Damage prediction in the multistep forging process of subminiature screws

In this study, a multi-step forging process of subminiature Torx screws is investigated. The production of subminiature screws requires precision forming technologies for the head forging process and the thread rolling process. In the head forging process, various defects such as micro-cracks or material folding are frequently observed due to the small size of screw heads and the highly-concaved geometry of the Torx pattern. To predict the formation of these defects, finite element (FE) analyses were performed for the multistep forging process of subminiature screws. The Cockcroft-Latham damage criterion was used to predict micro-cracks and the refined mesh was used to predict the folding defect, from which the possibilities of the crack and folding defects were numerically evaluated. Based on the FE analysis results, the effects of die design parameters and forging conditions were investigated for the purpose of reducing the amount of defects. The design of multistep dies was changed and analyzed through FE simulation. Experiments based on the analysis results were conducted, and the improved die can be used to prevent the formation of crack defects and folding defects.