Feed Continuity and Pellet Morphology in Compatibilizer and Plasticizer-Free High-Loading Soy Hull Fiber/PLA Single-Screw Compounding for Material Extrusion

High-loading plant-fiber/PLA feedstocks are often reported through final filament properties, leaving the upstream compounding mechanisms that determine pellet feedability in two-stage routes insufficiently resolved. This study developed a compatibilizer- and plasticizer-free single-screw compounding route for micronized soy hull fiber (SHF)/PLA through 42 sequential trials. Stable operation depended directly on continuous solids delivery. Reversible stop–restart observations and repeated defect patterns were consistent with a mechanism in which continuous feeding improves screw filling and solids-bed consolidation while limiting retained-air/gas carry-forward. Three-pass PLA milling reduced premix particle-scale mismatch and visible SHF/PLA agglomeration and enabled collective premix drying. The converged route used a double-mixing-zone screw, 2.75 mm die, 160/195/195/185 °C barrel profile, and 15 rev/min and produced downstream-usable pellets at 30 wt.% SHF, whereas 35 wt.% SHF defined a platform-specific processing boundary. Image analysis of 965 pellets and seven mapped pellet-to-filament lineages identified internal-texture P75 as the strongest route-level morphology marker; higher texture ranked with printing-filament readings above the 1.89 mm jamming threshold (ρ = 0.929, exact p = 0.0067). Compounded-pellet morphology therefore acts as a measurable process output linking upstream compounding history to downstream feed risk.