Tools and design considerations for improvements in fused filament fabrication 3D printing
Date
2020
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Publisher
University of Delaware
Abstract
Additive manufacturing, more commonly known as 3D printing, provides unlimited design freedom and access to geometries too complex for the more traditional means of manufacturing. Recent developments in this technique have contributed to a diverse palette of printable materials, some of which include metals, hydrogels, ceramics, and polymers. Complementary to this is a plethora of printing methods, such as the laser sintering of powders, the curing of photosensitive resins, and the controlled extrusion of thermoplastic materials. The last of these is often referred to as fused filament fabrication (FFF) and is the most commercially adopted printing technique. Despite the cost savings and design flexibility that FFF offers, products designed in this manner are weaker compared to parts made using other polymer processing operations. Addressing this shortcoming first requires an understanding of the strengthening mechanisms intrinsic to FFF, which is the primary objective of this dissertation, and is approached through the lens of rheological and heat transfer theory. ☐ First, a modified Cogswell rheological model is developed and is used to relate 3D printer extrudate temperatures to entry pressures developed within the nozzle flow field. Entry pressure measurements and calculations reveal unintuitive flow behavior, which are hypothesized to be attributable to heat transfer limitations within the melt zone. A dimensionless Nusselt-Graetz (Nu-Gz) number analysis is then presented and serves as strong evidence that heat transfer is a significant bottleneck in manufacturing the strongest printed parts. ☐ Next, computational fluid dynamics simulation tools are used to expand upon the understanding of the melting mechanism within the heated region. Extensive rheological characterization is used to guide computational efforts. Large pressure drops, which are observed in the simulations, are present through the system and are of similar magnitude to those obtained experimentally. These pressures are counteracted by the large viscosity within a recirculation vortex at the melting zone ingress. The large viscosity acts as a “seal,” preventing spillover and stabilizing the FFF operation. ☐ Then, polymer chain orientation as a means of imparting strength are explored. It was found that the deposition step in the FFF process is central to producing orientation in printed tracks. Thermal shrinkage and birefringence experiments are considered in tandem to quantify the degree of orientation for a given set of printing conditions. Low printing temperatures and high printing speeds are found to be critical in producing print tracks with the highest levels of orientation. ☐ Finally, in a slight departure from the previous topics, a study on six different commonly-used acrylonitrile butadiene styrene (ABS) 3D printing formulations was performed. ABS is a complex polymer which is composed of a styrene acrylonitrile (SAN) matrix that is then filled with butadiene spheres grafted with SAN (the grafted butadiene particles will referred to as the filler). The primary motivation for this work was to understand the rheological properties that would enable ABS to be drawn down for hybrid material applications. A secondary objective was to explore the effect of the rheological differences (if any) on end-use performance of objects fabricated with these different ABS formulations. Of the six ABS materials studied, four formulations had congruency of the storage and loss moduli at low frequencies, indicating gel-like behavior. One other formulation also had gel-like behavior, but due to parallel slopes in the moduli at low frequencies, and not because of congruency. The final formulation did not have any signs of gel-like behavior. The five gel-like ABS materials also never achieved steady-state elongational viscosity values. A modified Generalized Maxwell Model was used to separate the influence of the filler on the SAN matrix. The significant difference between the blend and matrix relaxation times is evidence of the filler’s enormous effect on the rheology. However,more work needs to be done to elucidate the structural reason for the filler’s influence. An analysis of the stress development down the draw line of a typical drawing process revealed that the formulation without gel-like behavior was most amenable to drawing. This was later confirmed experimentally by collaborators at the Army Research Laboratory (Aberdeen Proving Ground, Maryland, USA).
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Keywords
3D printing, Additive manufacturing, Fused filament fabrication, Heat transfer, Rheology, Thermoplastic materials