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Polylactic acid is a semi-crystalline polymer melt commonly used in extrusion-based 3D printing applications. This involves extruding molten polymer through a nozzle and depositing filaments layer-by-layer. Rapid cooling and solidification ensures the structural integrity of the printed part.

Since the crystallization kinetics are typically slow, there is insufficient time above the glass transition for crystallinity to develop. Thus, deposited filaments usually have an amorphous micro-structure upon solidification. Moreover, the deposition flow stretches and orients the polymer molecules, and residual alignment becomes trapped in the weld regions between deposited filaments at the glass transition. We propose that this micro-structure leads to reduced weld strength.

Post-processing thermal annealing may be employed to increase the crystal content and improve mechanical properties. However, we find that non-uniform properties are inevitable. Resdiual polymer stretch can “template” flow-induced nuclei into the weld regions. Upon annealing these nuclei grow into much smaller spherulites, whose size depend on both print speed and temperature.