Evidence of Flow-Induced Crystallization in Material Extrusion Additive Manufacturing

Material extrusion additive manufacturing processes force molten polymer through a printer nozzle at high (>100 s−1) 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(lactic acid), a common semicrystalline polymer used in material extrusion. The talk will encompass materials characterization (rheology and crystallization kinetics) relevant to printing, process line temperature measurements, and polarized optical microscopy techniques to characterize the final microstructure, which we compare with continuum modeling. Although no crystalline domains are seen in the as-printed part, 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. The higher nucleation density in these regions is not obvious in the as-printed part but is revealed only through the annealing process, which is captured in the model.