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The formability can also be estimate with thenumerical results. The method mainly involves the followingdetailed techniques: 3.1. Intermediate reference surfaceIn the calculation model, most of the intermediate referencesurface can be calculated automatically according to shapefeatures at local forming area. For example, the reference sur-face can be generated at the tangent direction along the borderof forming area. If the forming process is more complicated,the surface can also be generated manually with experience.However, it is difficult to design the intermediate shape of eachstep in the opposite sequence of processing step from finalshape to initial blank shape, especially for complex shape andforming process. When design the shape at current step, thereference surface should be generated from the shape of pre-vious step and the formability should be validated to avoiddefects. Sometimes not only the shape and forming processof current step but also the shapes of previous steps must bemodified.At the design stage of progressive die stamping part, thedesign engineers can generate relatively easily the intermedi-ate reference surface with the CAD software, such as UGNXand CATIA. It is difficult to predict precisely the interme-diate shape on the reference surface. Up to now, althoughthere are many authors try to optimize the shape of theintermediate reference surface with many optimization algo-rithms, the algorithms are only suit to the specific shape partand the optimization time is too long, which is unable tomeet the application requirements. Currently the intermedi-ate reference surface is generated by the designers and theintermediate shape is calculated using IA.3.2. Process conditionsIn the finite element model, many process conditions can beconsidered. For example, the fixed constrains will be addedat the nodes to assume that the relative displacement is verylittle at the connection area. At bending or flanging area, theinfluences of blank-holder force and friction are consideredto simulate the deformation of sheet metal. For large defor-mation at local area, the conditions treatment is differentbetween deep drawing and bulging. If the forming processis bulging, the restraint force or blank-holder force at flang-ing area is very big, the material of sheet metal is almoststiff, even fixed. If the forming process is deep drawing, therestraint force or blank-holder force at flanging area is rela-tively slightly, thematerial can flowat flanging area. InMUSM,the constraint or big blank-holder force should be added on thenodes at flanging area for bulging deformation.And the properblank-holder force will be added to corresponding nodes.3.3. Strain neutral layer offsettingAs shown in Fig. 3, the strain neutral layer (SNL) (Yingping andJun, 2002) will offset from the geometry neutral layer (GNL)to an inner layer along radial direction. Neutral layer offset-ting has a great influence on the blank shape size especiallywhen the rate between the thickness and radius of curvatureis bigger. When considering the SNL offsetting in MSUM, theshape size calculated and the experimental resultwill bemoreclosely. 3.4. Procedure of MUSMAswe know,material of sheetmetalwill flowin a differentwayfor different stamping process, and the formability of the finalpart will be also different. The sequence of MUSM must meetwith the requirement of actual stamping process in strip lay-out. Therefore, the stamping process must be designed firstlyaccording to the geometry feature and process requirementof the final part. Secondly, the intermediate shape will be cal-culated using MUSM. And the formability will be evaluated toavoid the defects such as crack and wrinkling. The referencesurface and boundary conditions should bemodified until theformability is perfect.Sometimes the surface should be modified again if theshapes calculated at previous steps are not good from thefull view of the strip layout. Finally, the optimum strip lay-out will be obtained after modify the stamping process manytimes.3.5. Formability analysisThe distribution of various physical quantities and the defor-mation of the inner holes can be calculated with the results.And the formability can be predicted to assist the design ofthe stamping process.3.6. LimitationsThe influence of restriking and springback are ignored duringunfolding process, and there are errors of the simulation resultfor the choice of shell element, which cannot simulate thedeformation of restriking and springback precisely.