The die-face shape control of a stretch-draw die

A computer model for the stretch-draw die assembly is designed using the developed die-face surface geometry for the punch, upper and lower binder elements. The dimensions of the die design are adopted for a single action type hydraulic press. The integration of the stamping die to the press machine is done with the lower and upper die adaptor plates, over which all other die construction elements are dimensioned and built following the guidelines in the in-house die design and construction standard. The major design elements of the upper and lower die assemblies are punch and binder castings, guide post and bushings, the lower and upper die adaptor plates, and wear plates. A geometric interference control under a ram-stroke of maximum 508mm is performed using a number of position analyses of the punch, die and blankholder elements during a complete forming cycle in order to make sure a consistent forming process. The height of the complete stamping die assembly is calculated to be 992mm at the forming position, and the overall dimensions are determined by the geometry of the ram and bolster plates on the manufacturing press.

In order to compute die-face distortions during the stretch-draw forming process, the finite element models are generated for both the lower and upper die assemblies. In the computational modeling, the dynamic effects of the inertial forces are neglected, and static stresses and deformation distributions of both upper and lower tooling assemblies are calculated as the self-weight of the construction elements and the forming loads determined previously with the process simulation models based on the ideally rigid forming interface. The mass of the stamping die assembly is computed to be approximately 8.9 ton including lower and upper adaptor plates, and in the finite element analysis the weight of the tooling due the gravitation acceleration in included as static distribute body forces. The forming loads applied during the stamping process, on the other hand, are changing during the forming process. With the assumption of ideally rigid forming interface, the forming loads generate variable normal and tangential forces acting on each rigid contact facet representing the die-face design. In order to reduce the computational cost of an incremental time-history analysis of the complete stamping die design, the maximum values are applied on the corresponding finite elements as static normal and tangential surface loads, and static stresses and deformation distributions of both upper and lower tooling assemblies are calculated at the instant of maximum loading.

The magnitude of the maximum displacement vector is observed near to the tip one of the punch element with a value of approximately 0.86mm, which is about half of the blank thickness. The maximum effective stress is calculated as approximately 509MPa and observed on the inner-ribs of the punch element due to the higher process loads acting during the forming the blank. Considering the typical yield stress values of die construction steels about 400 Mpa, a localized plasticity is expected on the punch casting. In order to increase the stiffness of the punch element, the wall thickness is increased from 4 to 6 mm. As a simply design modification, and the finite element analysis of the lower tooling assembly is repeated. In this case both the maximum face deformation and the maximum effective stress values are reduced to 0.61 mm and 348 Mpa, respectively, with a slight change in the overall distributions and the total weight of the lower die assembly.

An assessment of die-face distortions

The effect of the die-face distortions due to the tooling elasticity is investigated by conducting a sheet metal forming process simulated by conducting a sheet metal forming process simulation run using the updated forming interface mesh while all other computational parameters are kept the same of the simulation models assuming a rigid tooling construction. The numerical procedure uses the surface mesh nodal displacement vectors of the stamping die determined in the precious section in the updating of the nodal positions of the forming interface. The stamping form of the part is determined with the forming simulation run followed by a springback analysis after the forming simulation with a 200-ton blankholder force. A comparison of the equivalent plastic strain distribution computed with forming interface models with and without the die face distortions showed virtually no difference. A similar statement holds true for the distribution of the amount of thinning. There is a slight difference; however, in the computed stamping forms after the springback deformations.

Conclusions

An engineering methodology is proposed for the quantitative assessment for the accuracy of the ideally rigid die hypothesis typically employed in the forming process design of sheet metal stamping parts. Following a short review of the stamping die design practice, the computational approach based on the computer aided design. An industrial application is used to demonstrate the basic steps, and both the computation and the refinement analysis of the die-face deformations of a complete stamping die design process are presented. The formability and springback deformation analyses are conducted form the stamping process of an automotive structural part made of high strength steel. The geometric differences between the stamping forms determined with the ideally rigid and deformable forming interface are discussed, and the effect of die-face deformations on the final geometry of the stamping form is reduced by increasing the stiffness of the punch casting. The maximum deformation of the punch face is found to be less then the half of the thickness of the blank confirming the rigid forming interface hypothesis.