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1. Three-dimensional (3D) Simulation of Micro-Void Development within Large Scale Polymer Composite Deposition Beads

   Aigbe Awenlimobor, Zhaogui Wang (October 2022, 2022 International Solid Freeform Fabrication Symposium)

   Structural integrity and quality of short fiber composite parts produced by Big Area Additive Manufacturing (BAAM) are largely affected by inherent bead microstructural features such as voids. Unfortunately, our understanding of void nucleation and evolution during polymer deposition process is lacking. Flow modeling focused on the associated microstructural formation provides a means for better understanding the process-structure-properties relations in large area extrusion deposition additive manufacturing of fiber reinforced composites. Our prior computational effort that investigated mechanisms that may promote micro-void formation was based on 2-dimensional planar models of a single ellipsoidal fiber motion in purely viscous polymer extrusion/deposition flow through a BAAM nozzle. Here we present a 3D finite element modelling approach to simulate single fiber out-of-plane rotations utilizing velocity and velocity gradient values computed along streamlines obtained from a 3D extrusion/deposition simulation of the BAAM polymer deposition process. The pressure distribution on the fiber’s surface along the flow path provides new insight into potential micro-void nucleation mechanism. Results show low pressure regions occur near the fiber’s surface which varies across the printed bead and through its thickness. 

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2. Simulation of fiber-induced melt pressure fluctuations within large scale polymer composite deposition beads

   https://doi.org/10.1016/j.addma.2024.103980  

   Awenlimobor, Aigbe; Smith, Douglas E.; Wang, Zhaogui (January 2024, Additive Manufacturing)

   The process-structure-property relationship in Large Area Additive Manufacturing (LAAM) technology is an ongoing area of research as the inherent microstructural properties (chiefly fibers and voids) affect the performance of printed parts. Unfortunately, we currently lack adequate understanding of micro void nucleation and evolution during the LAAM and fused deposition modelling (FDM) processes. Modeling of the polymer melt flow during the extrusion process is important in understanding the underlying microstructural formation and associated properties of the print, that determines the part performance in service conditions. In this paper we compute fiber-induced local pressure fluctuations which may promote void formation in the bead’s microstructure. On a macro-scale, we determine flow fields of a purely viscous, Newtonian planar polymer deposition flow through a LAAM nozzle which are utilized on a micro-scale model where we simulate the evolution of a single ellipsoidal fiber along streamlines obtained from the macro-model. On the micro-scale, we determine instantaneous values of the translational and rotational velocities of the rigid ellipsoidal fiber that satisfies a balance of hydrodynamic forces and couples on the fiber’s surface based on a Newton Raphson algorithm and we track the fiber’s motion along the flow path via an explicit numerical iterative algorithm. Model verification is achieved by benchmarking results with solutions from well-known Jeffery’s equation of motion of a particle in homogeneous simple shear flow. We account for rotary diffusivity due to short-range fiber-fiber interaction in the FEA simulation by determining an effective fluid domain size representative of the interaction coefficient of the melt flow through a correlation analysis that yields an equivalent steady state orientation based on the Advani- Tucker equation. We also consider different possible motions of the fiber along individual LAAM flow paths from a given set of random initial fiber conditions to determine pressure bounds on the fiber surface along each streamline. For improved computational efficiency, calculations are carried out with respect to the fiber’s local coordinate axes to overcome the rigor of adaptive remeshing during the quasi-transient analysis. Results show low pressure extremes near the fiber’s surface which varies across the printed bead as well as through its thickness. Discussion is provided to gain insight into the effect of low-pressure extremes on micro void formation, particularly at the nozzle exit and during die swell/expansion. 

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3. A Comparative Study of Pellet-Based Extrusion Deposition of Short, Long, and Continuous Carbon Fiber-Reinforced Polymer Composites for Large-Scale Additive Manufacturing

   https://doi.org/10.1115/1.4049646  

   Pappas, John M.; Thakur, Aditya R.; Leu, Ming C.; Dong, Xiangyang (July 2021, Journal of Manufacturing Science and Engineering)

   null (Ed.)

