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The following study is a computerized analysis of the energy required to form a cross member with a hat-profile and a bottom embossment at the end of the stroke (Figure 1).

 

Figure 1: Cross section of a component having a longitudinal embossment to improve stiffness locally.1

 

Energy requirements increase with punch displacement, as shown in Figure 2, here energy curves are shown for Mild, HSLA 250/350, and DP 350/600 steels. The three dots indicate the start of the embossment formation at a punch depth of 85 mm.

Figure 2: Computerized analysis showing the increase in energy needed to form the component with different steel grades. Forming the embossment begins at 85 mm of punch travel.1

 

The last increment of punch travel to 98 mm requires significantly higher energy, as shown in Figure 3. Throughout the punch travel however, the two higher strength steels appear to maintain a constant proportional increase over the Mild steel.

Figure 3: A further increase in energy is required to finish embossing.1

Setting Draw Beads

A considerable force is required to set draw beads in AHSS before part formation begins. A nitrogen die cushion may be inadequate to fully set the beads, resulting in insufficient restraining force. Binder separation may occur, resulting in a loss of control for the stamping process and excessive wrinkling of the part or addendum.  Based on part geometry, the high impact load on the cushion may occur several inches from the bottom of the press stroke. Since the impact point in the stroke is both a higher velocity point and now at a de-rated press tonnage, shock loads develop and can cause damage to mechanical presses. Additional flywheel energy is dissipated by the high shock loads well above bottom dead center of the stroke.  Staggering the heights of the nitrogen cylinders so they do not all engage at the same time is one way to reduce the shock load (see Figure 4).

Figure 4: Staggered nitrogen cylinders designed to reduce the initial shock load when setting draw beads by engaging at different depths in the press stroke. 2

A double-action press will set the draw beads when the outer slide approaches bottom dead center, where the full tonnage rating is available and where the slide velocity is substantially lower. This minimizes any shock loads on die and press with resultant load spikes less likely to exceed the rated press capacity.

 

Sources:
1  G. Hartmann, “Das Spektrum Moderner Stahlfeinbleche-Festigkeiten und Auswirkungen auf die Umformung” Verschleißschutztechnik, Schopfheim, Germany (2004).
2  Courtesy of Peter J. Mooney, Managing Director, 3S Superior Stamping Solutions LLC

Header Image: Courtesy of Komatsu