The fused filament fabrication (FFF) process is similar to classic extrusion operations; solid polymer is melted, pressurized, and extruded to produce an object. At this level of investigation, it appears no new science or engineering is required. However, FFF has heat transfer limitations that are unique to it, due to its small throughput, not encountered in contemporary polymer processing, negating the use of present-day correlations or heuristics. Here, we quantify heat transfer by rheological modeling of the pressure drop data in the process to generate a general Nusselt number-Graetz number correlation. This is the first time the pressure has been measured in the die (nozzle) during normal printing that we accomplished by monitoring the power used to drive the hot end. Ultimately, we find that fouling within the region used to melt/soften the polymer significantly reduces the heat transfer rate. (C) 2018 The Society of Rheology.