Some factors affecting inter-layer weld strength in material extrusion 3D-printed amorphous and semicrystalline polymers*

Material extrusion (MatEx) 3D printing is an innovative polymer processing route which allows fabrication of parts with complex and customized geometries. Being based on a layer-by-layer deposition, the extent of welding occurring between the layers largely affects the mechanical response of the printed material.

To gain a molecular understanding of this process, we conducted experiments and molecular modeling on both amorphous and semicrystalline polymers. It is expected that the macromolecular diffusion process leading to inter-layer welding is arrested by the glass transition, for amorphous polymers, or by crystallization, for semicrystalline ones.

As reference polymers, poly(lactic acid) (PLA) and polypropylene (PP) are chosen. Experimentally, via birefringence and polarization modulated infra-red microspectroscopy measurements, it is revealed that some molecular orientation is frozen-in for PLA, particularly at the weld region. Such orientation increases with more severe printing conditions (higher printing speed and lower nozzle temperatures) and with polymer molar mass. Moreover, it is shown that the presence of residual alignment of the chains at the weld is detrimental for the weld strength, even though the modeling indicates that full macromolecular interdiffusion between the layers has occurred during cooling.

For a series of PP based materials, it is shown that the weld strength is limited with increasing molar mass and crystallization rate of the polymers. The modeling of the process demonstrates that, to capture the hindered interdiffusion between the layers, the role of flow-induced crystallization must be taken into account.

These results, spanning both the effect of molecular variables and processing conditions for amorphous and semicrystalline polymers, expands our understanding on the weld formation process in 3D printing, paving the way for improved control and optimization of polymer additive manufacturing.