Adaptive Extrusion Force Control of Freeze-Form Extrusion Fabrication Processes

Freeze-form extrusion fabrication is an additive manufacturing process that extrudes high solids loading aqueous ceramic pastes in a layer-by-layer fashion below the paste freezing temperature for component fabrication. Due to effects, such as the air bubble release, agglomerate breakdown, and change in paste properties during extrusion as a result of liquid phase migration, the extrusion force is difficult to control. In this paper, an adaptive controller is proposed to regulate the extrusion force. Recursive least-squares is used to estimate the extrusion force model parameters during fabrication and a low-order control scheme capable of tracking general reference trajectories is designed and implemented to regulate the extrusion process. The controller is implemented for sinusoidal reference trajectories and the results demonstrate excellent tracking performance of the adaptive extrusion force controller. Several parts were fabricated with the adaptive extrusion force controller. These results illustrate the need for extrusion force control and that variable reference extrusion force profiles are required to fabricate complex features.