The Condensed Structure and Property Evolution of Polyamide 6 Fiber in Industrial‐Scale Multistage Drawing and Heat‐Setting Processes

Post‐spinning processing profoundly influences the mechanical performance of polyamide 6 (PA6) fibers by reshaping their condensed structure. Herein, multistage drawing and heat‐setting post‐spinning processes were applied to as‐spun PA6 fibers prepared via industrial scale melt spinning. Fibers were obtained via both in‐process sampling at different stages of a single post‐spinning process and end‐product sampling under varied process parameters. DSC, FTIR, WAXD, and orientation characterization were utilized to track the evolution of the condensed structure, while mechanical properties and dry heat shrinkage were assessed to establish the structure‐performance relationship. During the two‐stage drawing, the crystallinity and orientation of fibers continually improve, accompanied by the transformation of metastable γ‐phase and amorphous phases to stable α‐phase. Increasing the drawing ratios enhances the content of rigid structures, thereby boosting fiber strength but elevating dry heat shrinkage. Two‐stage heat‐setting alleviates the internal stress induced by drawing. A moderate relaxation rate (2.50%) facilitates phase transition, enhancing crystalline perfection and balancing strength with shrinkage. In contrast, a 3.00% relaxation rate disrupts the crystalline structure and orientation, thereby impairing the overall fiber performance. This study provides a theoretical foundation for regulating PA6 fiber structure and properties, supporting industrial process optimization and high‐performance fiber development.