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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Filament-based material extrusion additive manufacturing (MEAM) is one of the most commonly used techniques in additive manufacturing (AM). In spite of recent notable development in the MEAM process, there is still a need to develop more materials that can be printed consistently using this technique. Isotactic polypropylene (PP), a popular thermoplastic material, undergoes rapid crystallization and subsequent volume contraction. This can lead to residual stress buildup in PP parts when processed using MEAM, resulting in poor adhesion to the printing platform, poor geometric tolerance, and mechanical performance. In this work, the effects of varying composition of low molecular weight hydrocarbon resins incorporated to PP are investigated. Specifically, the thermal behavior, crystallization, morphology, and printability of the blends are studied. The rapid crystallization of PP has been delayed by the addition of hydrocarbon resins that provided a larger time window for the residual stresses to relax. The addition of the resins to the pure PP matrix lowers the crystallization temperature of PP from 121.8 to 116.0 °C, which further enables additional diffusion during the solidification process. Polarized optical microscopy demonstrates the differences in crystalline morphology, which is expected to impact the structure at the interlayer boundaries between deposited layers. The combination of modifications in crystallization rate, time, and morphology significantly affects the interlayer adhesion and residual stress state, which directly controls the mechanical properties and part warpage of printed parts. Tensile bars of the different blends were printed in two different orientations to analyze the mechanical performance and study part anisotropy. The maximum tensile stress of pure PP (26.8 ± 2.1 MPa) printed at a ±45° raster angle increased with addition of 20 wt % hydrocarbon resin (32.4 ± 3.0 MPa) when printed under the same conditions. The improvement in the tensile strength is due to a combination of changes in crystallinity, morphology, and improved interlayer adhesion during printing. The parts were annealed postprinting to improve polymer chain diffusion across the layers, thereby improving interlayer adhesion and resulting in tensile modulus and strength values in excess of 90% of injection-molded PP parts.

Fused filament fabrication (FFF) has seen broad industrial adoption as it is capable of manufaturing large complex parts from robust thermoplastics in a cost-effective manner. However, the mechanical performance of the printed parts is limited due to poor interlayer bonding and the presence of voids. In order to overcome these drawbacks, the addition of short or continuous fibers into the polymer matrix has been investigated, as the fibers can act as a mechanical reinforcement while also mitigating residual stress resulting from the material's rapid solidification following extrusion. Therefore, understanding the implications of process parameters and fiber reinforcements on printed part properties through detailed crystallization analysis and rheological characterizations is of paramount importance. The goal of this study is to understand the process-structure-property relationships of short carbon fiberreinforced polyamide 6 (CF-PA6) by comparing the melt rheology and crystallinity of CF-PA6 versus a neat PA6 polymer. Differences in the melting and crystallization behavior resulting from the reinforcing fibers revealed an increased time window for crystallization in the fiber-reinforced matrix. Rheological characterizations at the recommended printing temperatures demonstrate the shear-thinning behavior of the samples at shear rates relevant to FFF. From a statistical design of experiments analysis, the layer thickness was found to be the most significant parameter affecting the tensile properties of a printed part at a constant printing temperature and printing speed. The tensile fracture surfaces of the printed specimens using scanning electron microscopy were analyzed to provide insights into the failure mechanisms as a function of AM processing variables.

Material-extrusion (MatEx) additive manufacturing involves layer-by-layer assembly of extruded material onto a printer bed and has found applications in rapid prototyping. Both material and machining limitations lead to poor mechanical properties of printed parts. Such problems may be addressed via an improved understanding of the complex transport processes and multiphysics associated with the MatEx process. Thereby, this review paper describes the current (last 5 years) state of the art modeling approaches based on momentum, heat and mass transfer that are employed in an effort to achieve this understanding. We describe how specific details regarding polymer chain orientation, viscoelastic behavior and crystallization are often neglected and demonstrate that there is a key need to couple the transport phenomena. Such a combined modeling approach can expand MatEx applicability to broader application space, thus we present prospective avenues to provide more comprehensive modeling and therefore new insights into enhancing MatEx performance.