Controlling Residual Stress in Material Extrusion

Fused filament fabrication (FFF) is a popular method of Material Extrusion additive manufacturing. Semi-crystalline polymers such as poly(lactic acid) (PLA) and poly(ether ether ketone) (PEEK) are easily processed using FFF; however, the slow mobility of polymer molecules, along with a complex thermal history, results in residual stress and limited interfilament diffusion within the printed parts. The impact of graphene incorporation into FFF feedstocks and in situ light-initiated crosslinking on residual stress is examined in this work. Stress release in FFF-printed PLA and PEEK monoliths was monitored over a 24-hour period by measuring their dimensional changes with thermal annealing (irreversible thermal strain). Filament formulation and light irradiation produced varying magnitudes of strain in the annealed monoliths, indicating that the residual stress of FFF-printed structures can be controlled. Differential scanning calorimetry provides evidence that polymer spherulite growth contributes to the irreversible strain; this is particularly important for PEEK, in which crystalline domains act as barriers to diffusion but may grow large enough to produce interlocked filament interfaces. Annealing printed structures with solvent vapors to release internal stress will also be discussed.