Die Science: Handling production splitting problems, Part I

I’ve heard it a thousand times: “This #%-*#*!*!* splits are killing us! Why can’t you die guys make the tooling more robust? The material is within spec. I checked it myself! Yesterday the die was running great, and now it’s only making scrap.”

Sheet metal, dies, presses, and lubricants respond only to simple physics, and when sheet metal meets physics, physics always wins. Understanding these physics principles is critical to a successful stamping operation, because it allows you to discover some possible causes of the splitting, and, therefore, ways to correct it.

Possible Cause 1—Unsuitable Material Properties

Bad or unsuitable material undoubtedly take step most blame for contributing the splits. Many drawing and starching operations are marginal at best, so any minor variation in incoming material properties most likely will cause a split. If the material is on the low end of the spec, you have problems.

Your choices are to modify your process to accommodate expected minor material variations or to hold your material to a tighter spec. However, holding tighter material specs will cost considerably more. The obvious long-term solution is to alter your process to fit the material better.

To begin altering the process, start with a few questions: Who defined your material specs? How were they defined? Was a formability study ever conducted on the formed part to determine if the material is suited to the process when it is on the low end of the spec?

When deep drawing, you need to understand the importance of critical mechanical properties such as elongation, n value, r value, and yield strength and the effects they have on your process.

Possible Cause 2— Blank Thickness Variability

On a large blank, thickness variation form edge to edge can be significant, especially if the material is a heavy gauge, less expensive product. Blanks varying in thickness will not flow and stretch consistently.

Whenever possible, check to make sure that your metal is at constant thickness. If server thickness variation is inevitable, make adjustments in the equalizer height to compensate for the differences.

Differences in a metal’s thickness from edge to edge often is called a right and left-handed material, and it is a result of the metal rolling process. When subjected to severe pressure, the rollers used to reduce the metal thickness deflect, causing thickness differences from edge to edge in the master coil. If the master coil then is slit into different coil widths, metal thicknesses will vary from one edge of the blank to the other (see Figure 2 )

Possible Cause 3 — Rust

Don’t run rusty metal through a drawing or forming operation. Rust is very abrasive and can cause serve die damage, as well as severe surface sects in class A exposed body panels.

Rust can result in tool steel galling; destruction of die coatings; and erosive damage to the die face, blank holder, punch, and cavity.

Possible Cause 4— Incorrect Blank Size and Shape

The blank size and shape are critical factors to consider in drawing operations. Stampers often make blanks a little too large, thinking that some extra trim stock won’t hurt and that it’s better to have too much to trim off than not enough.

In reality, though, this extra material is not only wasteful and costly, but it can contribute to splits during drawing. The basic principle of draw ratio theory is that the more surface area that is trapped between the die face and the blank holder, the greater resistance the metal has to flow. If the metal doesn’t flow, it will stretch; if you starch it too far, it will split (see Figure 3 ).

Don’t always assume that you metal is suited to the process or that you are getting what you asked for. Many successful stampers have an internal metals testing lab to verify a metal’s properties before it is released to production. Whenever possible, determine what mechanical parameters your process can accept, and qualify you material on mechanical values rather than on a general spec.

Best of luck!