Three Dimensional Printing is a solid freeform fabrication process that has extensive geometric flexibility. Intricate parts are built from powdered materials deposited in layers, and selectively joined with binder droplets from a continuous jet printhead.

One area of application is to generate accurate millimeter and sub-millimeter surface textures on ceramic parts. Used as molds, ceramic parts can transfer those accurate surface textures onto metal castings. An example of such application is for bone in-growth texture on cast orthopaedic prostheses.

This research established design rules and process control strategies to allow the production of functional orthopaedic implant with various porous textures from 3D printed alumina molds. The design rules outline the casting, printing and machine capabilities and limits with regard to the feature geometry, minimum size and accuracy that can be achieved effectively. Minimum mold protrusion size was constrained by the 200 μm primitive size while minimum cavities ≈150 μm relied on powder removal limitations. Mold dimensional integrity was achieved by accounting for the primitive size as tool offset and by developing drop placement control strategies.

An extensive analysis of the printhead was performed since it was found to be the major source of error in the process. Issues such as merged drops caused by air drag and induced charged caused by electrostatic crosstalk were compromising the integrity of the mold's small features. Physical models as well as simplified lookup tables were developed in order to predict merged drops and induced charge effects. It was found that merged drops could be delayed by either increasing the drop Reynolds number or selectively removing specific drops. Undesired induced charge was compensated by altering the charging voltage pattern according to inductive coefficient ratios that could be predicted or measured from drop position or charge information. As a result, printing accuracy was significantly improved in the range of $\pm$5 to $\pm$30 μm depending on the axis.

Casting conditions, related to metal head and freezing time, were also established to counteract metal surface tension and assure adequate mold filling of ≈350 μm cavity size. A variety of functional orthopaedic Co-Cr-Mo cast implants with 25 to 78% porous surface textures and 200 to 1000 μm pore size were produced successfully and submitted for evaluation against the FDA and ASTM standard requirements. Preliminary mechanical test on the textures exceeded easily both standards.