Die Science: Finding the root cause of stamping failures

Of the many process variables that interact to cause stamping failures, there three that stand out. Getty Images

“I just don't understand why we can't put this die in the press and make satisfactory parts without spending hours adjusting it. We paid good money for this die! They must have done something wrong when they designed or built it; otherwise we would be able to set it up and run it without making adjustments.”

Sound familiar? If you work in a sheet metal stamping operation or in a die building shop, you probably hear this frequently. Yes, die design is critical, but it’s only one element essential for manufacturing quality stamped parts. Variables such as the material’s mechanical properties, press speed and accuracy, lubricant type and application method, tool temperature, drawing ratio, punch and blank shape, holding pressure, and number of stations all play a part. Discovering how all process variables interact can help you find the root cause of typical stamping failures.

Failure 1: Parts Are Not Flat

When the material coming into the die isn’t flat, the pressure pads holding it down and securing it may not have enough force to hold it flat during cutting and forming operations. This often results in excessive cutting burrs, bending in improper locations, and nonconforming geometries.

When part flatness is lost, stampers often focus on the die. Tool- and diemakers shim stations and grind stations to try to “reflatten” the problem area. But dies can only slightly improve the flatness of the incoming sheet. They can be designed and engineered to retain sheet flatness, but it’s nearly impossible for the die to make a flat part out of incoming material—especially if the material has a very high tensile strength or is very thin.

In most of these cases, the problem is not the die, but the coil feeding and straightening equipment. The flatness and straightness of the incoming coil is a product of the straightener and leveler. Because the coil is wound tighter on the inside diameter than on the outside diameter, adjustments often need to be made to the coil feeder and straightener to compensate for coil set.

Failure 2: Parts Are Splitting or Wrinkling

Too much wrinkling can cause splitting in a stamping operation. For example, if the metal is allowed to wrinkle in the blank holder or draw pad area, it will be forced to unwrinkle before flowing into the draw die cavity. This will likely result in restricted metal flow, which in turn causes excessive stretching, thinning, and necking. Similarly, if the metal is allowed to wrinkle before flowing over a draw bead, the resulting restrictive force can cause excessive thinning or splitting. The diemaker must prevent wrinkling to stop the splitting by increasing blank holder force, adjusting the standoffs or equalizers, or changing the blank shape and/or size.

Conversely, splitting can result in wrinkling. If the metal is being overstretched and splits, sheet tension will decrease, allowing the sheet to wrinkle. It’s much like poking a finger into a piece of plastic wrap: Once your finger pushes the wrap, it wrinkles. To eliminate splitting, the diemaker can try decreasing holding forces, changing or adding lubricant, polishing the tool, or making changes in the blank shape or geometry.

Failure 3: Part Features Are Not in the Right Location

If the pitch or progression on a progressive die is not set up correctly or the pilot release is not properly calibrated, the die will not be able to locate and register the parts correctly in their respective stations. This will result in holes being pierced out of location and other features formed out of location.

An inexperienced diemaker may attempt to move the holes physically or make changes in the cutting or forming die geometry when the real problem lies with the timing of the feed release. For the die to register the strip properly within the tool, the feed release must let go of the material.

The Interactivity Maze

These are only a few examples of the interactivity that happens within a stamping process. Literally thousands of variables affect the stamping process, and a solution to a problem might involve a combination of several working together.

To find the root cause of stamping failure, you need to understand these variables and how they interact and take a systematic, data-based approach to the problem. Defect prevention is much better than defect correction.