To Stamp, or Not to Stamp?

How to determine when tool & die is your best option

A Few Initial Questions

If you want to produce a piece in volume, and you’re wondering whether to cast it or to invest in tooling and stamp it out, first take a look at the design itself. While stamping is suitable for many metals (including aluminum, steel, brass, copper, silver, karat golds, and platinum), that’s not always true when it comes to design.

One general rule applies: the more simple, geometric, and two-dimensional the design, the better. A basic, symmetrical shape with a simple cut in the center is ideal. Sculptural or three-dimensional pieces, as well as items with undercuts, are not suitable. In those cases, casting could be the preferred alternative.

If your design passes the first test for stamping—it has a clean, flat, symmetrical shape, no undercuts—you’re not done yet: You still must look a little further to decide if it’s the best option. Ask the following questions:

1. Does your design have crisp, sharp lines? For pieces with sharp details, such as engravings or other types of graphic decoration, stamping offers a crispness and consistency that may not be achieved as well with casting. Where stamping creates pieces that are machined to exact details, casting must move through several stages—from master to mold to wax to casting—with a little detail lost in each stage. And as long as the stamping tools are maintained and kept sharp, detail and clarity will be guaranteed in every piece.

2. Do you need a high polish finish? Stamping can enhance the aesthetics of your product. When you cast, you always have to anticipate porosity as the molten metal cools and contracts. Because stamping doesn’t require you to heat the metal and cool it down, it does not adversely alter the metal’s surface properties. If you start with clean, rolled sheets that have no marks or pits, you’ll end up with pieces that have a clean finish.

3. How many do you need? Although you can stamp as many pieces as you’d like—from 1 to 1 million—the initial tooling costs may not justify a smaller volume. The cost of creating a stamping tool is largely dependent on the manufacturing method (see “Tooling Options”), as well as the complexity of the job and how efficiently it can be done. Tools can range from under $100 to thousands of dollars.

4. What’s your price point? The cost of tooling can drive up costs, depending on the complexity of the design. However, stamping can produce thinner pieces than typically available through casting, which can help drive down metal costs. (A stamping can be as thin as 0.003 inch, although experts say it’s best to use this thickness only for specific designs, such as hollow pieces that comprise two halves soldered together.) Stamping also has a lower reject rate than casting, which further aids in quality-control costs. Some stamping companies may be able to “reverse engineer” a price point; starting with the price the client wants to achieve, they can work backwards to determine the best way to engineer the components to meet that goal.

5. What type of quality do you need? If you have a high-end client that will reject anything with even a hint of porosity, then stamping may be a better option than casting. Since stamping offers a better finish and lower reject rate, the initially high costs of tooling can be more than made up in the finishing and inspection stages.

Stamping Processes

Let’s say you’ve answered  all of the questions above and have decided stamping offers the best alternative. You decide to subcontract the work to a stamping company. At that point, the discussion may revolve around the type of stamping process that’s best suited to your design, as well as tooling options. The more informed you are about these topics, the better.

In and of itself, stamping is a relatively uncomplicated operation. It usually consists of three or more stages: blanking, in which the outline of a design is punched out of a flat sheet; striking, in which the details of the design are created when the patterned tool (the forcer) makes contact with the die; and trimming, in which the excess metal created as a result of striking is removed. Some designs may require additional piercing, in which cuts are made in the piece, or shaping, in which the flat piece takes on a form. But overall, stamping is a rather basic process—or rather, two basic processes.

In single-operation stamping, an operator manually resets the press with the tools for each step and runs them one at a time through the stamping stages. This stamping option is generally used for lower-volume items (anything under 50,000 pieces). Depending on quantity, customers should allow more time for delivery due to the manual nature of this process.

