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The upper and lower surfaces of sheet metalin the forming process are pressed by elastic poles, inorder to improve the stability of sheet metal.It can be seen from Fig. 6(a) that sheet metal ispressed by loads Px, the elastic pressing forces Fe arelocated in the upper and lower surfaces of sheet metal.Pressing forces are produced by springs in flexible poles.The upper elastic pressing force equals to the lower cor-responding force and the condition of sheet metal main-tains plane, as shown in Fig. 6(a).1—Coupled pole, 2—Blank, 3—Blank-holder, 4—Variant headsFig. 5 The stamping processes of the multi-point pressing and forming sheetPxPx PxPxPxzPx xxPx Pxbyaxx OObFe Fe FeΔFe1ΔFe2ΔFe3ΔFe4ΔFe FeKO LFeFe Fe Fe FezOOay(a) Pressing forces on surfaces of sheet(c) Pressure difference by pressure springs(b) Pressure differences of surfacesFig. 6 The relationship of elastic pressing forces on sheet and buckling critical stressK is the coefficient of elasticity of the spring insidethe flexible pole, L is the compressed length of thespring, as shown in Fig. 6(c). When sheet metal is gen-erated deflection δ, spring in the convex side of sheetcan be further compressed and elastic force made byspring is increasing. Springs on the concave side ofsheet will relax a bit. The elastic pressing force differ-ence ΔFe made by the two side springs can be obtainedas ΔFe =2Kδ.When sheet metal occurs bending made by the pres-sure load Px as shown in Fig. 6(b), it will produceforce differences ΔFe1,ΔFe2 ••• at the pressing points.These pressure differences may inhibit further bendingof sheet and can suppress wrinkling of sheet as well.The upper and lower surfaces of sheet metal are ap-plied loads Fe. When sheet metal occurs buckling, itmust overcome the work produced by pressure differ-ence ΔFe of load. The work is W = mΔFeδ/2.There m is the number of pressing points, δ is max-imum value of deflection after buckling and δ/2isthe average deflection of m pressing points approximately.When the surfaces of sheet aren’t applied with pres-sure forces Fe, the sheet metal is pressed with face-parallel load Px.Critical compressive stress is calculated with energyapproach, the formula of buckling critical stresses σcrof sheet[9-10]:σcr = π2Da2d 1+ a2b2 2. (1)When the surfaces of sheet are applied with pressureforces Fe, the buckling critical stress σcrq can be got.σcrq = π2Da2d 1+ a2b2 2+ 4mΔFeπ2δd,according to ΔFe =2Kδ,σcrq = π2Da2d 1+ a2b2 2+ 2mKπ2d, (2)where D = ETd312(1 − μ2), d is the sheet metal thickness,ET is the tangent modulus, μ is Poisson’s ratio.It is noted from Eqs. (1) and (2) that σcrq is greaterthan σcr obviously. Comparing Eq. (1) with Eq. (2), itcan be discovered from Eq. (2) that the buckling criticalstress of sheet metal increases along with the increasingof elastic coefficient of springs. Increasing the elasticcoefficient of springs will lead to enlarge the pressuredifference ΔFe and improve the buckling critical stress.It indicates that the elastic pressing force applied tothe surfaces of sheet can improve the buckling criticalstresses of sheet metal. The process of the multi-pointpressing and forming sheet can suppress buckling andwrinkling of sheet effectively.4TestsThe process of “the multi-point pressing and form-ing sheet” is carried out by the poles flexible die. Polesflexible die has the ability of quick reconfiguration. Thedie can be constructed by adjusting the length of polesand choosing sleeves and fitting the suitable spring inpoles and changing corresponding heads. Poles flexi-ble die has many advantages to surpass traditional die.It can effectively suppress wrinkle of sheet, also it canbring new method into the sheet metal forming. Follow-ing experiments will show the characteristics about theprocess of “the multi-point pressing and forming sheet”