Enabling New Material and Process Capabilities for Ultraviolet-Assisted Direct Ink Write Additive Manufacturing via Exploration of Material Rheology and Reactivity

Ultraviolet-Assisted Direct Ink Write (UV-DIW) is a material extrusion additive manufacturing (AM) technology in which a viscous ink, often at room temperature, is selectively extruded through a translating nozzle to selectively deposit material. The extruded ink is solidified via UV irradiation (photocuring) and three-dimensional parts are created by repeating the process in a layer-by-layer fashion. UV-DIW is an attractive AM technology due to its ability to (1) extrude highly viscous inks (i.e. >10,000 Pa·s if ink exhibits shearthinning behavior) (2) the promise of leveraging the broad photopolymer material library and chemistries established for other AM technologies capable of processing photopolymers and (3) the promise of processing a wide range of inks, which enables the fabrication of metal, ceramic, polymer, bio-based, and multi-material parts. Currently, the technology faces a few shortcomings including (1) limited material selection for UV-DIW due to requirement for inks to be photocurable and limited mechanical properties of photocurable materials (2) lack of feature resolution and topological complexity of printed parts and (3) lack of material screening models providing robust definition of the material requirements (e.g., viscosity, cure time, strength) for successful UV-DIW printing. To address these shortcomings, the goal of this work is to gain a fundamental understanding of the rheological and reactive properties required for successful Ultraviolet-Assisted Direct Ink Write (UV-DIW).

The first approach to answering the fundamental research question is establishing the existing rheology experiments used to characterize DIW inks and the relationships between rheology and printability. An in-depth literature review of the techniques and relationships was compiled to better understand ink requirements for successful printing (Chapter 2). This broad survey is not limited to only UV-DIW, but includes all variations of DIW. The first part of the review provides a summary of the rheological experiments that have been used to characterize a wide variety of DIW inks. The second part of this review focuses on the connections between rheology and printability. This survey helps identify the required rheological properties for successful printing that is then used throughout the rest of this work. Additionally, this review identifies shortcomings in current work and proposes areas for future work. From this exhaustive literature review, a systematic roadmap was developed that investigators can follow to quickly characterize the printability of new inks, independent of that ink's specific attributes (Chapter 3). The roadmap simplifies the trends identified in literature into a brief and intuitive guide to the rheology experiments relevant to DIW printing and the relationship between those experiment and printing results. The roadmap was demonstrated by evaluating the printability of two inks: (1) a silicone ink with both yield-stress and reactive curing behavior and (2) urethane acrylate inks with photocuring behavior. Experimental printing studies were used to support the conclusions on printability made in the roadmap.

The second main approach focuses on the development of three novel UV-DIW inks to address the current limited material selection for UV-DIW and help better understand the rheological and reactive properties required for successful printing. For the three novel UVDIW inks, the iterative process of ink synthesis, analysis of ink rheology, and printability evaluation was detailed. Data from the development process contributed to gaining a fundamental understanding of how rheology and reactivity affect printability. The three inks each had novel rheological properties that impacted their printing behavior: (1) photocuring (2) yield-stress behavior + photocuring and (3) photocuring + thermal curing. Additionally, each ink had unique properties that expands material selection for UV-DIW including (1) an all-aromatic polyimide possessing a storage modulus above 1 GP a up to 400 °C (Chapter 4), (2) a styrene butadiene rubber (SBR) nanocomposite with elongation at break exceeding 300 % (Chapter 5), and (3) a dual-cure ink enabling the printing of inks containing over 60 vol% highly opaque solids (Chapter 6).

The third approach details the development of two UV-DIW process models to better understand the process physics of the UV-DIW process and give insight to the properties of a successful ink. The first process model uses data from photorheology experiments to model how a photocurable ink spreads upon deposition from the nozzle, accounting for transient UV curing behavior (Chapter 7). This model allows for the rapid evaluation of an ink's behavior during the solidification sub-function of UV-DIW solely based on its rheology, without the time-consuming process of trial-and-error printing or complex computer simulations. The second process model combines modeling with a novel experimental method that uses a UV photorheometer to accurately characterize the relationship between cure depth and UV exposure for a wide range of photopolymers (Chapter 8). This model helps understand an inks photocuring behavior and ensure a sufficient cure depth is produced to adhere to the previous layer in UV-DIW printing.

Lastly, two UV-DIW process modifications are introduced to address research gaps of printing high resolution features and limited material selection. A hybrid DIW + Vat Photopolymerization system is presented to improve the feature size and topographical complexities of parts, while still retaining UV-DIW's ability to print with very high viscosity photoresins (Chapter 9). A high temperature Heated-DIW system is presented to heat inks to over 300 °C and ultimately enable printing of poly(phenylene sulfide) aerogels (Chapter 10). In enabling the DIW of poly(phenylene sulfide) aerogels, the production of ultra-lightweight thermally insulating components for applications in harsh environments is enabled. With the use of additive manufacturing, hierarchical porosity on the macroscale is enabled in addition to the meso-scale porosity inherent to the aerogels.