution eater than 500 dpi.