  Although 3D printing has the potential to transform manufacturing processes,
the strength of printed parts often does not rival that of
traditionally-manufactured parts. The fused-filament fabrication method
involves melting a thermoplastic, followed by layer-by-layer extrusion of the
molten viscoelastic material to fabricate a three-dimensional object. The
strength of the welds between layers is controlled by interdiffusion and
entanglement of the melt across the interface. However, diffusion slows down as
the printed layer cools towards the glass transition temperature. Diffusion is
also affected by high shear rates in the nozzle, which significantly deform and
disentangle the polymer microstructure prior to welding. In this paper, we
model non-isothermal polymer relaxation, entanglement recovery, and diffusion
processes that occur post-extrusion to investigate the effects that typical
printing conditions and amorphous (non-crystalline) polymer rheology have on
the ultimate weld structure. Although we find the weld thickness to be of the
order of the polymer size, the structure of the weld is anisotropic and
relatively disentangled; reduced mechanical strength at the weld is attributed
to this lower degree of entanglement.
