Process planning and control of functionally graded parts using freeze-form extrusion fabrication

Freeze-form Extrusion Fabrication (FEF) is an additive manufacturing technology that is capable of fabricating complex three-dimensional parts. FEF works by depositing an aqueous-based colloidal ceramic or metal paste in a layer-by-layer fashion below the freezing point of the aqueous medium as to rapidly freeze the paste. The FEF machine in the present study is equipped with three syringes and is capable of mixing each paste in a desired composition ratio by using an inline static mixer. Compensation for the transport delay caused by the static mixer is necessary; therefore, an algorithm was developed to apply a one-dimensional (1D) composition gradient to monolithic parts, adjusting the plunger velocities to account for the transport delay. Control of extrusion-based additive manufacturing techniques is difficult due to the uncertainties of the paste properties such as entrapped air bubbles, inhomogeneity, and inconsistency of the paste rheology from batch to batch. Other challenges are present such as starting and stopping extrusion on demand (EOD) and steady-state velocity control of a coupled triple extruder system. These issues have been addressed with the development and implementation of a hybrid extrusion force-velocity controller --Abstract, page iv