The forming process design of the part stamping form is done following a pure geometric modeling approach, and all die-face deformations are neglected assuming a rigid stamping tooling. Using the 3-CAD model of the part, firstly an undercut check is conducted in order to determine the possibility of forming the part in a single operation. Subsequent to the investigation of the tip angles for the part, the 3-D part geometry is rotated to the configuration of the ram motion direction, so that the part is positioned appropriately in the press coordinate system in which the pressing-axis became parallel to the part-drawing axis. Using the 3-D CAD model of the part, a set of surfaces for the upper die geometry is generated with an offset equal to the half of the sheet metal thickness, and a set of addendum areas are added to this set of surface geometries. Since the part sidelines follow approximately a similar form, and therefore a sweep binder surfaces and addendum flange are generated to complete the stamping die and upper binder surfaces of the stamping tooling. This 3-D composite surface forms the forming interface geometry from which the other tooling elements are obtained. The punch and binder interface are generated by a simple geometric duplication in the CAD environment, using their geometric counterparts through the thickness offsets in the pressing and normal directions. At this stage, the complete CAD description of the die-face design is obtained that can be employed in the finite element mesh generation for the formability assessment and springback analyses.
The formability assessment and springback deformation analyses are conducted using the concepts and methodologies. In the first finite element simulation run, the forming loads are defined using the predefined displacement-time history functions for the forming interface elements. During the forming process, the punch is kept fixed in its initial position, and both the binder and the die are loaded in displacement control, in which a constant clearance between the upper and lower binders of value 10% higher than the initial sheet thickness is kept constant. This type loading is intended to simulate a typical toggle draw press action. An investigation of the computed sheet metal deformations indicated the feasibility of a single stretch-draw forming process, and there is no material zone that tends to excessive stretching or wrinkle. However, the equivalent plastic strain distribution does show three localization zones that have relatively high values and may lead potential stamping failures due to the local material instability. The calculated amount of thinning is approximately 25%, which is not an acceptable stamping formability criterion for this part. An expected, the fixed clearance constraints between the upper and lower binders results in to the high contact forces along with higher frictional restraint force on the blank increasing the stretching deformations.
The plot of the forming loads variation indicated a maximum punch fore of 816 ton, while a remarkably high blank-holder force is predicted with a value of approximately half of the punch force. However, lowering the sheet metal holding force on the binder areas would significantly reduce the amount of stretching deformations, as well as the sheet metal thinning, and this can be achieved in the finite element simulation model by replacing the fixed clearance constraints between the upper and lower binders with a force-controlled blankholder force. Moreover, the overall sheet metal deformation pattern and the predicted localization zones indicate the feasibility of a single step stretch-draw forming process with an appropriate blankholder loading as a practical stamping design approach for this part.
A set of simulation runs with a force-controlled binder closure using a constant blankholder loads are performed in which the blankhollder force is increased from 100 to 300 ton with 50 ton increments. The feasibility of the sheet metal forming process conditions corresponding to each blankholder force value are evaluated using both the amount of thinning and the shape distortion after the springback deformations. In order to assess the shape distortion between the forming and the shape distortion between the forming geometry and the stamping geometry after the springback, the following parameter is introduced as a global measure of the springback deformation.
For higher blankholder force value, even though the shape of the stamping part, the material is exposed to relatively high stretching forces bringing about excessive thickness reductions which may lead forming failures in the form of tearing. On the other side, lower blankholder forces are good from the formability point of view; nevertheless the springback deformations are enhanced due to the reduced membrane plastic straining. For instance, in the case of 100-ton holding force, the amount of maximum thinning over the stamping form is reduced to 14.7% with a similar plastic strain distribution compared to 200-ton blankholder force case. However, there is a remarkable increase in the springback deformations when compared with that predicted under a 200-ton blankholder force, and consequently the shape distortion parameter is almost tripled between these two blankholder loading cases. A comparison of the formability and springback deformation indicated that a blankholder force of 200 ton sets the maximum allowable forming load considering the process and material related constraints that can be used in the computer aided design of the sheet metal forming die.
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