Correlating molecular and crystallization dynamics to macroscopic fusion and thermodynamic stability in fused deposition modeling; a model study on polylactides

Abstract

To define molecular parameters for fused deposition modeling of mechanically integral polylactide parts, the effect of intrinsic local heat fluctuations on morphology and structure evolution is studied. Macroscopic fusion during melt deposition is governed by molecular dynamics of solidification and positively affected by low print speed, low molar mass. However, low molar mass and high L-enantiomeric purity induces melt crystallization during deposition, limiting interfacial molecular diffusion. By increasing molar mass crystallization during melt deposition is suppressed, establishing interfacial molecular diffusion and mechanically effective interfaces. Further structure evolution via cold crystallization is timed in successive annealing cycles. Adding more layers entails a progressive decrease (i) in heat transfer to the build plate and (ii) number of annealing cycles per layer, inducing variations in crystallinity and thus thermodynamic instability. Consequently, macroscopic mechanics and geometrical stability of fused deposition modeled polylactides are compromised by judiciously timed crystallization and process design.