Die-integrated knurl formation in cold forging of automotive pin bolts: process integration and experimental validation

In the production of automotive fasteners, conventional cold forging processes typically require an additional burnishing step to form knurls, which increases process complexity, production time, and variability in dimensional accuracy. However, studies specifically investigating alternative approaches for directly integrating knurl formation within the cold forging sequence remain limited. To fill this gap, the present study introduces and validates a novel die-based cold forging process in which the knurl geometry at both ends of pin bolts is directly formed inside the die, thereby eliminating the need for post-forging burnishing. To evaluate the feasibility of the proposed method, finite element (FE) simulations were performed to analyse material flow, forging forces, process requirements, potential production problems, and die stresses. Prototype productions and experimental tests were conducted to validate the simulation results. Findings revealed that the proposed process not only ensures continuous material flow and superior dimensional accuracy, as confirmed by microstructural and morphological evaluations showing smoother transitions and reduced stress concentrations in the die-formed knurls, but also enhances mechanical reliability through more uniform hardness distribution across the knurl region. Most notably, the production efficiency increased significantly, with manufacturing rates rising from 25 to 55 units per minute, corresponding to an improvement of approximately 120%. Overall, the study demonstrates that integrating knurl formation directly into the die offers a promising alternative to conventional methods, improving efficiency, dimensional stability, and mechanical performance for large-scale automotive applications. The new cold forging method integrates knurl formation within the die, enhancing production efficiency by eliminating additional forging step. Prototype productions validates the cold forging process, confirming a 120% improvement in production rate, from 25 units to 55 units per minute. Comparative analysis shows better geometric continuity and morphology. Prototype productions confirm the reliability and accuracy of FEM-based software.