Material jetting (MJ) is an additive manufacturing (AM) process that involves the selective jetting of a liquid material into the shape of a layer and subsequent solidification, often via ultraviolet (UV) irradiation, in a layer-wise fashion. The MJ process has the potential to emerge as a robust fabrication method: the inherent, facile, multi-material capability in a high-resolution process should distinguish the technology as a competitive, multi-functional, manufacturing process. However, it is mainly constrained to prototyping use, limited by both material and process constraints. This research expands material and process knowledge by characterizing the multi-material process-structure-property relationships in photopolymer-based MJ, which provides a basis for advancing the capability of MJ to fabricate accurate and consistent multi-material parts for functional applications.
One of the challenges for advancing MJ is the general lack of processable materials. For example, MJ is increasingly being used for fabricating anatomic models for use as pre-procedural planning or medical student trainee tools, but commercial MJ elastomers are unable to mimic human tissues' mechanical properties, which limits the instructional value of printed anatomic models. By combining photo-curing and non-curing materials, a cardiac tissue-mimicking material was achieved and integrated into a fully-printed heart model used to practice the transseptal puncture procedure. Several mechanical properties of this multi-material combination were evaluated to facilitate quicker screening of future tissues that would be desired to be mimicked.
Also impeding technological advancement of MJ systems is a lack of understanding the effects of indiscriminate UV exposure on material properties. Depending on factors such as part design and build layout, an indiscriminate UV toolpathing strategy poses the risk for providing inconsistent UV dosing to parts and causing unintended variations in mechanical performance. Experiments were conducted to quantify these effects, and an empirical model was developed to predict the accumulated exposure parts receive. A connection was then made between accumulated exposure received by material voxels and final part properties, where it was observed that overexposure effects exist, and are largely dependent on material, build layout, and toolpathing. This work will lead to improved design guidelines and process modifications to ensure consistency of UV dosing and achieve desired mechanical performance. This knowledge will enable future photopolymer AM systems to account for potential overcuring effects toward fabricating repeatable and reproducible functional products.
Finally, documented in this work are efforts toward expanding the knowledge about the use of AM to safely produce personal protective equipment during the COVID-19 pandemic. Amid prospects of large-scale, distributed production of respirators via AM, the lack of filtration efficiency testing generated concerns about the respirators' effectiveness. The goal of this work was to measure particle transmission through respirators fabricated with powder bed fusion and fused filament fabrication processes and compare their performance to that of cloth masks and standardized N95 respirators. Through systematic post-processing, the connection between printed respirator deficiencies and changes in filtration efficiency were discerned. Identifying the system-level quality control challenges responsible for the respirator failure modes highlights some the current limitations in AM for fabricating functional parts. The findings will assist future efforts toward both creating enhanced designs and optimizing printer parameters, ultimately working toward qualifiable, end-use parts. / Doctor of Philosophy / The material jetting (MJ) additive manufacturing (AM) process operates in a similar fashion to inkjet printing. For MJ of photopolymer materials, liquid droplets are selectively deposited onto a build plate, and an ultraviolet (UV) light bulb provides the energy to solidify the droplets into a three-dimensional layer by curing the materials. Droplets are then deposited on top of these solidified droplets to fabricate a part layer by layer. Multiple materials and colors can be jetted simultaneously within a single part layer. If these materials exhibit different mechanical behavior, such as one material being rigid and another being flexible, a printed part could have regions with different material properties, as well as intermediate gradients of these properties. The MJ process offers high resolution, smooth surface finishing, a large build volume, and the opportunity to print multiple parts in one build. However, the process is mainly limited to prototypes and non-functional applications.
One of the challenges for advancing MJ is the general lack of processable materials. In the medical field, surgeons are increasingly looking to MJ to fabricate physical, patient-specific models to assist in pre-surgical planning and to serve as practice models for medical student trainees. In particular, a printed cardiovascular model was sought to enable the practice of the transseptal puncture procedure; however, the available materials were not able to mimic the heart tissue. In this work, a non-curing liquid was patterned into an elastomer to soften the material and attain tissue-mimicking performance for a model to practice the transseptal puncture procedure. By characterizing this expanded material space, this work enables the potential for mimicking a broader spectrum of tissues in future anatomic models.
Another aspect limiting widespread functional use for MJ is the lack of understanding how UV exposure affects material performance. For the MJ process, the UV light is on the same assembly as the printheads and remains on throughout the duration of a print, which means that the amount of administered energy is not consistent across the build plate. If, for example, parts have different heights, the shorter part will finish printing first and receive excess UV exposure, which has been shown to alter the mechanical performance for some materials. A model was developed to predict the accumulated exposure received by parts of different materials and build scenarios. Observed changes in mechanical properties could then be connected to specific instances of overexposure. With this knowledge, future strategies can be implemented to achieve consistency of UV exposure and thus better ensure reliable, functional parts.
Additionally presented in this work is a study involving the use of AM to safely produce personal protective equipment for COVID-19 relief efforts. During the initial stages of the pandemic, AM was sought to address respirator shortages; however, there were no studies measuring printed respirators' effectiveness. By measuring particle transmission through respirators fabricated with a variety of AM processes, it was found that even when N95 filters were inserted, printed respirators were not able to consistently filter 95% of virus-sized particles, even with modifications. The quality control challenges for the AM processes identified in this study will assist future efforts in part design and printer parameter optimization to work toward accurate and qualifiable products.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110917 |
| Date | 23 June 2022 |
| Creators | Bezek, Lindsey Bernadette |
| Contributors | Mechanical Engineering, Williams, Christopher Bryant, Li, Ling, Dillard, David A., Boreyko, Jonathan B. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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