Numerical Simulation Of The Effect Of Elastic Cushion In The Process Of Multi-Point Forming

Multi-point forming(MPF) is a new means for sheet metal forming. Its mainidea is dividing the curved surface of the traditional stamping dies into many pairsof matrices of elements, and then the three-dimensional shape of sheet metal isformed by the curved surface of elements. Relative to the traditional stampingtechnique, the MPF has saved a lot of time for die design, die manufacturing anddebugging. It has shortened the exploiting cycle and reduced the costs. Now, theMPF technology can be applied extensively in many fields, such as the skin ofhigh-speed locomotive, ship shell plates, automobile panel, large sculpture piecesin the city.In the MPF the principal loading part is elements, so the part possiblyappears dimples owing to the excessively centralizing load on the sheet metal. Tosuppress dimples, elastic cushion technology is adopted, which is the blanketbetween punches and blank to distribute the pressure on the blank by itself largedeformation filling the interspaces among elements during the forming course. Butthe parameters and thickness of elastic cushion have great impact on not onlydimples but the precision of the part. So to obtain the ideal forming results, elasticcushion should be selected reasonably.With computer applications and the maturity of nonlinear finite element, thenumerical simulation technology of sheet metal forming is developed quickly. Nowthe 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 thesheet metal forming because the contact between the elements and the sheet isdiscontinuous and the loading condition is very complicated. So a dynamic explicitfinite element software—ABAQUS is introduced to simulate the MPF for sheetmetal forming, including the creation of finite element model, the choice ofmaterial model, the establishment of boundary conditions and the treatment ofcontact friction and so on. The dimples defect and its controlling methods aredetailedly analyzed and studied in this paper.The main contents and results are as follows:1) Control of dimple and method to eliminate error with elastic cushion inmulti-point formingBlankholder must be used in punching. To get good result, accurate force onblankholder is of great important. A lot of time will be lost to confirm magnitude ofthe force with numerical simulation in many times. Fix up and down blankholderand calculate the anti-force on blankholders. Thus more exact force on blankholderwill be got. It can save much time and efficiency is improved greatly.Dimples will come into being when punching with multi-point forming.Dimple can be eliminated to a certain extent using elastic cushion. Compared withideal shape little error appears when using elastic cushion. The method to correctthe error is that first, calculate the value of elastic cushion's changed thickness withnumerical simulation after part is formed sufficiently, second, conform everyelement's displacement of multi-point mould by thickness change of elasticcushion to build new surface, last, use new multi-point mould in punching andmore exact part will be got.2) The numerical simulation of the effect on elastic cushion suppressingdimples when stretchingThrough changing the thickness of elastic cushion with different shape ofmulti-point mould when stretching, the effect of elastic cushion controlling dimplesis studied. The results indicate that elastic cushion can effectively suppress dimples.If the thickness is small, dimples will occur on the deformed parts. With thethickness of elastic cushion increasing, the dimples become shallow obviously, andthe dimples are apparently suppressed when the thickness reaches a fixed value.And in the same time the elastic cushion distortion and its effect on the part shapeare investigated. The results show that with appropriate elastic cushion the surfaceof part is smooth and the forming result is very similar to that formed byconventional solid die. It illuminates that multi-point die can be applied to theactual stretching process.3) Numerical analysis of springback about stretching with multi-point mouldFor springback analysis of stretching, ABAQUS/Explicit is used in simulatingforming process and ABAQUS/Standard is used in springback. Numericalsimulation is done for two-direction paraboloid parts with different thickness anddifferent pre-stretching, conclusion is as followed: With the increase ofpre-stretching, the springback on stretched parts decreases and with the increase ofsheet metal's thickness, the springback decreases. In stretching the springback instretching direction is far larger than shrinking direction. So to get precise resultsuitable blank thickness and pre-stretching magnitude should be selected in actualapplication.