Mechanisms of elastoviscoplastic polymer flows in material extrusion additive manufacturing

Understanding the coupled effects of elasticity and plasticity in polymeric inks is crucial for improving strand quality and residual stress management in material extrusion (MEX) additive manufacturing (AM). However, these effects have been overlooked in existing studies. In this work, the Saramito model is employed to characterize the complex rheological properties of elastoviscoplastic (EVP) inks in extrusion and deposition. Parametric studies are conducted by varying the Bingham number (Bi), Weissenberg number (Wi), and the ratio of printing speed to extrusion speed (v/u). Results identify four distinct printing modes and reveal that high Bi leads to poor printing quality at non-optimal Wi and v/u. Additionally, an unbalanced distribution of residual stress is observed in the strand at higher Bi, and the shape of strands tends to be unsmooth. The normalized height (H/D) of the stable area decreases with an increase in Wi, while the normalized width (W/D) shows the opposite trend. Furthermore, the unyielded area and the distribution of residual stress inside strands show that the dominant mechanism transitions from plasticity and elasticity as Bi increases. The larger v/u leads to an elongated shape of the stable area, and an obvious neck appears between the head and the stable area. This work contributes to the understanding of plastic deformation of complex polymers in various flow states, thereby providing valuable guidance to MEX-AM.