We consider the interplay between heat transfer, rheological properties and the processing performance of a 3D printer. Heat transfer limits the processing rate achievable by any FFF 3D printer and we have developed a technique to quantify it. The first step was to determine the pressure drop in the nozzle which we do by monitoring the power required to extrude polymer out of the nozzle. Since the rheological properties are known, the pressure drop can be modeled to find the temperature of the polymer as it exits the printer. This allowed us to develop a dimensionless correlation that should be applicable to any 3D printer aiding design in the future. Finally, since the outlet temperature is known, the welding behavior of deposited tracks (or roads) can be measured and modeled by a simple reptation-based theory. Good agreement between the model and the measured weld strength are found. These two aspects of 3D printing any material are critical to fabricate a strong object and with the models one can then optimize the printer and/or material.