Temperature gradient at the nozzle outlet in material extrusion additive manufacturing with thermoplastic filament

The extrudate temperature at the nozzle outlet is a crucial indicator of the process limit in material extrusion additive manufacturing with thermoplastic filament. However, there is a lack of accurate information on its radial distribution. This study bridges the gap by monitoring the extrudate surface temperature and studying the role of radial thermal conduction in extrudate cooling. The radial temperature distribution was determined with a parabola model under different poly(lactic acid) flow conditions through a 0.4 mm nozzle at 210 °C. The study found that both the surface and core temperature at the nozzle outlet decreased with the Graetz number Gz. The decrease was negligible at low flow rates when Gz < 1. At the process up-limit of stable extrusion, Gz = 3.2, the exit surface temperature was 194 °C while the exit core temperature was 164 °C, with a maximum radial gradient of 300 °C/mm at the nozzle outlet. This gradient empowered local thermal conduction from the extrudate surface layer towards its core for the first 0.3 s outside the nozzle. These results manifest the inability of the hot-end to sufficiently heat the core of filament feedstock from its circumference at high flow rates in material extrusion additive manufacturing.