The strengths of additive manufacturing (AM), especially the tool-less manufacturing paradigm and rapid production of low-volume products, are well-aligned with the needs of manufacturing of expensive, high-temperature resistant, engineering thermoplastic polymers. High temperature polymer parts made with AM for either tooling or end-use applications have been implemented in the aerospace, automotive, and biomedical fields. However, parts made from these polymers using traditional manufacturing processes are generally high-value parts in low-quantity production runs. Moreover, AM processing of these polymers present significant challenges due to limitations associated with large thermal gradients, residual stress buildup, and interlayer adhesion as well as the inability of the printers to consistently maintain required high processing temperatures. This review highlights the current state of the art for processing high-temperature (i.e., traditional processing temperatures exceeding 250°C) thermoplastic polymers by the melt-based, AM processes of material extrusion (MatEx) and laser powder bed fusion (PBF). The authors address common challenges to AM of high-temperature polymers and gaps in fundamental understanding of the process-structure-property relationships needed to identify the machine design, process parameter selection, and synthetic modifications to enable processing.

Current understanding and challenges in high temperature additive manufacturing of engineering thermoplastic polymers

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 manufa...

Importance of Polymer Rheology on Material Extrusion Additive Manufacturing: Correlating Process Physics to Print Properties

This review documents the current state of the art of highly filled (HF) polymer composite systems used in additive manufacturing (AM), with a core focus on short and continuous fiber filled composites for use in material extrusion (MatEx) AM. Current state of the art composite materials systems have been succesfully incorporated into the MatEx process with loading percentages up to approximately 45 vol. %. Further increasing the loading percentage would afford significant mechanical improvements that could enable structural performance, however there are currently a number of limitations that need to be addressed. Viscosity limitations associated with MatEx processing of HF composites originate from the low processing pressure inherent to filament feed MatEx processes. Restrictive hot end nozzle geometries (high volume contractions) increase required processing pressure and potential for nozzle clogging failures. The increased viscosity of the HF composite reduces interlayer contact and adhesion during the printing process, resulting in increased void spacing. Mechanical limitations of HF composites processed using MatEx are also reviewed. Flow induced fiber orientation and its influence on viscosity and mechanical anisotropy are investigated. Fiber breakage as well as strain to break limitations are reviewed. Lastly, the geometric packing limitations of fibers within a circular bound nozzle are reviewed. This review takes an in-depth look at recent advances in addressing these challenges, and discusses opportunities for realizing MatEx processing of HF composite systems with significantly improved performance.

Highly Loaded Fiber Filled Polymers for Material Extrusion: A Review of Current Understanding

Material extrusion additive manufacturing utilizes a thermoplastic polymer in the form of a solid filament as a built material. The polymer melts inside the hot-end channel and flows under the pressure generated by the filament feeding force. The flow of polymer through the hot-end is not fully understood yet, as it involves many complex phenomena, such as phase transition, shear rate and temperature dependent viscosity, as well as viscoelastic effects. In this paper, we investigate experimentally the filament feeding force, as a function of the feeding rate, for different materials (PLA and ABS), liquefier temperatures, nozzle diameters, and lengths of the liquefier. Two extrusion regimes are identified: a linear regime with a stable flow, and a non-linear regime with fluctuations in the feeding force, which are concomitant to unstable extrusion. Increasing the liquefier length and liquefier temperature are found to extend the linear extrusion regime. It is shown that the filament feeding force predicted by the analytical models available in the literature deviates from the measurements, especially at high feeding rates. A model solely based on heat transfer considerations is proposed to estimate the maximum feeding rate before the extrusion becomes unstable. The modelling results agree well with the measurements. The model can be used to select the hot-end design as well as appropriate printing parameters.

Experimental and analytical study of the polymer melt flow through the hot-end in material extrusion additive manufacturing

This work presents Computational Fluid Dynamics (CFD) simulations of the polymer flow inside the hot-end during material extrusion additive manufacturing. Two CFD models are investigated: a previously-published one-phase model, where the entire domain is filled with liquid, and a novel model, where the free surface of the polymer inside the channel is resolved. Both models predict a recirculation region between the nozzle wall and the incoming filament. With the free surface resolved, melting of the solid filament and filling of an empty liquefier channel are shown in detail. Moreover, the simulations predict the pressure and temperature distributions inside the channel. The molten polymer (ABS) is simulated as a Generalized Newtonian Fluid with shear- and temperature-dependent viscosity. The numerical results are compared with the experimental measurements of the filament feeding force, which relates to the pressure inside the flow channel. An inverse analysis of the heat transfer coefficient is performed to estimate the thermal resistance at the channel’s wall. It is shown that the model which resolves the free surface is able to predict the feeding force for typical working conditions with a reasonable accuracy. Moreover, it captures the change of the flow regime from stable to unstable extrusion at high feeding rates. A hypothesis that explains the pressure and melt zone oscillations that occur during unstable extrusion is given. The influence of the liquefier temperature, liquefier length and nozzle diameter on the flow are discussed. The predictions of the model become less accurate when different channel geometries are simulated, which is attributed to the simplified material model that does not capture viscoelastic effects and possible buckling of the solid filament.

Numerical modeling of the polymer flow through the hot-end in filament-based material extrusion additive manufacturing

We demonstrate a novel Fused Filament Fabrication (FFF) nozzle design to enable measurements of in-situ conditions inside FFF nozzles, which is critical to ensuring that the polymer extrudate is flowing at appropriate temperature and flow rate during the part build process. Testing was performed with ABS filament using a modified Monoprice Maker Select 3D printer. In-situ measurements using the printer’s default temperature control settings showed an 11 °C decrease in temperature and significant fluctuation in pressure during printing as well as fluctuations while idle of ± 2 °C and ±14 kPa. These deviations were eliminated at lower flow rates with a properly calibrated proportional–integral–derivative (PID) system. At the highest tested flow rates, decreases in melt temperature as high as 6.5 °C were observed, even with a properly calibrated PID, providing critical insight into the significance of flow rate and PID calibration on actual polymer melt temperature inside the FFF nozzle. Pressure readings ranging from 140 to 6900 kPa were measured over a range of filament feed rates and corresponding extrusion flow rates. In-situ pressure measurements were higher than theoretical predictions using a power-law fluid model, suggesting that the assumptions used for theoretical calculations may not be completely capturing the dynamics in the FFF liquefier. Our nozzle prototype succeeded in measuring the internal conditions of FFF nozzles, thereby providing a number of important insights into the printing process which are vital for monitoring and improving FFF printed parts.

Devante C. Ruffin

Papers

In-situ monitoring of polymer flow temperature and pressure in extrusion based additive manufacturing