Nozzle-based additive manufacturing of fiber-filled thermosets: a case study on soft and hard materials

The intent of this work is to develop a framework towards nozzle-based additive manufacturing (AM) of thermal curing polymers embedded with fibers, which could enable functional tools and actuators made of hard and soft materials. Thermosetting systems, such as epoxies, cyanate esters (CE), and silicones, represent a class of materials that do not allow reversible liquid-solid phase changes at an elevated temperature. Extruded thermosetting composites have the potential to overcome issues with limited mechanical stability and strength as presented in thermoplastic materials at high temperatures; it also allows further tuning the characteristics of extruded parts by leveraging the filler materials' unique properties. A nozzle-based approach becomes preferable not only because of its simplicity in configuration and ease of use, but also its additional layer of protection as the extruding materials are in closed containers, which is beneficial for unique applications, such as in-space additive manufacturing (ISAM). This work focused on a proof-of-concept solution for AM of thermosetting composites, accompanied with material characterizations, experimental validations and numerical analysis of a scalable, reproducible system. It consists of systematic reviews and studies of the mechanical, thermal, and rheological properties of two representative thermosetting systems, i.e., Ecoflex 0050 and G/Flex. The experimental study includes material processing, extrusion of fiber-embedded polymers, characterization of cure kinetics, rheology of polymers, and evaluation of printed parts. With milled carbon fibers (up to 20 wt%), the experimental results also showed tunable mechanical and thermal performance by using shear-induced alignment of filler materials. Finally, this work suggests two schemes for 2-D and 3-D multiphysics simulations based on the developed models for rheological properties and cure kinetics. The predictive tools could be advantageous in design of parts, elimination of errors, and tuning of processing parameters.Ph.D.Includes bibliographical referencesby Jingjin Xi