Which of the following is NOT one of the key process variables in FDM?

In Fused Deposition Modeling (FDM), several key process variables significantly influence the quality, precision, and characteristics of the printed object. One of these variables is the nozzle tip diameter, as it affects the extrusion width and layer adhesion. The feed rate of filament material is crucial because it determines the amount of material that is fed into the nozzle, influencing the extrusion consistency and overall print quality. Additionally, nozzle transverse velocity, or the speed at which the nozzle moves while extruding, directly impacts layer deposition and surface finish.

The printer color setting, however, does not influence the mechanical or physical aspects of the printing process itself. It relates more to the aesthetic or appearance of the final print rather than how the materials are processed or laid down during printing. Thus, it is not considered a key process variable in the context of FDM, which focuses on mechanical parameters critical for successful print outcomes.