Jackson S. Bryant’s research while affiliated with Virginia Tech and other places

Powder bed fusion (PBF) is a promising technology in polymeric additive manufacturing, whose growth is inhibited by limited material options. In this manuscript, we report the use of thermally induced phase separation (TIPS) to controllably produce polymer powders of polypropylene that are suitable for PBF. Moreover, these studies provide crucial insight into the factors that govern the final size of the produced powder, offering fundamental insight that fosters the rational control of powder fabrication for PBF. More precisely, the impact of polymer concentration, molecular weight, and quench temperature on the size of the powder is demonstrated, where the particle size increases with solution concentration, quench temperature and molecular weight. The molecular weight dependence is consistent with a decrease in polymer solubility with an increase in chain length, while the solution concentration dependence can be explained by the relative fractions of the two phases in the precipitation process of polymer solutions. Careful analysis of the temperature and solution concentration dependence of the powder verifies that droplet coalescence is the governing mechanism in the phase separation‐based particle formation process. Therefore, this fundamental understanding provides pathways to use TIPS to produce powders suitable for PBF from a broad range of polymer solutions.

Polyolefin-based thermoplastics such as polyethylene and polypropylene constitute a major fraction of the polymers employed in commodity applications due to their ease of processability, durability, and economic viability. Additive manufacturing (AM) of polyolefins offers both a viable path toward functional prototyping of design concepts and direct manufacturing of end-use parts. Melt-based AM of polyolefins is more challenging than other semicrystalline polymers (polyamides) due to the relatively high levels of volumetric shrinkage encountered during crystallization of such polymers that lead to significant issues related to warpage and interlayer adhesion. The focus of this review is to evaluate the latest state-of-the-art for processing polyolefins by powder bed fusion (PBF) and material extrusion (MatEx) AM modalities. Recent progress in processing neat, filled, and blends of polyolefins using PBF and MatEx are discussed to highlight the importance of the rheological and morphological characteristics of the polymer melt on the printed parts performance. The existing challenges to AM of polyolefins are emphasized and strategies to address the limitations are recommended through a better understanding of the associated process-structure-property relationships. A holistic approach spanning synthetic modifications for feedstock development, improved system design, and physics-guided process parameter selection is required to broadly adopt melt-based AM of polyolefins.

We introduce a novel experimental method that uses an ultraviolet (UV) photorheometer to enable accurate and repeatable measurements of cure depth for a wide range of photopolymers. When exposed to UV irradiation within the rheometer, the cured thickness of the photoresin is measured by lowering the upper plate while monitoring the commensurate axial force. The encoder-measured distance is correlated with axial force to measure cure depth under different UV curing conditions. This technique enables precise measurements of photocured films thinner than 50 µm, which can then be used to establish working curve equations for photopolymerization-based additive manufacturing (AM) processes. This process is validated through the evaluation of three unique photoresins, including a commercially available VP stiff photoresin, a highly-viscous photoresin that cures into a “soft” gel, and a highly solids-loaded photoresin with particles that greatly limit maximum cure depth. In addition, the technique is employed to explore the impact of UV irradiance on curing behavior of photoresins. This technique enables measurement of AM process-relevant cure depths with process-relevant irradiance and wavelengths and has applications in advanced material development for a range of AM and traditional UV processing technologies. This technique meets identified gaps in current cure depth characterization techniques, including the need for automated instrumental approaches to reduce human error in measurements, the need to directly measure cure depths less than 150 µm, and the need to measure curing behavior at different UV irradiances/wavelengths. Due to the non-linear cure depth versus exposure behavior of some photoresins, it is important to directly measure cure depths at AM process-relevant sizes (<150 µm), as a working curve made from measurements of thicker films leads to more inaccurate predictions than those made from measuring thinner films.