Die Science: 7 ways that part material affects die design and the forming process

I recently had an opportunity to train some young product designers. During this training session, one of the product designers brought me a print of a project he was working on. It was a contoured, deep-drawn part with many intricate features, and he had a lot of questions about the manufacturability of the design.

The title block of the print listed a long number. It was a typical automotive specification that indicated 1.5-millimeter, DP600 material was to be used to make the part. I asked the designer to tell me what the material was. He said, “Some kind of steel.” When I asked if he could give me more detail about the material, he admitted that he knew very little about it—just that his supervisor wanted him to use it for the part.

It’s All About the Material

I explained that he was inadvertently and unknowingly making all sorts of decisions about his part forming process, because material choice determines multiple aspects of part production:

1. Part manufacturability. All metals have a certain amount of ductility, stretchability, and malleability that makes them more or less suitable for deep forming. For example, designing a multiple-step, deep-drawn part to be made out of DP980 would defy the metal’s physical capabilities and likely cause part failure. Designing the same part out of a low-carbon, deep-drawing steel would be a much better choice.

2. Tool steel type. Parts made out of abrasive materials—such as high-strength steel; spring steel; stronger-grade high-strength, low-alloy steel; or superalloys such as INCONEL®will require tools made of premium grades of tool steel or solid carbide. Of course, this affects the tooling cost significantly.

3. Die coating type. Depending on the metal being formed, the die might have to be coated with special antifriction, high-wear surface coatings such as titanium carbide or specialized duplex coatings.

4. Press capacity. High-strength steel takes more tonnage to form and cut than lower-strength steel. Although the product designer might not select the actual press the part will be run on, his or her decisions certainly are going to affect the amount of tonnage and type of drive needed. In addition, some materials are very strain rate-sensitive or sensitive to forming speeds, so they lend themselves to a slower-speed press.

5. Die component thickness and geometry. High-strength materials require greater forces to cut and form. Therefore, the tool steel sections must be thicker and more rigid than the sections used for forming low-strength mild steel.

6. Lubricant chemistry. High-strength materials generate a lot of heat when being formed and cut. For this reason, additives such as chlorine, sulfur, fats, waxes, and oils often are added to the lubricant to help reduce friction and reduce the amount of heat. Some of these additives must be removed from the formed part before painting, which adds expense to the manufacturing process.

7. Number of stations. High-strength materials most likely will require more stations to make the part geometry than their low-strength counterparts. Higher-strength metal bottles, for instance, probably will require secondary restrike or requalifying stations to compensate for the material’s extensive amount of springback.

A Few Final Tips

Every feature and every tolerance that a product designer assigns to a part has a certain cost associated with it. Sometimes, reducing the geometric dimensioning and tolerancing on a part can double or even triple the tooling and manufacturing cost. Choosing to make a part out of high-strength material can have a similar effect.

Make sure you fully understand how the material from which you are designing a part will affect the entire process. Avoid overdesigning your part with features and tolerances the aren’t absolutely necessary. Choose a metal that suits the fit and function of your part.

Take the time to learn about the metals you’re using in your product designs. It will pay off in the long run.