FDM liquefies solid thermoplastic and extrudes it through nozzles not much thicker than a human hair. Each layer of plastic fuses to the previous one and is immediately solidified.

FDM first became widely used because of its ability to model with engineering thermoplastics that create tough, durable parts for functional testing. However, the process has become known for its accuracy, which today rivals injection molding tolerances. FDM produces accuracies as high as 0.003 in. The trade-off with FDM is that it cannot create a surface finish with the crisp appearance and sharp glass-like edges like a photopolymer prototyping system can.

However surface finishes can be smoothed, if needed, in a separate, optional smoothing process. The FDM process offers nine varieties of engineering thermoplastics. It should be noted that a smooth surface finish or sharp edge is not an indicator of accuracy. Accuracy is the ability to hold to a given tolerance, which FDM excels at.