Numerical Simulation Of The Multi-Point Stretch-Forming For Sheet Metal

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 designing, 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 panels, and large sculpture pieces in the city. The craft skin stretching technology is an important developing direction for MPF. Applying the MPF to craft skin stretching has realized rapid and flexible forming which is suitable for the diversification,varieties and less amount.Hangs in the air region of blank and the shape of clamp have much effect on simulation results.In Stretch-Forming (MPSF),the pull is transferd by the hangs in the air region of blank,the pull can be transferd uniformity,but it is decreasing uniformity because the friction between the mold and the clamp.So the stretching of hangs in the air of blank is longer than other parts of blank,this situation always bring on rupturing of blank.The different shape of clamp has direct effect of simulation results on stress-strain. The stretch-forming of Saddle,it is important that if the blank has technologic allowance. The main contents and results are as follows:(1)In stretch-forming the length of hangs in the air region has much effect on the Von-mises,thickness and the precision of skin-stretching.Building the finite element model of Stretch-Forming (MPSF),using the different length of hangs in the air region on the simulation of saddle and sphericity. Analysing the effect on stretch-forming.The result shows that,when the length is 100mm,the change of stress,thickness are acuteness in blank.When the length is 150mm,the change of stress is little acuteness than when the length is 100mm,the max mises also declining.When the length is 200mm,the simulation result is the best of all these situation. Contrast the precision of stretch-forming between these three situation.The result shows that when the length of hangs in the air region increasing,the error between blank and target surface is reducing.So it is better to increasing the length of hangs in the air region when stretch-forming.(2)The analysis on stress,thickness and precison which casused by the changing the shape of clamp.The shape of clamp has important effect in stretch-forming.Contrast the straight clamp and curving clamp we can come to the conclusion that,when using the curving clamp the max stress is much lower than using the straight clamp.Thickness is uniformity.It always have the stress concentrate on the edge of clamp when using the straight clamp.This phenomena happens when using the curving clamp but the numerical value of stress is much lower.Reducing the risk of rupture. The error between blank and target surface is reducing when using the curving clamp.(3) Study the influence on the quality and precision of saddle blank when the blank has the technologic allowance.When the saddle stretch-forming,the technologic allowance has important effect on the results.Have the technologic allowance can restrain the shrinking of blank. When thire are no technologic allowance ,the change of stress,thickness are acuteness in blank.It is more uniformity by using the technologic allowance.To consider the precison of stretch-forming, the error between blank and target surface is reducing when using the technologic allowance.It is not appropriate to setting the technologic allowance too long or too short,it is such a waste to setting it too long but it can not encasing the model when it is too short.The appropriate technologic allowance length is between 30mm~50mm.(4) The simulation on actual Stretch-Forming (MPSF)The actual skin-stretch of blank always has big dimension and complex shape.Building the finite element model by considering these characteristics,choose the rational technics and load.Using the technics that mentioned in this article,to testifing that these technics can be used in actual stretch-forming and get a better effect.