Effective Drawbead Modeling in Stamping Simulations

To achieve the optimal forming process of a stretched part without any cracks and wrinkles, control of material flow during the drawing operation is required. Such control is often achieved by using drawbeads in combination with a binder force. During the process design, the position, length and strength of drawbeads are modified to produce an optimal stamped part with a minimum use of material. Also during tryout, the drawbeads may be manually modified to obtain the desired process. In general, when using drawbeads several aspects must be taken into consideration in order to achieve optimal forming conditions.

The intensive usage of drawbeads as well as the need to carry out various modifications requires a structured approach to model drawbeads in a forming simulation. The modification of such simulation models should be fast and easy and should not influence the computation time significantly. With the latest release AutoFormplus R6, AutoForm Engineering introduced a method to effectively model the behavior of drawbeads. The methodology consists of:

  • 3D bead generation
  • Drawbead flattening
  • Accurate drawbead behavior
  • Drawbead unflattening

3D bead generation

For 3D bead generation, two aspects need to be considered: position and geometry of the bead. First, the position of the bead is defined by means of a curve at the location of the bead. This curve represents the center of the bead. Second, the geometry of the bead is defined based on a 2D profile of the bead cross section. Finally, the bead profile is modeled in the tool surfaces at the location of the drawbead curve.

Drawbead flattening

Drawbeads can influence the shape of the sheet after binder closing and wrinkles may occur. The appearance of wrinkles can only be detected by using geometric drawbeads in the simulation. These geometric drawbeads also allow for more accurate blank outline determination because the material used by the drawbead is taken into account. Geometric drawbeads are therefore used during binder closing, when the computation time is relatively short. After binder closing, the bead area is flattened and a line bead is used during the drawing process. Effects such as blank draw-in and the appearance of wrinkles remain present after drawbead flattening.

Accurate drawbead behavior

Adaptive line bead is a sophisticated line bead model that uses current process conditions in the calculation of drawbead effects. The drawbead restraining and uplifting factors (fR and fU) are constantly recalculated during the simulation based on the following parameters:

  • Drawbead geometry, which is influenced by the current gap between the tools. The gap may change during the simulation as a result of tool opening, leading to reduction of drawbead effects.
  • Current sheet thickness
  • Local properties of the material (work hardening). If, for example, the sheet hardens before entering the drawbead, then the restraining force that the material generates in the bead will increase.
  • In-plane forces (membrane stress) due to binder pressure or due to restraining in the double bead area

This means that, similar to what happens in reality, the drawbead effects can change at any moment in the drawing process. One of the major advantages of the adaptive line bead model, therefore, is that once the user defines the geometry of the drawbead, all of the factors listed above are automatically considered in the calculation. There is no need to perform manual adjustments of the restraining conditions as with double beads or inclined areas of the binder.

Drawbead unflattening

Unflattening is an option which allows the reverse flattening of geometric beads at the end of the drawing operation. In some cases, it may be necessary to reintroduce geometric beads to consider, for example:

  • The influence of structural stiffness of the part on its shape after springback following the drawing operation
  • Any influence of drawbead shape during locating in the secondary operation


Concluding Remark

In order to effectively incorporate the effects of drawbeads during a stamping simulation, it is recommended to use the adaptive line bead model in combination with the flattening functionality. If there are also secondary forming operations to be simulated, the adaptive line bead model should be used in combination with the flattening and unflattening steps.