Die Science: Preventing springback in thin, high-strength metals

Most stampers and die builders have experienced a gradual change in the metals they work with. As the automotive industry continues to strive to reduce vehicle weight while maintaining or improving performance and fuel efficiency, material thickness has decreased, and strength has increased.

In addition, stampers are focusing more on material savings. When I was a young apprentice, it was a reasonable engineering decision to give a cutting punch strength by leaving an extra quarter-inch of material between each part in a progressive die strip layout. Today that would be viewed as a major blunder.

These changes in material thickness, strength, and usage present challenges to stampers who are trying to prevent springback and form difficult geometries.

Work Hardening – No Strain, No Gain

For a stamping to retain its shape, it must be adequately strained. This means you must form the metal in a way that imparts work hardening, also known as strain hardening. If you don’t have adequate strain in the metal to meet its yield point—the point at which it permanently plastically deforms—elastic recovery, or springback, will occur.

There are two basic types of permanent strain. Tensile strain happens when the metal is stretched, and compressive strain happens when the metal is compressed or squeezed together. Generally, the higher the strain levels are in a part and the more uniform the strain is distributed, the lower the springback and greater the part consistency.

The metal’s ability to distribute strain through work hardening is commonly known as its work hardening exponent. This numeric value—typically 0.10 to 0.26 for carbon steel, for instance—is known as the metal’s “n” value. The higher the number, the greater the strain distribution and the less likelihood for metal splitting or springback. As the strength of the metal increases, the metal work hardening exponent typically decreases.

The material’s thickness also affects the metal forming process. Thicker metals stretch farther than thinner metals of the same type, and they resist wrinkling and buckling better. For these reasons, creating a die to produce a part made of thin, high-strength material can be a challenge, especially if the part is to be formed into a difficult, contoured geometry.

Three Blank Designs

Undeveloped Blank. An undeveloped blank starts off as a basic shape such as a square, rectangle, trapezoid, or any other shape that can be created using straight-line cuts. You can use a simple shearing die to cut a special shape from an undeveloped blank—there is no need to purchase a special blanking die.

An undeveloped blank typically provides a lot of extra material to grip outside the product geometry, and this allows the metal to be adequately stretched and achieve the necessary strain levels to reduce springback, control wrinkling, and produce a smooth part surface. However, the extra material, especially in the corners, can be trapped between the die face and pressure or drawing pad, creating a resistance to flow in the die. If the metal doesn't flow, it will ultimately overstretch and potentially split. When using an undeveloped blank, you likely will need to perform secondary trimming.

Semideveloped Blank. A partially or semideveloped blank is shaped so that just enough material is used, allowing for minimal material trimming later. A semideveloped blank requires slightly less material to produce a part than a fully developed blank, but secondary trimming still is required.

When a part is drawn, the blank must be shaped with the blank edge close enough to the forming punch to allow metal to flow inward toward the punch. Unlike a fully developed blank, a semideveloped blank provides just enough material outside of the forming punch to obtain adequate amounts of strain and flow.

Although a semideveloped blank requires both a blanking die and secondary trimming, it is considered to be the optimum blank style for most metal forming operations, especially deep drawing.

Fully Developed Blank. A fully developed blank is shaped so that after forming, no trimming is required. The net shape of the part is already established. This blank usually is susceptible to high springback because there’s no additional metal in the blank to strain using stretching, drawing, or compression.

A fully developed blank can reduce material consumption and tooling cost. It is smaller to begin with, and trimming dies are eliminated. Fully developed blanks lend themselves to simple geometry, so they can be formed with simple bending, flanging, and embossing processes.

However, because of normal sheet metal variables, such as small differences in mechanical properties, thicknesses, and frictional values, using a fully developed blank often results in mild to severe variation and inconsistency in the edge dimensions (trimlines) of the part. When severe drawing and stretching are used to form the part, something as simple as mixing and applying the forming lubricant can result in significant trimline inconsistency.

Of course, a fully developed blank is the best choice for certain applications. But you always need to consider the part geometry and material when choosing a blank style and shape. Designing and engineering a part from thin, high-strength material often comes at the cost of additional material and added forming stations.