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Connor Armstrong

Mechanical Engineering Ph.D. Student

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Previous Work

Dynamic Control of Fiber Orientation for Additive Manufacturing via a Soft-Actuating Nozzle

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Additive manufacturing (AM) of short fiber reinforced composites offers unprecedented material tunability including improvements in tensile strength, flexural strength, and electrical conductivity. Spatially varied fiber orientation distributions provide even greater material tunability; acoustical perturbations, magnetic fields, and rotational shearing have been used to control fiber orientation for composite AM. However, acoustical and magnetic field approaches require strong field interactions are therefore limited in scalability. Here we present a novel method for continuous control of fiber orientation for direct ink writing (DIW) AM using a soft-actuating dynamic extrusion nozzle. We explored the relationship between actuation pressure and extrusion channel inlet area as well as extrusion channel divergence angle impact on fiber orientation. We found that inlet area could be varied down to 12% of the original area which resulted in a reduction of fibers aligned to the direction of flow by 40%. To demonstrate the continuous mechanical tunability of extruded material, we investigated the impact variable fiber orientation on swelling strain relating to hydration triggered 4D printing. By creating an extrusion nozzle with varying extensional flow zones via channel divergence angle, we were able to produce fibrous hydrogel composites with spatially varied fiber orientations using DIW.

 

Continuous Polymer Compounding Fuse Filament Modeling

Controlled processing of carbon microfiber (CMF) reinforced polymers widens control of material properties of fabricated parts. Continuous transfer from compounding to Fused Filament Modeling (FFM) platform brings this advantage to additive manufacturing. CMF reinforced composites are compounded using a co-rotating twin screw extrusion machine (CoTSE). Controlled, direct transfer from the CoTSE to FFM is accomplished using a mechanical system comprised of interconnected feedback control subsystems. Controlled transfer of CMF reinforced composite polymers is studied over a selected range of temperatures, volumetric flow conditions, and microfiber weight fractions using the system. Characteristics of the produced materials are discussed with respect to CMF weight fractions and processing conditions.

 

 

Characterization of Stress in A Twin-Screw Extruder for Processing and Extrusion of Extrinsically Self-Healing Thermoplastics

Capsule breakup percentage in a co-rotating twin screw extruder is studied for the purpose of producing extrinsically self-healing polymers. A method of real-time characterization of stresses using calibrated stress beads and an optical probe was devised for this research. Three different strengths of stress beads are used to represent poly(urea-formaldehyde) (PUF) encapsulated healing agents. Stress bead breakup percentage was depicted over a selected range of statistically significant operating conditions: screw speed (N) and specific throughput (Q/N). Central composite design grids were created to analyze experimental results and generate a set of predictive equations for stress bead percent breakup. This paper examines the relationship between co-rotating twin screw extrusion operating conditions and breakup of PUF encapsulated extrinsic healing agents. It also marks the first step in understanding extrusion of efficient self-healing polymer composites.

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