Melt Rheology to Guide Commercial Polypropylene Grade Selection in Material-Extrusion Additive Manufacturing

Filament-based material extrusion additive manufacturing (MEAM) is a well-established technique in additive manufacturing (AM). However, semicrystalline polymers, such as polypropylene (PP), have seen limited commercial use in MEAM processes historically because of their rapid crystallization kinetics, which adversely affect the integrity of printed structures. While prior research has explored PP blends and composites to improve printability, the influence of varying properties among commercial PP grades, despite similar molecular weights (MWs) and molecular weight distributions (MWDs), remains underexplored. These grades differ in properties such as melt flow index (MFI), additive content, and polymer architecture. Although MFI is widely used as a rapid measure of processability and, indirectly, mechanical performance, it does not account for the effects of additives, crystallinity, or molecular architecture. As such, MFI alone may not reliably predict the print quality or mechanical properties. In this work, melt rheological parameters, particularly the storage/loss modulus crossover point, are evaluated as complementary indicators for selecting PP grades for MEAM. Different commercial PP grades, blended with a fixed loading of a hydrocarbon resin to control crystallization rates and enhance interlayer adhesion by extending re-entanglement times, were subjected to melt rheological measurements to assess their suitability for MEAM. The resulting rheological profiles correlate with increased polymer chain interdiffusion and reduced warpage in printed structures while also revealing trade-offs such as reduced tensile strength and increased sensitivity to print orientation. Notably, PP1B and PP2 exhibit similar MFI values but markedly different rheological responses and structural properties of the prints, demonstrating that MFI alone is insufficient to predict MEAM performance across neat and modified commercial PP grades. This rheological approach provides a practical method for predicting the structural integrity and mechanical performance of PP-based MEAM parts, enabling more informed material selection than relying on MFI alone.