Process-induced morphological features in material extrusion-based additive manufacturing of polypropylene

Abstract Fused filament fabrication is a material extrusion-based additive manufacturing technique that has been strongly growing in popularity, for it is accessible, versatile, and affordable. The 3D-printing of semi-crystalline polymers still faces major challenges, though. Apart from common issues such as shrinkage and warpage phenomena, a variety of process-related morphological and crystallographic changes can occur. Since these changes strongly influence the resulting material properties, it is crucial to understand the complex relationships between the material, its processing and final properties on a fundamental level. In this context, the present work examines the impact of different nozzle temperatures and printing speeds on 3D-printed polypropylene (PP) samples. One extreme parameter set (high nozzle temperature, low printing speed) reveals a homogeneous morphology, weak flow-induced orientations, isotropic thermal conductivities and a strong inter-layer diffusion. In contrast, the other extreme parameter set (low nozzle temperature, high printing speed) forms an inhomogeneous morphology with a complex growth of shish-kebab structures, a pronounced weld line morphology and a highly anisotropic behaviour. By in-depth analyses of four parameter sets, this paper offers novel insights into the complex formation of crystalline structures in 3D-printed semi-crystalline polymers and suggests how to purposefully design the property portfolio of these materials.