Numerical Simulation Of Control Of Dimple By Elastic Cushion During Multi-point Forming Process Of Sheet

Multi-point forming (MPF) is a new means for sheet metal forming. Its main idea is dividing the curved surface of the traditional stamping dies into many pairs of matrices of elements, and then the three-dimensional shape of sheet metal is formed by the curved surface of elements. Relative to the traditional stamping technique, the MPF has saved a lot of time for die design, die manufacturing and debugging. It has shortened the exploiting cycle and reduced the costs. Now, the MPF technology can be applied extensively in many fields, such as the skin of high-speed locomotive, ship shell plates, automobile panel, large sculpture pieces in the city.In the MPF the principal loading part is elements, so the part possibly appears dimples owing to the excessively centralizing load on the sheet metal. To suppress dimples, elastic cushion technology is adopted, which is the blanket between punches and blank to distribute the pressure on the blank by itself large deformation filling the interspaces among elements during the forming course. Bu the parameters and thickness of elastic cushion have great impact on not only dimples but the precision of the part. So to obtain the ideal forming results, elastic cushion should be selected reasonably.With computer applications and the maturity of nonlinear finite element, the numerical simulation technology of sheet metal forming is developed quickly. Now the technology is widely applied in the die design for forecasting forming defects. In MPF, the dynamic explicit finite element arithmetic is very suited to simulate the sheet metal forming because the contact between the elements and the sheet is discontinuous and the loading condition is very complicated.So a dynamic explicit finite element software—LS-DYNA is introduced to simulate the MPF for sheet metal forming, including the creation of finite element model, the choice of material model, the establishment of boundary conditions and so on. The dimples defect and its controlling methods are detailedly analyzed and studied in this paper. The main contents and results are as follows: on the process of Multi-Point Forming, a certain radius of curvature on the different surfaces, either steel or aluminum, spherical or cylindrical surface, a small contact area exists between sheet metal and elements, so the part appears dimples owing to the excessively centralizing load on the sheet metal; Through changing the thickness of elastic cushion with different shape of multi-point forming when stretching, the effect of elastic cushion controlling dimples is studied. The results indicate that elastic cushion can effectively suppress dimples. If the thickness is small, dimples will occur on the deformed parts. With the thickness of elastic cushion increasing, the dimples become shallow obviously, and the dimples are apparently suppressed when the thickness reaches a fixed value; Dimples will come into being when punching with multi-point forming. Dimple can be eliminated to a certain extent using elastic cushion. Compared with ideal shape little error appears when using elastic cushion. The method to correct the error is that first, calculate the value of elastic cushion's changed thickness with numerical simulation after part is formed sufficiently, second, conform every element's displacement of multi-point mould by thickness change of elastic cushion to build new surface, last, use new multi-point mould in punching and more exact part will be got, the results of the multi-point forming technology with the reference value and significance.