Recent advances in the field of additive manufacturing (AM) or 3D printing, have garnered serious interest for its potential to substitute time-consuming and costly subtractive and formative manufacturing techniques. Material extrusion (MatEx), employing filament and pelletbased feedstocks, is an AM technique for fabricating three-dimensional objects dictated by a computer-aided design (CAD) file in a layer-by-layer manner. Being inherently a "melt-and-form" technique, the physics of MatEx is strongly dependent on the melt flow behavior of the polymers and hence on their rheology. The focus of this review article is to analyze the current progress in rheological characterizations of filament and pellet-based polymeric feedstocks for application in MatEx. The importance of shear and temperature-dependent viscosities in relation to consistent extrusion through the print nozzle and in the standoff region between nozzle and bed will be highlighted. The importance of shear and/or extensional viscosities and extent of die swell (upon exit from the nozzle) experienced by the polymers under processing parameters relevant to MatEx will be investigated. Postextrusion from the nozzle, the rheological characteristics of the viscous polymer melt as it cools once deposited on the print bed governs the degree of interlayer welding, that impacts the mechanical performance of the printed parts. Controlling and monitoring rheological properties such as zero-shear viscosities and shear moduli of the melt is of significant importance in this region in order to ensure proper mechanical robustness and shape integrity of the deposited layers. Both experimental and theoretical approaches based on polymer chain reptation mechanisms will be reviewed in detail and suggestions to address the existing limitations associated with the process will be provided. Fundamental understanding of the correlation between the classical theories and current understanding based on recent experimentation and analysis is expected to assist the design and development of the next generation of polymer feedstocks and machine designs for MatEx-based AM.

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