Printability of highly viscous composite slurries via reactive extrusion additive manufacturing
Additive manufacturing (AM) can enable the fabrication of strong and stiff composite parts with complex geometries and without the need for expensive tools and molds. Current composite AM technologies are slow, energy-intensive, and their resulting parts usually suffer poor inter-laminar properties. This paper investigates the printability of carbon fiber filled thermosetting polymers via a novel AM technique, reactive extrusion AM (REAM). In REAM the resin and curing agent from two separate reservoirs are pumped and mixed directly inside the nozzle and deposited layer-by-layer onto a print-bed. The process utilizes exothermic resin/catalyst systems with fast curing cycles, eliminating the need for post-curing and external energy sources. Chemical cross-linking occurs between the printed layers, resulting in parts with relatively isotropic properties. Viscous properties of the thermosetting polymer are tuned using additives (e.g., fumed silica) to ensure printability while achieving sufficient storage modulus/shear yield strength to hold the shape after printing. Similarly, carbon fibers can be added to tune the viscosity and reinforce the resin. While high carbon fiber loadings are desired for improved mechanical properties, it is difficult to print highly loaded resins due to their excessive viscosity. An integrated experimental/numerical study is performed to examine the printability of a highly viscous 30 wt.% carbon fiber filled resin based on the rheology and pump pressure required for the REAM. Temperature and shear thinning were found to control the process and allow for tuning the rheology of the slurries for REAM printability.