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Introduction
Additive manufacturing (AM), also known as 3D printing, offers the ability to produce complex components of arbitrary shape and design directly from a solid model. This technology was initially developed in the 1980s and has been widely used for the production of prototypes (Turner et al. , 2014; Wendel et al. , 2008; The Economist , 2012; Scott et al. , 2012). As the mechanical properties of printed parts have improved over the past decade, it has become possible to directly fabricate finished components suitable for functional service in the field. However, 3D printing technologies are generally restricted to small build volumes (0.03-0.3 m3 ), have slow deposition rates (15-85 cm3 /h) and are very expensive ($100-200/kg).
The Oak Ridge National Laboratory (ORNL) is developing a large-scale 3D printer for thermoplastics and composite materials, called Big Area Additive Manufacturing (BAAM) (Holshouser et al. , 2013). The BAAM system is similar to fused deposition modeling (FDM(TM)) in that both technologies extrude hot thermoplastic material along specific tool paths to generate three-dimensional shapes. However, instead of resistively melting a thin filament feedstock, the BAAM technology uses a single-screw extruder to melt a pelletized feedstock similar to that used in the injection molding industry. Not only does this make a wide variety of common thermoplastic materials readily available for BAAM, but it also reduces the cost of feedstock materials by a factor of 20× ($2-$10/kg). The single-screw extruder is also capable of depositing material at a rate that is >200× faster (up to 50 kg/h) than conventional polymer AM systems. The BAAM technology has been released as a commercial product by Cincinnati Inc. (BAAM-CI) (Krassenstein, 2015; Cincinnati Inc), and similar approaches for large-scale printing of extruded thermoplastics are currently under investigation by various industry, government and academic entities (McNabb et al. , 2013; Duffy, 2015; Millsaps, 2015).
The BAAM build platform can accommodate structures as large as 6 m in length, 2.4 m in width and 1.8 m in height, which is ~10× larger than most commercial systems. As the BAAM system operates outside of an oven, it is critical to deposit with reinforced thermoplastic materials to reduce distortion and warping (Love et al. , 2014). Compared to unfilled resins, fiber-reinforced thermoplastics have been...