Measuring & Predicting Crystal Morphology in Fused Deposition Modeling
Claire McIlroy, Jonathan Seppala, Anthony Kotula
Semi-crystalline polymer melts are commonly used in fused deposition modeling. Although flows have a profound effect on polymer crystallization, the relationship between typical fused deposition modeling (FDM) deformation rates and printed-part crystal morphology is yet to be understood. Here FDM is used to print a wall of poly-lactic acid filaments. The linear rheology and quiescent crystallization kinetics are characterized, infra-red imaging is used to measure temperature variations during the FDM process, and optical microscopy is employed to determine the resulting crystal morphology. Our flow-enhanced crystallization model demonstrates that FDM-induced polymer stretch leads to higher nucleation density and greater space filling in the weld regions during printing. Consequently, after a post-printing thermal annealing process, the weld region of a deposited filament features smaller spherulites than the center, in agreement with optical microscopy data. Finally, flow-induced crystallization is proposed as a method to improve weld toughness.