Positive pressure drop-on-demand printhead for Three-Dimensional Printing

Abstract

In drop-on-demand printing, wetting out of the binder fluid onto the orifice face typically has been prevented by some combination of the following: coatings on the orifice face, high surface energy fluids, and negative pressure. This non-wetting approach is not practical for low surface energy fluids. A new positive pressure method of drop-on-demand printing has been developed which eliminates the requirement for a non-wetting system; modifications to the binder fluid properties or to the orifice material to ensure non-wetting are no longer necessary for reliable drop formation. In fact, slight positive pressure (relative to atmosphere) is maintained in the binder fluid at the orifice to achieve intentional wetting of a small area around the orifice. The pressure causes the binder meniscus to bulge out from the orifice until it detaches from the orifice edge and wets the orifice face. The binder wetting continues outward from the orifice edge along the plane of the orifice face, and would continue uncontrolled were it not for an additional constraint: the orifice face ends abruptly with a sharp corner to which the meniscus attaches, thus defining the wetting boundary for the binder. With such a boundary in place, the fluid meniscus forms a spherical cap on the orifice face. The thickness of the cap can be varied by adjusting the level of positive pressure. Benefits of this controlled wetting include successful drop-on-demand printing with low surface energy fluids, including ethanol and chloroform, and also successful drop formation using polymer-loaded binder without the build-up of solid polymer deposits at the orifice.

(cont.) Polyacrylic acid (PAA) and polyetherimide (PEI) have been printed in both aqueous and alcoholic binder fluids using an alumina positive pressure nozzle. Forty micrometer diameter drops can be ejected reliably at five meters per second velocity and one kilohertz frequency.