Melt exit flow modelling and experimental validation for fused filament fabrication: From Newtonian to non-Newtonian effects

Fused filament fabrication (FFF) is an important additive manufacturing technique to produce complex poly-meric parts in fields such as prototyping, medical devices and sporting equipment. An outstanding challenge is the detailed understanding of the flow behaviour once the filament has been molten, as it determines the structure and stability. Here one can distinguish between the flow downstream the nozzle exit and the shape variations once the flow is deposited on the printing plate. Advanced modelling techniques are recommended but currently one typically assumes Newtonian flow, despite that conventional large-scale extrusion studies have highlighted the relevance of the viscoelastic nature of thermoplastic polymers, specifically upon the die exit. In the present contribution, polypropylene (PP) FFF flow is simulated at a reference FFF temperature of 473 K with ANSYS Polyflow by inputting Phan-Thien-Tanner (PTT) viscoelastic parameters that have been previously validated, based on both rheological and conventional extrusion slit die swell measurements. The suitability of these PTT parameters is further confirmed by comparing simulated and experimental FFF swelling data, with a coefficient of determination of 0.916. Benefiting from this validation, the flow model is reliably used to highlight that both the FFF processing parameters, e.g. temperature and printing speed, and machine parameters, e.g. nozzle length and diameter, have a large impact on the viscoelastic exit flow. A sensitivity analysis on the viscoelastic parameters additionally highlights the relevance of dedicated rheological data to understand FFF swelling and to accurately simulate the subsequent deposition stage.