Structural stability of thermosets during material extrusion additive manufacturing

Over the past decade, the scale of polymer additive manufacturing has been revolutionized with machines that print massive thermoplastic parts with greater geometric complexity than can be achieved by traditional manufacturing methods. However, the heat required to print thermoplastics consumes energy and induces thermal gradients that can reduce manufacturing flexibility and final mechanical properties. With the ability to be extruded at room temperature and excellent compatibility with fibers and fillers, thermoset resins show promise to decrease the energy consumption, expand the manufacturing flexibility, and broaden the material palette offered by large-scale polymer additive manufacturing. However, structural instability in the uncured state has limited the scale of thermoset printing. This dissertation develops models to predict instabilities in printed thermoset structures and evaluates in-process chemically initiated curing to suppress instability.

First, instability is studied using direct ink writing to print thin walls with epoxy-based composite resins. Assuming these complex, viscoelastic resins exhibit elastic-plastic behavior, mechanical models based on measured rheological properties are shown to accurately predict collapse height, linking rheological properties to stability. This study helps estimate the scaling limits of thermoset additive manufacturing.

Second, unsupported, overhanging walls are printed with an epoxy-based composite resin. Again, assuming elastic-plastic behavior, an analytical beam bending model, finite element analysis, and rheological properties are used to predict the yield height and deflected profile of an unsupported overhanging wall. The predicted yield height and deflected profile agree with experimental observations. This study generalizes the link between rheological properties and structural stability, provides tools to analyze overhanging walls, and emphasizes scaling limitations.

Finally, a custom-built, large-scale printer is used to mix organic peroxide with vinyl ester resin immediately before deposition to initiate curing and suppress collapse. A stable processing window is revealed, where curing keeps pace with the rate at which layers are stacked on the print. Outside the stable processing window, successful printing is affected by pre-gel collapse, transient thermal behavior, and warpage. Preliminary modeling efforts are presented and motivate future research.

These studies advance large-scale thermoset additive manufacturing by providing ways to predict structural instabilities and evaluating one way to address instability.