Ask the Stamping Expert: Forming stainless steel foil

A: I’m sorry to say, I tend to agree with the FABTECH press manufacturers. I think you are pushing the limits of formability of 0.002-in.-thick stainless steel material at a depth of 0.5 in. However, I can address forming a 0.004-in.-thick rectangular cavity to a depth of about 0.25 in. in 316L grade stainless steel. There are a few points that are critical to success in drawing a flat, square, wrinkle-free cup:

1. Base material
2. Material handling
3. Lubrication type and application
4. Strengthening rib in the bottom of the can
5. Nitrogen or hydraulic pressure pads in the draw station

Grade 316 is molybdenum-bearing stainless steel. Molybdenum gives 316 improved corrosion resistance properties over the grade 304. What you really want to use is grade 316L, as it is tougher and has higher tensile strength than chrome- and nickel-bearing stainless steels. It also will not rupture and tear as easily as standard 316 stainless. From an industry standards point of view, 316L is interchangeable with 316, but 316L actually has lower carbon content. Price also is about the same for both.

In a progressive die, getting the material from the feed through the tool is quite tricky. The material guide liners must support the material top and bottom very tightly, with just a small opening cut in the liners top and bottom for lubricant application. Start with a 0.025-in. gap between the top and bottom liner plates and adjust from there accordingly. Try these two important tricks:

1. Relieve the surface area of the plates with a series of undercuts along the length. This is called “scalloping.” When you are finished, less than 1 percent of the total surface area of the plate should touch the material. The flat surface between undercuts should be only 0.01 in. wide.
2. Add compressed air assist to the liners. Drill and tap a hole in the center of the liner to connect an air line so that applying light air pressure will cause the material to float on a cushion of air.

The bottom line is there can be no resistance if you plan to push-feed such light-gauge material. Pull feeding is an option, but it has more negatives than positives.

For proper lubrication, consult a lubricant supplier for advice. The application is critical. I recommend high-end lubricators that provide precise control of a lubricant mist applied lightly and evenly across the entire strip. You can purchase a $5,000 applicator or a $25,000 applicator; trust me when I say you get what you pay for. We use the type that applies light oil and then suctions the excess off the material, leaving a perfect, even coating. You need to do your homework.

When drawing the cup, the goal is to do it in as few steps as possible. You need to minimize work-hardening of the material. In this application we draw 80 percent of the final depth in the first draw. The second draw brings the rectangle to the final size and depth.

Three key details make it possible to achieve a square, straight-walled, flat cup:

1. You need to clamp the material between the draw punch and draw die pressure pad with sufficient pressure (in this application, 2 to 3 tons) so that when you draw into the draw die, the material is held flat and does not wrinkle.
2. In the second draw only, relieve the face of the punch by one material thickness, leaving a raised ledge of material around the entire perimeter about 0.010 in. wide. Relieve the corresponding pressure pad by the same geometry. When the punch and pressure pad hit home at the bottom of the press stroke, you create a setting/strengthening rib.
3. The length and width of the draw dies should be 0.0005 in. larger in the second draw to allow the material to flow freely to the completed depth.

Good luck, and happy stamping!