Manufacturing-ready hemp fiber–blended composites: extrusion compounding, pelletization, and injection-molding outcomes

This study investigates the industrial-scale production and mechanical performance of hemp fiber–reinforced thermoplastic composites. High-density polyethylene (HDPE), polylactic acid (PLA), acrylonitrile styrene acrylate (ASA), and polybutylene terephthalate (PBT) were compounded with unretted and field-retted hemp fibers (powder, ground, and chopped) at 30–50 wt% using extrusion and injection molding process. Injection-molded specimens were tested to verify processability and usability. For HDPE composites at 30 wt% hemp, the elastic modulus increased from 1.1 GPa for neat HDPE to 2.9–3.1 GPa with unretted fibers, while tensile strength decreased from 22 MPa to 17.5–21.5 MPa, depending on fiber size. In PLA, adding 30 wt% field-retted hemp powder obtained from dust collection reduced UTS from 50 MPa to 38 MPa, whereas PLA containing powder produced from long bast fibers retained higher strength (∼43 MPa). The modulus increased by up to 160 % at 50 wt% fiber loading. Elongation at break decreased significantly, with HDPE dropping from 290 % to 1.42 % at 50 wt% fiber content, while hardness increased by 10–12 % across all systems. Field-retted fibers from dust collection showed lower performance than long bast fibers, whereas unretted fibers maintained higher modulus and ductility. Overall, powdered hemp fibers at 30 wt% provided the best balance between stiffness enhancement and processability.