A Combined Molecular Dynamics and Experimental Study on the Processing and Property Modification of UHMWPE / HDPE Blends by CaSt 2

High melt viscosity limits the processability of ultra‐high‐molecular‐weight polyethylene (UHMWPE). Here, UHMWPE/high‐density polyethylene (HDPE) blends modified with calcium stearate (CaSt 2 ) were studied by combined molecular dynamics (MD) simulation and experiment. MD results show that 0.5 wt% CaSt 2 maximizes the fractional free volume and diffusion coefficient, with increases of 8.06% and 27.17%, respectively. Interaction strength increases monotonically with CaSt 2 content, while chain mobility peaks at 0.5 wt%, indicating that excessive interactions restrict segmental motion at higher loading. Experimentally, CaSt 2 increases the melt flow rate by 23.53%, reduces processing torque, improves extrusion stability, and increases tensile strength and crystallinity by 4.01% and 5.18%, respectively. These results show that appropriate CaSt 2 incorporation improves melt processability while maintaining mechanical performance.