Factors Affecting the Pillow Effect in Single-Point Incremental Forming
Authors: Jiang Guoping , Liu Xiaojun , Shi Jin 
Published: June 2024

Single-point incremental forming (SPIF) is a machining process that uses a tool to perform extrusion motion on a metal sheet blank, causing plastic deformation of the metal sheet blank to achieve the target model size accuracy requirement. The pillow effect, which occurs in the unformed region at the bottom of the part, significantly impacts the accuracy of the final product. Optimizing process parameters is necessary to minimize the impact of the pillow effect on accuracy. Conducting experiments to optimize the process parameters will inevitably increase research and development costs and extend the research and development cycle. However, using simulation models to simulate and optimize process parameters can effectively reduce research and development costs and shorten the research and development cycle. This work aims to study the effects of seven factors that affect the formation of the pillow effect in SPIF machining parts, including wall angle, diameter, height, wall thickness, downward step size, tool path interlayer connection method, and side wall shape. First, based on the ANSYS Workbench/LS-DYNA platform, a simulation model is constructed, and machining experiments and simulations are conducted to compare the results of the experiments and simulations by comparing the key points of the pillow effect profile curve to verify the feasibility of the model. Due to the layered machining characteristics of the single-point system, the pillow effect feature is easily formed in the unformed area at the bottom, and the pillow effect can be clearly seen to experience an increase zone, an upward zone, and a steady zone from the cross-sectional profile chart through the center axis. The simulation results show that the minimum pillow effect amount can be obtained when the process parameters are wall angle of 30°, bottom diameter of Φ50mm, forming depth of 30mm, plate thickness of 1mm, downward step size of 1mm, straight-line interlayer connection method, and hyperbolic side wall shape. Then, using the Taguchi method to set up an orthogonal experiment of L18(3^7), conduct simulation experiments, and obtain the measurement data of the pillow effect. The software used for the analysis of experimental signal-to-noise ratio data is Minitab-19, and the signal-to-noise ratio refers to the ratio of useful data read in the experiment to interference noise data. Variance analysis is performed on the obtained results to determine the optimal process parameter configuration of the seven factors. A prediction model is generated, and a confirmation experiment is conducted to validate the model. The confirmation experiment was conducted three times, and the average value of the measurement peak results was taken. The measurement results of the confirmation experiment were within the confidence interval of 95% confidence level. The results showed that the pillow effect decreases with an increase in wall angle and downward step size, and increases with an increase in bottom diameter, forming depth, and plate thickness. When the tool path is selected to be a straight-line interlayer connection method and the part side wall shape is a hyperbolic shape, a smaller pillow effect can be obtained.