Fused deposition modeling (FDM) is the additive manufacturing process that builds an object by extruding melted thermoplastic filament through a heated nozzle and depositing it in stacked layers.
The mechanism is continuous rather than stepwise. An extruder drives solid filament into a heated block where it melts, the nozzle traces the cross-section of the current layer onto a build plate, and the toolhead or plate then steps along the Z axis so the next layer fuses onto the one beneath it. Geometry that overhangs unsupported air requires printed support structures, which are removed after the part cools.
The process carries two names for legal rather than technical reasons. Scott Crump patented it in 1989 and founded Stratasys to commercialize it, and FDM remains a Stratasys trademark; the RepRap project coined fused filament fabrication, or FFF, as an unencumbered synonym. Expiry of the core patents in the late 2000s opened the consumer market that produced machines like the 📝Bambu Lab X2D.
FDM is the cheapest and most widely deployed form of 📝3D Printing, and it accepts the broadest range of materials. Its defining limitation is anisotropy: a printed part is meaningfully weaker across its layer lines than along them, because layers are fused rather than continuous, which makes print orientation a structural decision rather than a convenience. Resin processes produce finer detail and powder-bed processes produce more isotropic parts, both at higher cost and handling burden.