Increased fracture toughness of additively manufactured semi-crystalline thermoplastics via thermal annealing

Polymeric components manufactured via freeform fabrication (FFF) typically have poor inter-laminar toughness resulting from incomplete bonding across layers during production. Here we study the effect of printing and post-processing on the inter-laminar toughness of additively manufactured semi-crystalline (poly-lactide (PLA)) structures. Specimens were subject to post-print thermal annealing to promote inter-laminar bonding, while post-annealing quenching rates were chosen to vary the induced degree of crystallinity in the final structure, as characterized via dynamic scanning calorimetry (DSC). Critical elastic-plastic strain energy release rates (JIc) of annealed samples were evaluated using the single edge notched bend (SENB) geometry and post-testing fractography. The results show that as-printed PLA adopts an amorphous character with good inter-laminar toughness and ductility. Post-print annealing can double the toughness via increased interfacial wetting, but only if the material is quenched rapidly to preserve the amorphous character. In contrast, post-print annealing followed by slow cooling results in a semi-crystalline state (≈25% crystallinity) with low fracture toughness and brittle fracture behavior.