Die Science: The top 5 drawing die violations

Deep-drawing failures—from splitting to scoring to shock lines—often are a result of common engineering mistakes. These problems can occur whether you are using a single-station drawing die, multiple drawing stations, or drawing reductions.

Let’s take a look at the five most common engineering mistakes that can cause deep-drawing failure.

1. Wrong Material Quality

Material that is described as “deep-draw quality” must have the appropriate formability properties for the deep-drawing process. Just because a material has this label does not mean it has the specific mechanical ability to be drawn. This description instead just means that the material has some drawing qualities.

Take a look at the material’s tensile strength, yield strength, n and r values, and elongation. These basic values define the metal’s forming characteristics. Also, keep in mind that thinner metals typically present more forming challenges than thicker metals of the same type.

As a rule, to be drawn with reasonable ease, metal must have a minimum elongation of 40 percent, a minimum r value of 1.6, and a minimum n value of 0.2.

2. Inaccurately Sized Die Entry Radius

If a die entry radius is too small, material will not flow easily, resulting in stretching and, most likely, fracturing of the cup. If a die entry radius is too large, particularly when deep drawing thin-gauge stock, material begins to wrinkle after it leaves the pinch point between the draw ring surface and the binder. Severe wrinkling can restrict flow when the material is pulled through the die entry radius.

The die entry must be produced accurately, hook-free, and polished in the direction of flow. A good starting place is to use high-wear tool steel for die entry radii, and make the die entry radius four to six times the material thickness.

3. Incorrect Binder Pressure Setting

If binder pressure is too low, the material wrinkles during metal flow and compression. The wrinkles cause the binder to separate further from the draw ring surface, and control of the material is lost. The material is forced to unwrinkle when it is squeezed between the post and cavity walls, which can pull metal on top of the cup and cause fracturing.

Binder pressure that is too high also causes problems. The pressure can be lowered with standoffs, which maintain a given space between the draw ring surface and the binder. Set your standoffs at 105 percent of the metal thickness to allow for compressive thickening. If the standoff gap is too small, the material will be pinched tightly between the draw ring and the binder surface, reducing its ability to flow freely. If the standoff gap is too large, the material will wrinkle during circumferential compression.

The recommended binder pressure for round draws of low-carbon, draw-quality steel is 600 pounds per lineal inch around the post (draw post diameter x 3.141). For high-strength, low-alloy, and stainless steels, use 1,800 lbs. of pressure per lineal inch.

4. Wrong Limiting Draw Ratio

The limiting draw ratio (LDR)—the direct relationship between the draw punch and the blank edge—is perhaps one of the most important items to consider during product and process design. If the blank edge is too far from the draw punch, the metal will not flow inward and will be forced to stretch.

Metals with poor stretchability, such as high-strength steel, will be subject to failure. Metals with good stretchability will be less likely to fail. The material’s elongation and n and r values are very important in this case.

When designing deep-drawn parts, keep in mind where the forming punch contacts the metal relative to the blank edge. Very deep forms far from the blank edge are risky and may require additional preforms. This will add cost to the tooling and product.

Also avoid sharp profile radii, because they force the metal to compress severely before flowing inward. This severe compression may cause embrittlement fractures. Areas in compression also have a great resistance to flow, which can result in excessive thinning or splitting.

As a general rule, the drawing punch should be about 60 percent of the blank size.

5. Incorrect Lubricant Choice

Nothing affects the friction between the sheet metal and die in a deep-drawing operation more than the lubricant being used.

Most lubricants have special additives that allow them to change their frictional value with respect to heat. You need to determine the amount of heat generated during your drawing operation and choose a lubricant with the proper additives to control heat and friction.

For example, chlorinated lubricants have a much lower coefficient of friction when heated to about 350 degrees F than when they are cool. A great deal of friction is required to create enough heat to activate the chlorine, so these lubricants are best suited to deep drawing and forming of higher-strength steels. However, too much heat causes the lubricant to burn off, resulting in drawing failures.