Masters Theses

Date of Award

8-2020

Degree Type

Thesis

Degree Name

Master of Science

Major

Engineering Science

Major Professor

Brett G. Compton

Committee Members

Chad Duty, Christopher Wetteland

Abstract

The geometric design freedom, short lead time, and customization make additive manufacturing (AM) increasingly popular. In addition to rapid prototyping and three-dimensional molds, additive manufacturing has created wind turbine blades, robotic arms, and custom medical implants. Major manufacturing companies such as Porsche and Aetrex are utilizing AM to customize automotive seats and orthopedic footwear. However, available materials limit AM applications. Currently, the high-temperature requirements from the aerospace and automotive industries provide additional, unmet challenges. Many high-temperature epoxies have high pre-polymer viscosities and produce highly exothermic cure reactions, which limits volumetric scaling. Traditionally, fast, high-temperature processing reduces the viscosity, filling a mold before crosslinking initiation; however, this is not possible for AM. Currently, epoxy-fiber composites replace many traditional materials, such as aluminum, in applications where their high strength-to-weight ratios reduce lifetime energy costs. Fiber composites are limited by current fabrication methods, which can be expensive with limited geometric adaptability. Direct ink write (DIW) AM extrudes viscoelastic feedstock, creating parts layer-by-layer. The ink feedstock can readily incorporate fibers while AM produces parts without a mold reducing start-up requirements. This work develops a high temperature, heated cure epoxy feedstock for DIW applications achieving strength and modulus values of 145 MPa and 4.9 GPa, respectively. Two pre-polymers are combined, to maintain a glass transition temperature upwards of 285°C while reducing the viscosity. A heated deposition system requires understanding the thermal viscosity and cure profiles. With a viscosity of 5.4 Pa.s and an 18-hour pot life, 70°C allows for shear flow without premature cure during extrusion. An upper loading limit of 30 vol% glass fibers was determined. The fibers improve the heat deflection temperature by 100°C to 320°C and yield a 160% increase in flexure modulus; however, a 34% reduction in strength occurs. While processing did not decrease the fiber length as observed with carbon, the initial distribution contained 15% of fibers shorter than the critical length. The short fibers and pores that arose from both processing and dissimilar fiber-matrix expansion can account for the reduction. This work aims to develop a hightemperature fiber-filled feedstock while broadly considering print and extrusion parameters of viscous inks.

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