Paper Number
AM6 

Session
Additive Manufacturing

Title
From liquid ink to solid object: Tailoring the microstructure of additively manufactured freeform macrostructures through extensional rheology-mediated extrusion

Presentation Date and Time
October 15, 2018 (Monday) 1:30

Track / Room
Track 3 / Bellaire

Authors (Click on name to view author profile)

1. Owens, Crystal E. (Massachusetts Institute of Technology, Mechanical Engineering)
2. Hart, A. John (Massachusetts Institute of Technology, Mechanical Engineering)
3. McKinley, Gareth H. (Massachusetts Institute of Technology)

Author and Affiliation Lines (in printed abstract book)
Crystal E. Owens, A. John Hart, and Gareth H. McKinley
Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

Speaker / Presenter 
Owens, Crystal E.

Text of Abstract
Additive manufacturing through direct-ink writing constructs 3D objects via the buildup of a filament, and the capabilities of the final part rely directly on the properties of the ink. Commonly, rigid rod-like particles such as carbon nanotubes or glass fibers are added to starting materials in low concentrations as reinforcement or “filler” for end-use 3D-printed parts, substantially improving strength and toughness. In addition, direct printing of electrically conductive materials can enable entirely new functional geometries, such as field emitting devices.

Meanwhile, fiber-spinning processes can create processed structures with the highest specific strength relative to other traditional manufacturing methods (such as molding or extrusion), when using the same starting material. This is because of the additional external control over the development of microstructure during the processing operation, particularly from flow-induced crystallization of polymer chains and/or orientation of rigid rod-like structures from the strong extensional kinematics. Using these extensional flows along with 3D printing can enable microstructure control.

In this work, we study the flow-mediated structure evolution of rigid rod-like particle-based inks, especially using single-walled carbon nanotubes (SWCNTs) dispersed in water, dimethylformamide, or ionic liquids at concentrations from 0.1-1%wt, inside a bespoke microfluidic spinneret nozzle designed to impose extensional flow and control the tension at all locations along the forming filament. We report the influence of processing parameters such as the ratio of the sheath and filament flow rates, rate of elongation, and rate of solvent exchange, on the strength, conductivity, and dimension of the extruded filament and of the final printed object, and develop scaling relationships that connect these to the rheological properties of the ink.