Die Science: Solving stretch flange problems (Part I)

Cracking failure in a part’s stretch flange is one of the most common problems stampers face. And even if the flange is not splitting, it might not be flat, which also can cause problems.

What is a Stretch Flange

Stretch flanging is a deformation process in which the metal is bent along a concave cured axis. This curvature forces the material to stretch from a short length of line to a longer length of line (see Figure 1 ). Because the metal is in tension, it has a tendency to crack or split along the curved surface.

Stretch flanges are commonly designed in stamped parts. Features such as the length of flange and the radial profile determine the difficulty of the flange. Long flanges formed in an area with a small radial profile are the most difficult (see Figure 2 ).

Material Effects

Of all the factors controlling the success of a stretch flanging operation, material type and mechanical properties are the most influential. Materials exhibiting good stretchability and stretch distributions characteristics are best-suited for stretch flanging operations.

In the cause of ferrous metals, such as plain carbon steels, the metal’s stretching characteristic is indicated by its n value, or work-hardening exponent. This numeric value ranges from about 0.1 to 0.3. The higher the number, the more evenly and uniformly the material will stretch. In simple terms, it is defined as a metal’s ability to work-harden and stretch uniformly.

As odd as it may seem, steel needs to work-harden to exhibit good stretchability. One of the unique properties of steel is its ability to increase its hardness and strength as it plastically deforms. If a steel work-hardens quickly during plastic deformation, more surface area will be forced to deform (in other words, more steel will stretch), resulting in good strain distributions.

A metal’s stretch distribution characteristics and thickness are two of the most important factors controlling the success of a stretching operation. Metal with poor stretch distribution characteristics do not stretch over a large surface area; instead, stretch is confined to an isolated area. If only a small area of the cursed flange stretches, it will most likely fail.

Figure 3 shows two different stretch flanges, as well as the difference between metal with good stretch distribution and poor stretch distribution. The figure obviously shows that the metal with poor stretch distribution resulted in a split. However, note that the metal did not distribute the stretch over the strength of the flange. Solving such problems often requires a special preform operation to force the metal to distribute the stretch more evenly.

The figure also shows how successful stretch flange distributed the stretch more evenly over a large surface area.

The Effects of Cutting on the Flanging Process

Cutting metal before flanging can affect the results. A normal metal cutting process, with the exception of speciality operations such as grip flow or fineblanking, involves exceeding the metal’s shear strength, which causes metal to fracture or break free.

Before the metal fractures, however, the punch must cut or extrude the metal. This basic process results in a cut metal edge that has a portion of shear and a portion of fracture. This shear-to-fracture relationship (often referred to as the metal’s cut band) can be seen easily on the edge of any conventionally cut part (see Figure 4 ).

Stampers often find that grinding or sanding the edge of the blank before forming results in a more successful stretch flanging operations for two reasons: The smooth portion of the cut has more edge-stretching capability than the

1. fracture portion of the cut. Smoothing out this fracture zone results win more edge-stretching ability for the material.
2. During the metal cutting process, the metal often is deformed in compression around the punch or cutting section. This metal deformation usually results in work hardening of the material at the cut edge. This cold-worked material has less stretching ability than unworked material. Grinding the edge of the of the blank often removes the cold-worked material (see Figure 5 ).

Many stampers resort to shaving the cut edge in an effort to smoothen the cut edge and remove the work hardened edge.

Stampers don’t often get to use metals with good stretch distribution, nor do they typically find it economical to grind the edges of their blanks. Nevertheless, these factors can and do contribute to the success of a stretch flanging operation.

What should a stamper do with metal that is not very stretchable? What if grinding the blank edge still results in failure? Those questions will be answered in Part II.

Until next time… Best of luck!