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1. Non-Newtonian modeling of contact pressure in fused filament fabrication

   https://doi.org/https://doi.org/10.1122/8.0000052  

   Kim, Sun K; Kazmer, David O; Colon, Austin R; Coogan, Timothy J; Peterson, Amp M (January 2021, Journal of rheology)

   null (Ed.)

   The nozzle pressure was monitored in a fused filament fabrication process for the printing of high impact polystyrene. The contact pressure, defined as the pressure applied by the newly deposited layer onto the previous layer, is experimentally calculated as the difference between the pressure during printing and open discharge at the same volumetric flow rates. An analytical method for estimating the contact pressure, assuming one-dimensional steady isothermal flow, is derived for the Newtonian, power-law, and Cross model dependence of shear rates. A design of experiments was performed to characterize the contact pressure as a function of the road width, road height, and print speed. Statistical analysis of the results suggests that the contribution of the pressure driven flow is about twice that of the drag flow in determining contact pressure, which together describe about 60% of the variation in the observed contact pressure behavior. Modeling of the elastic and normal stresses at the nozzle orifice explains an additional 30% of the observed behavior, indicating that careful rheological modeling is required to successfully predict contact pressure. 

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2. The dependency chain in material extrusion additive manufacturing: Shaft torque, infeed load, melt pressure, and melt temperature

   https://doi.org/10.1016/j.addma.2023.103780  

   Colon, Austin R; Kazmer, David O; Peterson, Amy M (September 2023, Additive Manufacturing)

   Current material extrusion systems can produce complex parts but lack instrumentation for observability and control. To investigate methods for observing the material extrusion process, a printer is instrumented to examine the dependency chain from the motor shaft torque to the infeed load and finally the melt pressure and temperature. The transient rheological and thermal behavior of the material extrusion process and the effect of volumetric flow rate, nozzle orifice diameter, and temperature setpoint on the pressure estimate from each point in the dependency chain are reported. The work also presents pressure predictions from COMSOL Multiphysics non-isothermal flow simulations and an analytical (Poiseuille) model. The pressure estimated by the motor shaft torque is greater than the downstream pressure estimated by the infeed load, which is greater than the downstream melt pressure in the hot end. In other words, both the torque sensor and the infeed load significantly overpredict the melt pressure. Significant variations in the pressures are also observed and explained. The findings demonstrate low and high frequency variation in the process, which can be attributed to gear eccentricity and teeth-to-filament engagement. The melt pressure variation is also observed to increase significantly at lower temperature set-points and higher flow rates, both of which reduce the melt temperature and thereby increase the viscosity. The increase in viscosity tends to reduce the viscous damping such that the variations in the filament infeed are transmitted through the hot end to the extrudate. 

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3. Effect of shear-thinning rheology on the dynamics and pressure distribution of a single rigid ellipsoidal particle in viscous fluid flow

   https://doi.org/10.1063/5.0242953  

   Awenlimobor, A; Smith, D E (December 2024, Physics of Fluids)

   This paper evaluates the behavior of a single rigid ellipsoidal particle suspended in homogeneous viscous flow with a power-law generalized Newtonian fluid rheology using a custom-built finite element analysis (FEA) simulation. The combined effects of the shear-thinning fluid rheology, the particle aspect ratio, the initial particle orientation, and the shear-extensional rate factor in various homogeneous flow regimes on the particles dynamics and surface pressure evolution are investigated. The shear-thinning fluid behavior was found to modify the particle’s trajectory and alter the particle’s kinematic response. Moreover, the pressure distribution over the particle’s surface is significantly reduced by the shear-thinning fluid rheology. The FEA model is validated by comparing results of the Newtonian case with results obtained from the well-known Jeffery’s analytical model. Furthermore, Jeffery’s model is extended to define the particle’s trajectory in a special class of homogeneous Newtonian flows with combined extension and shear rate components typically found in axisymmetric nozzle flow contractions. The findings provide an improved understanding of key transport phenomenon related to physical processes involving fluid–structure interaction such as that which occurs within the flow field developed during material extrusion–deposition additive manufacturing of fiber reinforced polymeric composites. These results provide insight into important microstructural formations within the print beads. 

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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. Analytical Study of Shear-Thinning Fluid Flow in Direct Ink Writing Process

   https://doi.org/10.1115/MSEC2022-85747  

   Guo, Zipeng; Fei, Fan; Song, Xuan; Zhou, Chi (June 2022, ASME 2022 17th International Manufacturing Science and Engineering Conference)

   Abstract As a facile and versatile additive manufacturing technology, direct ink writing (DIW) has attracted considerable interest in academia and industry to fabricate three-dimensional structures with unique properties and functionalities. However, so far, the physical phenomena during the DIW process are not revealed in detail, leaving a research gap between the physical experiments and the underlying theories. Here, we presented a comprehensive simulation study of non-Newtonian ink flow during the DIW process. We used the computational fluid dynamics (CFD) method and revealed the shear-thinning behavior during the extrusion process. Different nozzle geometry models were adopted in the simulation. The advantages and drawbacks of each syringe-nozzle geometry were analyzed. In addition, the ink shear stress and velocity fields were investigated and compared in the case studies. Based on these investigations and analysis, we proposed an improved syringe-nozzle geometry towards high-resolution DIW. Consequently, the high-resolution and high shape fidelity DIW could enhance the DIW product performance. The results developed in this work offer valuable guidelines and could accelerate further advancement of DIW. 

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