In automatic-operation stamping, a metal sheet is automatically fed through a press (eliminating the blanking stage) and advanced through the steps in the stamping operation. This procedure does not require resetting the press; the process is performed in consecutive steps, or stations, that are all incorporated into one tool. This stamping option is generally used for higher-volume items (anything over 50,000 pieces), especially when more than one run is anticipated. Automatic operation mandates higher tooling costs—they’re about double those of single operation—but due to the larger volumes, customers generally make up their costs in saved labor charges. Depending on the design, automatic operation may generate more scrap.

Tooling Options

When the discussion comes to tooling options, know that traditionally there are three options:

Hand tooling, which involves the skilled labor of a hub and die cutter and a tool maker, who create the tools using techniques such as die marking, filing, grinding, and punching.

Conventional electrical discharge machining (EDM), in which an electrode controlled by a CNC system burns a design into a die block.

Wire EDM, in which a traveling wire electrode controlled by a CNC system cuts a die block that has been pre-drilled to allow the wire to shape internal contours.

Nowadays, as with so many other artisan-based techniques, hand tooling has been largely replaced by EDM. Both wire and conventional EDM employ electrical discharges—or spark erosion—to melt and vaporize metal; they generate thousands of rapid DC or AC electrical impulses per second that, with the aid of de-ionized fluid, travel between an electrode and a workpiece. They differ in how to apply those charges, though.

Conventional EDM uses an electrode with a contoured head to create a negative shape in a die block; it is largely used for creating dies, hubs, and forcers. It’s an older and somewhat more labor-intensive technology than its counterpart, but it’s more suited to creating designs with textures or raised details. When a tool is made through conventional EDM, the machinery works the whole surface of the tool face. This isn’t the case with wire EDM.

For designs that don’t require raised details, or that need tighter tolerances at the edges, wire EDM is the preferred choice. It’s largely a cutting tool—like a very expensive pair of metal shears. Given a CAD-designed toolpath, wire EDM uses a traveling wire electrode to trim out a shape. It works well for particularly intricate shapes, and can also be used to cut out internal details. What it can’t do is put a raised cheekbone or the texture of hair on a baby’s-head charm.

Once again, the demands of your design will largely dictate whether conventional or wire EDM is the best option. In either case, the computer-driven tooling offers greater accuracy and tighter tolerances than hand tooling. While the best craftsmen can hand-work a stamping die or metal model piece to an accuracy in the range of 0.001 inch, EDM systems can easily drive that number down to ±0.00001  inch—every time and with perfect repeatability, regardless of the job. In addition, you’ll get more life out of the tool if it’s made with EDM rather than hand tooling, since EDM can cut the steel longer and deeper than is possible by hand.

Regardless of which method is used, you should be aware that tools can sometimes break—and that some designs are more prone to breakage than others. For instance, dies for designs with many pointed corners, prongs, or letters tend to break more frequently. Despite their basic geometric forms, which are generally ideal for stamping, these types of designs draw pressure to their weak corners and points, sometimes causing the die to crack. It’s not that you can’t or shouldn’t stamp such pieces, you simply must be aware of the greater potential for problems—and for increased expense—inherent to those designs.

Communication Is Key

As in all cases with subcontracting, you must give your stamper as much information as possible: Good communication at the initial stages of the process can prevent glitches down the road. Tell your stamper all of the answers to the questions asked earlier, as well as those to the following:

What is the function of the piece? If it is going to bend, for instance, you’ll have to use a thinner metal with an appropriate temper to make the piece function properly.

What metal or metals would you like to use? Knowing all of the metals you are considering for the piece will help the stamper to decide which temper will work best.

How thick do you want the piece? If your goal is to get a “cast” look with a stamping, you may be overestimating the amount of metal you need to get that look. Your stamper might suggest using a lighter-weight alloy, which will also be lighter on your pocketbook.

Answering these questions—and, most important, relaying the information to your stamping company—will help to prevent any misunderstandings, and put you on the road to stamping success.

Adapted from the articles “Stamp of Approval” by Edward N. DeCristofaro, which appeared in the April 2001 issue of MJSA Journal (formerly AJM Magazine), and “Tool and Die Today” by John Shanahan, from the April 2007 issue of MJSA Journal.