If compression is not controlled during drawing, the flange will be severely wrinkled.

On a reasonably frequent basis, I receive an email with a photo of a failed stamped part. Usually the failure is an open split or excessive thinning, and the descriptive text undoubtedly explains that the failure is happening in the drawing die.

I can assure you with a great deal of confidence that the failure is not the result of drawing. Even though it might be happening in the draw die, it’s not a drawing failure. It’s a stretching failure.

To find the best solution to a stamped part failure, you need to understand the difference between drawing and stretching.

In the drawing process, the surface area of a blank is displaced by tension into an alternative shape via controlled metal flow (the feeding of metal into the cavity). Basically, if the blank is changing its outer profile or getting smaller as the metal is formed, drawing is taking place.

Drawing is one of the most complicated yet effective means of shaping sheet metal. Although some stretching of the metal does occur during this process, the key is to try to form the sheet metal by feeding metal inward and avoiding excess stretch that may result in the metal splitting or weakening in a localized area.

For the part to be produced, the metal must flow or feed over punch and die radii. This usually involves a lot of bending and unbending of the metal. And not all part shapes can be made using a single drawing process; some require as many as 10 or more drawing reductions.

The drawing process, used to form such items as oil filters, deep-formed auto parts, kitchen sinks, cookware, and motorcycle gas tanks and fenders, is highly reliant on the limiting drawing ratio (LDR). The LDR is the relationship between the edge of the draw punch and the edge of the blank—in other words, the maximum blank diameter that can be drawn safely into a cup.

For instance, a cylinder starts out as a simple round blank. For that round blank to transform into the smaller cylinder shape, it must compress radially: The metal must flow inward toward the centerline of the cup simultaneously as it compresses. If the compression is controlled, the result will be a part with a flat flange. If it’s not controlled, the flange will be severely wrinkled (see Figure 1).

Metal in compression has a great resistance to flow. If there is too much surface area outside of the punch, the metal will resist flowing inward, causing excessive stretching of the material and possibly splitting. Although not right for every application, a standard rule is to use a blank no larger than two times the punch diameter. For example, if the punch is 2 in. in diameter, use a blank no larger than 4 in. If the blank to make the part needs to be bigger than 4 in., you might need to use more than a single drawing station to make the part. If you need more than one operation, your LDR percentage will change. This punch-to-blank relationship is critical for making all shaped parts that require metal flow.

During stretching, the surface area of a metal blank is increased as a result of tension. Unlike drawing, there is no inward movement of the blank edge. Stretching dies are very similar to embossing dies, except unlike most embossing operations, stretch dies use a high-pressure binder to restrict and stop metal flow. A binder is similar to a drawing pad, but it intentionally restricts the metal from moving inward.

Stretching often is used to produce parts such as automobile hoods, roofs, and fenders that require a smooth Class A surface finish. The metal deformation also work-hardens the metal to impart dent resistance.

Because drawing uses tension to pull the metal inward, a little stretching will occur. The lack of inward flow is what contributes to splitting, and different corrective actions often are required to resolve that problem. Stretching failures typically can be resolved with product radii changes, polishing, and getting more material to flow inward.

See More by Art Hedrick

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