When sourcing milled carbon fiber, the most critical factor is the fiber length (typically 50-150 microns), as this determines the reinforcement efficiency and surface finish of the final composite. You must also verify the bulk density, which affects the ease of mixing in resin or thermoplastic systems. Ensure the carbon content is ≥95% for high-performance applications, and check the sizing/coating type (e.g., epoxy-compatible or polyurethane-compatible) to ensure chemical bonding with your specific matrix material.