Computational fluid dynamics simulation of the melting process in the fused filament fabrication additive manufacturing technique

Numerical simulation is used to understand the melting and pressurization mechanism in fused filament fabrication (FFF). The results show the incoming fiber melts axisymmetrically, forming a cone of unmelted material in the center surrounded by melted polymer. Details of the simulation reveal that a recirculating vortex of melted polymer is formed at the fiber entrance to the hot end. The large viscosity within this vortex acts to effectively seal the system against back-pressures of order 1000 psi (10 MPa), which are typical under standard printing conditions. The Generalized Newtonian Fluid (GNF) model was appropriate for simulation within the region that melts the fiber, however, a viscoelastic model, the Phan-Thien-Tanner (PTT) model, was required to capture flow within the nozzle. This is due to the presence of an elongational flow as molten material transitions from the melting region (diameter of 3 mm) to the nozzle at the exit (diameter of 0.5 mm). Remarkably, almost half the pressure drop occurs over the short capillary (0.5 mm in length) attached to the end of the converging flow region. Increased manufacturing rates are limited by high pressures, necessitating more consideration in the nozzle design of future FFF printers.