Springback in Stamped Parts and How Dies Fix It
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Bend a piece of sheet metal and let go, and it opens up a little. That elastic recovery is springback. In stamping, it's the main reason parts from a new die don't match the CAD, and it's a bigger problem than it used to be.
What drives it
Springback grows with the material's yield strength and shrinks with its stiffness. Higher strength means more elastic strain stored in the part when the die opens. That's why advanced high-strength steels spring back much more than mild steel, and why aluminum, with about a third of steel's stiffness, springs back roughly three times as much as a steel of the same yield strength. Larger bend radii relative to the sheet thickness make it worse too.
Ways to fix it
- Overbend: form past the target so the part springs back to it.
- Restrike or coin: hit the bend again, or squeeze the radius, to set it.
- Add stretch: lock the sheet with draw beads or stake beads so it stretches over the punch. A few percent of stretch reduces springback a lot, because the whole section yields in tension.
- Change the part: ribs, darts, flanges and steps stiffen a part and hold its shape. They are best added at the design stage.
- Compensate the die: simulate forming and springback, then adjust the die surface so the part springs back into shape.
Simulate first
Forming simulation software predicts thinning, splits, wrinkles and springback before any steel is cut. It also produces the forming limit diagram, the chart that shows how close each area of the part is to splitting. Compensating the die surface from simulation, then fine-tuning in tryout, is now standard practice for high-strength steel parts.
Tryout still matters
Simulation gets the die close. Real coils vary in thickness and properties from batch to batch, so the die is tuned in tryout and the parts are checked on a checking fixture or scanned against the CAD.
Forming simulation is part of how we design stamping dies. See tooling.