In the automotive industries, the design and construction practice of stamping dies is adopted according to the sheet metal type and draw action in accordance with the type of the chosen manufacturing press. Usually, an in-house die design and construction standard is followed in the material selection for the die elements including the detailed specification of casting and heat treatment procedures. Using the developed die-face design as the starting dimensions for the punch, upper and lower binder elements, the guidelines given in the in-house standard are employed in the dimensioning and integration of the major structural elements, such as the lower and upper die adaptor plates, punch and binder casting, guide post and bushings and wear plates. Additionally, by selecting press tool action, the bolster-ram geometry and punch stroke completely define the forming die construction. The use of a CAD system at this phase allows the design engineer to build a virtual prototype of the upper and lower halves of the forming die using a number of geometry parameters such as the inner ram shut height and the geometry of the adaptor plate, punch and binder wall thickness or position of blankholder balance blocks. A number of position analyses of the punch, die and blankholder elements during a complete forming cycle are conducted to control the interference and overlap to eliminate any inconsistency between the ram stroke and amount of drawing.

The die-face shape control

The sheet metal forming process is a compound system made up of the stamping di and the blank, and involves a set of mechanical interactions with the press and the foundation structure that provide the necessary forming energy. Assuming an ideally rigid die construction connected to the ram and bolster plates of an ideally rigid press and neglecting all die-face distortions help the methods engineer designing the forming process following a pure geometric modeling procedure only. Otherwise, it would be an enormous engineering effort to include all of these mechanical systems in a computational model that is intended to simulate the sheet metal deformation response during the forming process. It is therefore the most practical approach to isolate the forming interface, i.e., the die-face design and the blank, from the remaining, and to model the blank deformations under the forming forces generated during the frictional contact with the purely geometric description of the die-face design. Moreover, this proposition has found widespread use in the industry for even large inner panel draw-dies in the case of conventional sheet metals. The notion of an ideally rigid draw-die construction, nevertheless, may become questionable when it comes to the forming of high strength steels due to the higher forming loads needed. In addition, a side trust is generated in the forming of large-scale structural parts with non-symmetric profiles, which may apply remarkably high loads on the balancer blocks and o the wear plates between the punch and binder elements increasing the wear and distortion of guideposts. In these cases the deformations of the production tooling should be included in the computational modeling of the forming process.

Presently, building a computer model in order to simulate the complete process system is achievable considering the advancements in computer hardware and finite element software, nonetheless it is hardly feasible from an industrial perspective due to the high computer analysis times. Instead a rather simple but a practical engineering approach may be the decoupling the stamping system in to the process-only part composed of the blank plus the die-face design and tooling-only part containing complete draw-die design, based on the individual characteristics of the deformations experienced during a complete pressing-cycle, respectively. Considering process-only part, there are time-dependent interactions of the sheet metal blank and the forming interface bringing about large changes in the blank shape when compared with the scale of die-face distortions. Consequently, the process-only part should be simulated using a finite element formulation based on large-strain and finite incremental deformation theory due to the kinematic characteristics of the blank deformation. On the other hand, small deformation transients superimposed on the large displacements histories characterize the deformation of the draw-die elements during a single forming cycle. Therefore, a small strain elastic-plastic finite element analysis of the draw-die construction may be appropriate.

The interaction of both computational sides is defined in terms of an appropriate data transfer routines. For the process-only part, the forming simulation uses the geometry information from die-face design as rigid surface entities and the blank as an elastic-plastic deforming body, and the time-dependent displacement-driven binder and punch motion realize the forming process. The major outputs are deformed geometry and production stress distributions of the blank after spring back and the forming load histories as well as the frictional contact stress distributions over the die-face elements. On the other side, for the tooling-only part, the forming load histories are the basic input for the assessment of the die-face deformation analysis. The finite element analysis of the complete draw-die design for a given press cycle provides the displacements of die-face material points, and the updated die-face design may be fed back to process-only part for the next iteration. Also the computed elastic-only part for the next iteration. Also the computed elastic-plastic stress-strain histories during a complete press cycle and the contact forces between the punch and binder elements through the wear plates and balancer blocks bring about a significant insight to the interaction of draw-die construction elements.

Industrial application

The engineering methodology outlined in the previous section is employed in the structural assessment for the stamping die design of a front-side cab body member before the tooling production. The strength of this part is an essential feature in the crash-energy management of a cab frame. The conventional design practice for this type of large-scale structural elements is to use draw-quality steels of thickness. The commercial availability of 1.5mm HSLA steel with a higher yield stress and moderate level formability, introduced as a feasible alternative with enhanced strength properties along with an approximately 10-15% weight reduction.