Effect of Chain Alignment and Entanglements on Thermal Welding in Fused Filament Fabrication

3D printed structures are often substantially weaker than those made with conventional techniques, but the reason has been unclear. Indeed alignment from flow during deposition was expected to reduce the number of entanglements and thus accelerate welding of filaments through interdiffusion. To address this puzzle we performed molecular dynamics simulations of welding between polymer melts with different degrees of initial alignment. Standard measures of the entanglement density, such as primitive path analysis, indicate significant entanglement loss with increasing alignment, but contrary to existing theories there is no change in the rate of interdiffusion or reorientation. The distance polymers have diffused and the number of entanglements formed across the interface are independent of initial alignment, and chain reorientation always occurs on the equilibrium disentanglement time. Despite this, mechanical tests show that parts welded from aligned states are weaker. We show that this is due to residual alignment in regions near the weld that yield before the weld. The maximum shear strength and fracture energy of systems containing a weld are the same as systems starting from uniform melts with the same alignment.