A recent article in Polymer Composites examined the use of recycled milled carbon fibers (rCFs) as fillers in polyamide-6,6 (PA66) filaments for fused filament fabrication (FFF), targeting automotive part production. The rheological and thermal properties of PA66 filaments loaded with 5 and 10 wt.% rCFs were analyzed experimentally.
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Background: Fiber-Reinforced Filaments for FFF
FFF, also known as fused deposition modeling (FDM), is a widely used 3D printing technique in industries including automotive, aerospace, medical, and consumer goods. The properties of the filament material heavily influence the mechanical and thermal performance of printed parts.
Reinforcing fibers are often added to thermoplastics to enhance filament performance. rCFs have gained attention for offering environmental, economic, and technical benefits.
While rCFs have proven effective in improving the performance of printed samples, their processability in industrial-scale FFF for functional components remains underexplored.
This study addresses that gap by producing and characterizing rCF-loaded PA66 filaments and testing them in automotive part prototyping.
Methods: From Waste Fiber to Functional Filament
The base material used for filament production was PA66. The rCFs were obtained as a byproduct from an industrial process that converts pyrolyzed carbon fibers into non-woven fabrics. After drying, the fibers were blended with PA66 in two compositions:
- 5 wt.% rCF (PA-rCF5)
- 10 wt.% rCF (PA-rCF10)
These mixtures were extruded into composite filaments. A neat PA66 filament was also produced as a reference.
The thermal properties of the filaments and printed samples were analyzed using thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). Melt flow rate (MFR) testing was conducted to assess the flow characteristics. Morphology was examined through scanning electron microscopy (SEM).
To test printability, the rCF-reinforced filaments were used to fabricate a shark fin antenna cover prototype via FFF. The slicing process was completed with KISSlicer PRO v1.6.3, and printing parameters such as temperature and flow rate were optimized using the built-in tuning wizard. Two prototypes were produced:
- One with 25 % infill
- One hollow version (0 % infill) that was double the size
Results and Discussion: Material Behavior and Print Quality
Thermal analysis showed that rCFs caused only minor changes to the thermal behavior of the PA66 matrix. Slight increases in melting and crystallization temperatures were observed, likely due to the nucleation effect of the rCFs.
The main influence of the rCFs was seen in rheological behavior. The composites showed higher melt viscosity and Vicat softening temperature (VST) with increased rCF content, improving the material’s heat resistance and stability.
Importantly, the inclusion of rCFs did not significantly compromise the thermal characteristics of the PA66. The modest improvements in mechanical and thermal performance, along with ease of processing, suggest that rCF-based filaments could be integrated into existing manufacturing workflows without significant changes to equipment or settings.
The 3D-printed shark fin antenna covers showed good dimensional accuracy and low porosity (<4 %), indicating consistent print quality. SEM images revealed minor structural defects, such as delamination and small air pockets, but no major issues like fiber clustering or filament gaps.
Overall, the material's printability was confirmed, and the resulting parts met baseline requirements for automotive prototypes. Some surface and structural imperfections could be resolved through further tuning of the printing process.
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Conclusion: Toward Circular and Scalable Automotive Manufacturing
This study demonstrated the potential of using rCF-reinforced PA66 filaments in the additive manufacturing of automotive components. The filaments showed favorable rheological and thermal properties and performed well in 3D printing tests, producing functional and dimensionally accurate prototypes.
rCFs offer a sustainable, circular approach to reinforcing thermoplastics. Their integration into 3D printing materials not only adds value to industrial waste but also aligns with broader goals of reducing material costs and environmental impact.
Future work should focus on refining the print process to address the minor defects observed and to optimize mechanical performance. Broader application testing and scaling the production process could support the transition to full-scale FFF use in automotive manufacturing, helping reduce waste while advancing sustainable design.
Journal Reference
Sambucci, M., et al. (2025). Recycled milled carbon fibers in fused filament fabrication of composite filaments: Thermophysical analysis and 3D printability assessment for automotive parts manufacturing. Polymer Composites. DOI: 10.1002/pc.29909, https://4spepublications.onlinelibrary.wiley.com/doi/10.1002/pc.29909
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