New perspectives on intra-layer reheating in fused filament fabrication

• Intra-layer reheating peak height may exceed that of inter-layer reheating • Intra-layer reheating may compensate for >50% of initial thermal energy loss • Nozzle makes direct physical contact with nearby strands when moving around In fused filament fabrication, part cooling is commonly described as an initial and boundary value problem of heat conduction in an evolving geometry, where the nozzle defines the initial condition of polymer strand segments with a nominal temperature upon deposition. Recently, two anomalous thermal behaviours were observed during the part cooling of a double-wall: the height of the intra-layer reheating peak exceeded that of the inter-layer reheating, and it even surpassed half of the temperature drop since deposition. These observations contradict the energy conservation principle under the current strand-strand conduction-based understanding of reheatings. To explain these anomalies, the random error, measurement error, nozzle radiation, and conduction anisotropy were excluded in high-resolution infrared thermography. A newly proposed heat transfer path way of direct physical contact between the hot nozzle tip and neighbouring strands was then implemented into the existing free software T4F3, thereby extending the physical model of part cooling in fused deposition 3D printing. With this extension, the anomalous reheating behaviours were successfully reproduced. As a first application, the interfacial heat transfer coefficient between the nozzle bottom surface and the polymer was estimated to be 2.0 ± 0.1 kW/m 2 /K. This insight provides a refined thermal framework that is critical for accurately predicting bond formation and optimising mechanical properties in fused deposition 3D printing.