   Abstract Pellet-based extrusion deposition of carbon fiber-reinforced composites at high material deposition rates has recently gained much attention due to its applications in large-scale additive manufacturing. The mechanical and physical properties of large-volume components largely depend on their reinforcing fiber length. However, very few studies have been done thus far to have a direct comparison of additively fabricated composites reinforced with different carbon fiber lengths. In this study, a new additive manufacturing (AM) approach to fabricate long fiber-reinforced polymer (LFRP) was first proposed. A pellet-based extrusion deposition method was implemented, which directly used thermoplastic pellets and continuous fiber tows as feedstock materials. Discontinuous long carbon fibers, with an average fiber length of 20.1 mm, were successfully incorporated into printed LFRP samples. The printed LFRP samples were compared with short fiber-reinforced polymer (SFRP) and continuous fiber-reinforced polymer (CFRP) counterparts through mechanical tests and microstructural analyses. The carbon fiber dispersion, distribution of carbon fiber length and orientation, and fiber wetting were studied. As expected, a steady increase in flexural strength was observed with increasing fiber length. The carbon fibers were highly oriented along the printing direction. A more uniformly distributed discontinuous fiber reinforcement was found within printed SFRP and LFRP samples. Due to decreased fiber impregnation time and lowered impregnation rate, the printed CFRP samples showed a lower degree of impregnation and worse fiber wetting conditions. The feasibility of the proposed AM methods was further demonstrated by fabricating large-volume components with complex geometries. 

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4. Investigating the Effect of Generalized Newtonian Fluid on the Micro-Void Development within Large Scale Polymer Composite Deposition Beads

   Aigbe Awenlimobor, Douglas E. (October 2023, Proceedings 2023 International Solid Freeform Fabrication Symposium)

   The formation and development of micro-voids within the bead microstructure of a polymer composite during the extrusion/deposition additive manufacturing process continues to be of interest given the adverse effect these features have on part quality. A computational method is employed here to investigate potential volatile-induced micro-void nucleation mechanism which simulates the evolution of a single rigid ellipsoidal fiber in purely viscous polymer extrusion/deposition flow through a Large Area Additive Manufacturing (LAAM) nozzle. Our previous studies on potential micro-void nucleation mechanisms have assumed a Newtonian fluid property definition for the polymer melt flow, the current study assesses the effect of assuming a generalized Newtonian fluid (GNF) model on the fiber’s response. Preliminary findings based on Jeffery’s flow assumption reveal the fiber’s orientation kinetics are unaffected by the shear thinning fluid behavior, however there is a reduction in the pressure distribution on the fiber’s surface as the power law index is decreased which is expected to reduce the likelihood for microvoid nucleation. 

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5. High through-thickness thermal conductivity of 3D-printed composites via rotational direct ink writing

   https://doi.org/10.1016/j.addlet.2023.100167  

   Wilt, Jackson K.; Hmeidat, Nadim S.; Bohling, John W.; Compton, Brett G. (December 2023, Additive Manufacturing Letters)

   Composites printed using material extrusion additive manufacturing (AM) typically exhibit alignment of high- aspect-ratio reinforcements parallel to the print direction. This alignment leads to highly anisotropic stiffness, strength, and transport properties. In many cases, it would be desirable to increase mechanical and transport properties transverse to the print direction, for example, in 3D-printed heat sinks or heat exchangers where heat must be moved efficiently between printed roads or layers. Rotational direct ink writing (RDIW), where the deposition nozzle simultaneously rotates and translates during deposition, provides a method to reorient fibers transverse to the print direction during the printing process. In the present work, carbon fiber-reinforced epoxy composites were printed by RDIW with a range of nozzle rotation rates and the in-plane and through-thickness thermal conductivity was measured. In addition, the orientation of carbon fiber (CF) in the composites was measured using optical microscopy and image analysis, from which second-order fiber orientation tensors were calculated. These results showed that the orientation of CF became less anisotropic as nozzle rotation rate increased, leading to increased through-thickness thermal conductivity, which increased by 40% at the highest rotation rate. The orientation tensors also showed that RDIW was more effective at reorienting fibers within the in-plane transverse direction compared to the through-thickness transverse direction. The results presented here demonstrate that a current weakness of material extrusion AM composites—poor thermal conductivity in the through-thickness direction—can be significantly improved with RDIW. 

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