Recycled milled carbon fibers in fused filament fabrication of composite filaments: Thermophysical analysis and 3D printability assessment for automotive parts manufacturing

Recycled fibers like glass, carbon, and basalt are increasingly used in 3D-printed composites due to their high performance and sustainability. This study explores the use of recycled milled carbon fibers (rCFs) as fillers in polyamide-6,6 (PA66) filaments for automotive parts via fused filament fabrication (FFF). Building on prior research, the focus was on analyzing the rheological (melt mass flow rate) and thermal properties (thermogravimetry, differential scanning calorimetry, Vicat softening temperature) of PA66 filaments loaded with 5 and 10 wt% rCFs. Results showed that rCFs did not significantly affect the thermal stability of the PA66 matrix, with glass transition temperature and crystallinity remaining constant. A slight increase in melting and crystallization temperatures was observed, attributed to rCFs promoting nucleation and restricting molecular movement. The melt flow rate decreased with increasing rCF content, and the Vicat softening temperature increased by 3°C for 5 wt% rCF and 9°C for 10 wt% rCF. Then, prototypes of automotive shark fin antenna covers were 3D printed, showing good resolution and dimensional accuracy. However, structural defects could be improved by optimizing print parameters. Thermo-mechanical analysis showed no significant changes in thermal stability and transition temperatures. While minimal porosity (<4%) was observed, the Vicat softening temperature of the printed parts dropped by approximately 10°C, still within acceptable limits for automotive applications. Scanning electron microscopy images revealed slight defects such as air pockets and delamination but no fiber clustering.