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The use of fused deposition modeling (FDM) in printing polymers for various applications has been ever increasing. However, its utilization in printing polymers for high-strength and superior surface finish applications is still a challenge, primarily due to process intrinsic defects, i.e., voids between the layers and the rough exterior arising from unrestrained deposition of molten polymer. This research hypothesizes that application of ultrasonic vibration (USV) post-fabrication could minimize these shortcomings. For this investigation, ASTM D638 Type IV samples were FDM-printed using poly(lactic) acid (PLA). Through screening experiments, an optimized set of ultrasonic parameters was determined. Then, the effect of both-sided ultrasonic application was characterized. Subsequently, the impact of USV on the samples’ physical, tensile, and morphological properties was examined by varying the layer height, infill patterns, and % infill density. Up to 70% roughness reduction was observed as a result of post-FDM ultrasonic application. Additionally, the tensile strength of the samples increased by up to 15.31%. Moreover, for some lower % infill samples, post-ultrasonic tensile strengths were higher than 100% infill control samples. Analysis of scanning electron microscopy (SEM) and X-ray computed tomography (CT) imagery indicated enhanced layer consolidation and reduced void presence in samples treated with ultrasonic. The combination of ultrasonic-generated heat and downward pressure promoted a synergistic squeeze flow and intermolecular diffusion across consecutive layers of polymers. As a result, increased tensile strength and surface finish were achieved while dimensional change was marginal.

FDM is the most widely used AM process due to its ability to fabricate complex geometries at a lower cost. However, its process characteristics introduce anisotropy in the Z-direction due to interlayer voids and result in a stepped surface finish. Global research efforts are addressing these limitations through techniques such as infrared pre-heating, plasma heating, localized laser heating, and ultrasonic vibration, with the latter not investigated in the way this dissertation explores. The effect of applying ultrasonic vibration in FDM through three distinct approaches by focusing on interlayer voids, surface finish, and dimensional accuracy has been investigated thoroughly in this study. The study utilizes PLA as the material of interest and employs mechanical property testing, X-ray CT, SEM, and other physical characterization methods. In the first approach, ultrasonic vibration was applied to both sides of completed FDM parts as a post-processing technique. This method reduced surface roughness by up to 70%, decreased thickness by 5.2%, and increased tensile strength by 15.31%. The second approach was an in-situ procedure where printing was repeatedly paused after every few layers, and ultrasonic vibration was applied on the whole layer of material in a scanning mode. Impact resistance of the parts increased by 54% as a result, but the tensile strength was mostly unaffected. In the final approach, the printing was repeatedly paused, similar to the previous approach, but an image processing method was implemented to identify the part’s contour shape and position on the bed. Further processing of the image was done to generate tool paths inside the contour. Following the distance calculation of the tool paths from the home position of the ultrasonic horn, the ultrasonic vibration was applied in a contour scanning mode on different layers. This approach resulted in a 36% increase in maximum tensile strength and a 73% improvement in impact resistance. These findings highlight the potential of ultrasonic vibration in enhancing the mechanical properties and surface finish of FDM-printed parts from PLA. This hybrid approach will increase the usage of PLA in high-strength applications. Improvements are in progress to develop an industry-friendly hybrid FDM system.

Moisture absorption into hygroscopic/hydrophilic materials used in fused deposition modeling (FDM) and ultrasonic welding (USW) can diminish desired mechanical properties. Sensitivity to moisture is dependent on material properties and environmental factors and needs characterization. In this thesis, moisture sensitivity of PLA filaments and PLA/PBS blended filaments was characterized in FDM printed ASTM test samples post-conditioning the filaments at different relative humidity levels. Tensile strength decreased with increase in moisture content. Parts made with PLA 4043D, PLA/PBS 75/25 filaments were most sensitive to moisture. Investigation of tensile properties of parts made with PLA filaments exposed to room temperature and humidity conditions for three months showed a more significant decrease. Moisture sensitivity of PLA, PBS, and PLA/PBS 25/75 blend characterized for USW using injection-molded industrial standard test parts (ISTeP) showed a downward trend in weld strength for 100% PLA and PLA/PBS 25/75 while 100% PBS was significantly affected at high moisture conditions.