Evidence of Flow-Induced Crystallization in Material Extrusion Additive Manufacturing

Material extrusion additive manufacturing processes force molten polymer through a printer nozzle at high shear rates prior to cooling and crystallization. This can lead to flow-induced crystallization in common polymer processing techniques, but the magnitude of this effect is unknown for additive manufacturing. Here, we will show the effect of the material extrusion on the morphology of poly(caprolactone) and poly(lactic acid), two common semicrystalline polymers used in material extrusion. The talk will encompass materials characterization (rheology and crystallization kinetics) relevant to printing, process line temperature and crystallinity measurements, and polarized optical microscopy techniques to characterize the final microstructure, which we compare with continuum modeling. When crystallization kinetics are fast compared to the timescale of the print process, the crystallization rate near the extrudate surface can be tracked in situ using Raman spectroscopy. Even when the as-printed part has negligible crystallinity, a post-print annealing process reveals spherulitic domains with sizes that dramatically decrease near the weld line. Our results show that residual chain stretch from the extrusion and deposition process enhances the nucleation rate in the weld regions between extruded layers.