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1. Introduction
Among the most widely used and rapidly growing rapid prototyping or additive manufacturing (AM) technologies are extrusion deposition processes such as fused deposition modeling (FDM® ), fused filament fabrication and melt extrusion manufacturing (MEM) ([38] Wohlers, 2011). In a typical process, a filament of material is fed into a machine via a pinch roller mechanism. The feedstock is melted in a heated liquefier with the solid portion of the filament acting as a piston to push the melt through a print nozzle. A gantry moves the print nozzle in the horizontal x-y plane as the material is deposited on a build surface that can be moved in the vertical z direction. This enables complex 3D objects to be produced as the melted bead leaving the nozzle solidifies. The most common materials used in this type of process are amorphous thermoplastics, with acrylonitrile butadiene styrene (ABS) being the most common. This article provides a more detailed overview of the typical components of extrusion-based AM processes and, more importantly, gives a review of the state of the art in process modeling and science for these processes.
1.1 Commercial market
Technologies first developed in the 1990s for rapid prototyping have grown in sophistication and their applications. Most parts produced with rapid prototyping in the past have been for use as visual aids, presentation models and rapidly produced, but not necessarily functional prototypes. While these applications remain a major part of the market for parts produced by melt extrusion AM and other 3D printing processes, a growing number of applications are for end-use parts that must meet stringent functional design requirements for mechanical properties and dimensional tolerances. The overall market for AM products and services has grown into a $1.325 billion industry (2010 estimate) and is projected to grow to over $5 billion by 2020 ([38] Wohlers, 2011). AM machines can be found in a number of settings worldwide such as in industrial plants, homes/offices, service providers, academic institutions and government/military settings. Investments in AM research and development from both government agencies and the private sector have grown rapidly in recent years ([30] Scott et al. , 2012), including the recent establishment of the National Additive Manufacturing Innovation Institute ([25] NAMII, 2012).
Extrusion-based processes are among...