Mechanical recycling of PA 12 from the multi jet fusion process: Influence of screw rotation speed on degradation and crystallization behavior

Mechanical recycling represents a viable strategy for reprocessing polyamide 12 (PA 12) powder waste generated from the Multi Jet Fusion (MJF) additive manufacturing process, enabling its reuse in the production of new materials beyond the additive manufacturing cycle. However, during mechanical recycling the polymer undergoes melting and shear stages during extrusion, which may promote degradation of the polymer matrix, resulting in molecular weight reduction and changes in thermal properties. These effects are associated with processing conditions such as temperature and screw rotation speed, which influence shear rate and the residence time of the material inside the extruder. In this context, this study evaluated the effect of screw rotation speed (100-200 rpm) on the rheological and thermal properties of recycled PA 12. The results showed a significant increase in the melt flow index (MFI) after extrusion, indicating molecular weight reduction due to chain scission during reprocessing. The degradation index (DI) ranged from 3.94 to 4.30, with the highest degradation observed at 150 rpm (DI = 4.30) and the lowest at 200 rpm (DI = 3.94), suggesting improved molecular retention at higher screw speeds. Linear regression analysis (DI = −0.0023x + 4.48; R 2 = 0.40) indicated a negative correlation between screw speed and degradation, attributed to reduced residence time at higher rotation speeds. Differential Scanning Calorimetry (DSC), Thermogravimetry (TGA) and Fourier Transform Infrared Spectroscopy (FTIR) analyses revealed thermal and chemical modifications associated with degradation. The results demonstrate that screw rotation speed controls the balance between chain scission and molecular reorganization during recycling.