Feed-Controlled Filament Extrusion of High-Loading Micronized Soy Hull Fiber/PLA Biocomposites for Fused Deposition Modeling

This study reports the filament-making stage of a sequential single-screw process-development pathway for compatibilizer- and plasticizer-free soy hull fiber (SHF)/PLA biocomposites used in fused deposition modeling. Thirty-three filament-making trials were interpreted through an event-linked process chain, and 14 trials were evaluated using phase-resolved in-line diameter records and capability-style Cp/Cpk metrics. Filament-making converged on a single-mixing-zone screw, a 3.85 mm orifice/5.75 mm land die, 10 rev/min, and a 160/170/180/195 °C barrel profile for 10–30 wt.% SHF, whereas neat PLA required 180/185/200/205 °C. The strongest sustained benchmark was a 10SHF filament produced under converged settings, with a mean diameter of 1.7411 mm, a standard deviation of 0.0236 mm, 95.45% of readings within 1.70–1.80 mm, and only 0.013% above 1.89 mm. Feed replenishment, depletion, irregular pellets, recycled material, and fines-rich feed shifted the same nominal configuration among controlled and unstable states. The highest reliably spool-fed formulation was 30 wt.% SHF. The 35SHF filament remained nozzle-depositable from loose coils but fractured repeatedly during take-up and direct spool unwinding in 3D printing. Operational validation of all four converged filament formulations comprised 720 printed mechanical-test specimens over approximately 936 h. The reported conditions define platform-specific operating windows, but the process insights hold global relevance for pellet-based extrusion systems.