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1. Concurrent characterization of compressibility and viscosity in extrusion-based additive manufacturing of acrylonitrile butadiene styrene with fault diagnoses

   https://doi.org/https://doi.org/10.1016/j.addma.2021.102106  

   Kazmer, David O; Colon, Austin R; Peterson, Amy M; Kim, Sun K (June 2021, Additive manufacturing)

   null (Ed.)

   Compressibility and viscosity of polymer feedstock are critical to their volumetric flow rate, weld strength, and dimensional accuracy in material extrusion additive manufacturing. In this work, the compressibility and viscosity of an acrylonitrile butadiene styrene (ABS) material is characterized with an instrumented hot end design. Experiments are first performed with a blocked nozzle to characterize the compressibility behavior. The results closely emulate the pressure-volume-temperature (PVT) behavior of a characterized generic ABS. Experiments are then performed with an open nozzle over a range of volumetric flow rates and temperatures. The static pressure data is fit to power-law, Ellis, and Cross viscosity models and the dynamic melt pressure data is then used to jointly fit material constitutive models for compressibility and viscosity. The results suggest that the joint fitting substantially improves the fidelity relative to the separately characterized viscosity and compressibility. The implemented methods support material extrusion process simulation and control including real-time identification of process faults such as (1) limited melting capacity of the hot end, (2) skipping (grinding) of the extruder drive gears, (3) low initial nozzle temperature, (4) varying flow rates associated with the intermeshing gear tooth velocity profile, and (5) delays and reduced melt pressures due to drool prior to extrusion. The ability to monitor the printing process for faults in real time, such as that presented in this work, is critical to born qualified parts. Additionally, these approaches can be used to screen new materials and identify optimal processing conditions that avoid these process faults. 

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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. Transient torque, pressure, and temperature data for material extrusion additive manufacturing of acrylonitrile butadiene styrene

   https://doi.org/10.18126/FU80-S9BT  

   Colon, Austin R; Kazmer, David O; Peterson, Amy M (January 2023, Materials Data Facility)

   Dataset includes transient torque, infeed pressure, melt pressure, and melt temperature that were acquired by an instrumented hot end for material extrusion additive manufacturing of acrylonitrile butadiene styrene (ABS). Data were collected according to a design of experiments wherein the volumetric flow rate, temperature setpoint, and the nozzle orifice diameter were varied one factor at a time. 

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4. Non-isothermal non-Newtonian three-dimensional flow simulation of fused filament fabrication

   https://doi.org/https://doi.org/10.1016/j.addma.2022.102833  

   Sun Kyoung Kim; David O. Kazmer (October 2022, Additive manufacturing)

   This work investigates three-dimensional simulation of fused filament fabrication using the Cross-WLF model for the non-isothermal and shear thinning behavior of the melt. To realistically simulate the deposition flow, the acceleration, viscosity evolution, and flow front tracking models have been included with the pressure gradient in the deposited road and boundary modeling of the melt and air interface. The results indicate that the non-isothermal and shear thinning behaviors greatly affect the geometry of the deposited roads including the flow front and trailing cross-section shapes. The thermal footprint of the interface between the deposited melt and the substrate is also predicted as a function of the thermal contact conductance. The pressure distribution within the deposited road is also modeled and is found to be not symmetric with respect to the nozzle center-line. Rather, the pressure peak shifts slightly downstream due to redirection of the melt around a stagnation point opposite the nozzle exit. Furthermore, a negative stress is observed downstream the exterior nozzle face associated with the free expansion of the melt as the extruded material climbs and releases from the exterior nozzle face. The developed simulation is verified by comparison with experimental results providing contact pressures ranging from 5 to 132 kPa. 

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5. Pressure-driven gas ow in viscously deformable porous media: application to lava domes

   https://doi.org/10.1017/jfm2019-211  

   Hyman, D.; Bursik, M.; Pitman, E.B (June 2019, Journal of fluid mechanics)

   The behaviour of low-viscosity, pressure-driven compressible pore fluid flows in viscously deformable porous media is studied here with specific application to gas flow in lava domes. The combined flow of gas and lava is shown to be governed by a two-equation set of nonlinear mixed hyperbolic–parabolic type partial differential equations describing the evolution of gas pore pressure and lava porosity. Steady state solution of this system is achieved when the gas pore pressure is magmastatic and the porosity profile accommodates the magmastatic pressure condition by increased compaction of the medium with depth. A one-dimensional (vertical) numerical linear stability analysis (LSA) is presented here. As a consequence of the pore-fluid compressibility and the presence of gravitation compaction, the gradients present in the steady-state solution cause variable coefficients in the linearized equations which generate instability in the LSA despite the diffusion-like and dissipative terms in the original system. The onset of this instability is shown to be strongly controlled by the thickness of the flow and the maximum porosity, itself a function of the mass flow rate of gas. Numerical solutions of the fully nonlinear system are also presented and exhibit nonlinear wave propagation features such as shock formation. As applied to gas flow within lava domes, the details of this dynamics help explain observations of cyclic lava dome extrusion and explosion episodes. Because the instability is stronger in thicker flows, the continued extrusion and thickening of a lava dome constitutes an increasing likelihood of instability onset, pressure wave growth and ultimately explosion. 

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