3D printing utilizing the extrusion of thermoplastic material is easily the most common and recognizable 3DP process. The most popular name for the process is Fused Deposition Modelling (FDM).
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The process works by melting plastic filament that is deposited, via a heated extruder, a layer at a time, onto a build platform according to the 3D data supplied to the printer. Each layer hardens as it is deposited and bonds to the previous layer. The FDM/FFF processes require support structures for any applications with overhanging geometries.
The great advantage of FDM is the durable materials it uses, the stability of their mechanical properties over time, and the quality of the parts. The production-grade thermoplastic materials used in FDM are suitable for detailed functional prototypes, durable manufacturing tools and low-volume manufacturing parts.
Support structures, or lack thereof, have generally been a limitation of the entry level FFF 3D printers. However, as the systems have evolved and improved to incorporate dual extrusion heads, it has become less of an issue. Occasionally, the process can be slow for some part geometries and layer-to-layer adhesion can be a problem, resulting in parts that are not watertight.