Hybrid Extrusion Force-Velocity Control Using Freeze-Form Extrusion Fabrication for Functionally Graded Material Parts

Freeze-form extrusion fabrication (FEF) is an additive manufacturing (AM) process that uses an aqueous-based paste loaded with ceramic or metal powder to build complex, three-dimensional parts by extruding the material from a syringe onto a solid substrate in a subzero temperature environment. This paper describes the development of an intelligent control methodology for paste extrusion that utilizes a hybrid extrusion force-velocity controller. A plunger velocity controller was used to ensure a steady extrusion flow rate, and an extrusion force controller was developed to precisely regulate the starting and stopping of the extrusion process. Both controllers were coupled with a hybrid control scheme for extrusion-on-demand and air bubble release compensation. The plunger velocity controller successfully regulated the output material composition from two syringes, and the extrusion force controller precisely controlled the extrusion start and stop. Air bubble release compensation reduced the severity of extrusion gap defects and extrusion track thinning resulting from air bubble releases. Monolithic and functionally graded parts were fabricated to illustrate the functionality of the hybrid extrusion force-velocity controller.