Coining is a form of precision sheet metal stamping in which a piece of metal is subjected to an enormously high stress in order to induce plastic flow on the surface of the material. One beneficial feature is that in many metals, the plastic flow reduces the metal’s surface grain size, which, in turn, work hardens the surface of the metal, while the deeper material of the part retains its toughness and ductility. The term “coining” comes from the initial use of the process which was in the manufacture of coins.
A coin die is one of the two metallic parts that are used to strike each side of a coin. The die contains an inverse version of the image that is to be struck on the coin. In order to imagine what the inverse version looks like, press a coin into some clay or wax and look at the resulting inverted image. Modern dies are usually made out of hardened steel and are capable of producing many hundreds of thousands of coins before they must be retired and defaced to prevent unauthorized use.
Coining is also used to manufacture small parts, such as washers, for all industries and is usually used when a high relief and very fine features are required. For example, not only is it used to produce coins, but also to produce medals, badges, and buttons, as well as precision energy springs and precision parts with small or polished surface features.
Coining is a cold working process rather than the somewhat similar process of forging, which takes place at an elevated temperature in order to make the metal more pliable. Coining uses a great deal of force to plastically deform a piece of metal, so that it conforms to a die and therefore takes on it’s shape. Coining can be done using a gear driven press or a mechanical press, but more commonly is accomplished with a hydraulically actuated press.
Coining typically requires a much higher tonnage for it’s presses than stamping, because the workpiece is plastically deformed and not actually cut, as in some other forms of precision metal stamping.
Coining is also used in electrical components. When electronic components are soldered, bumps are formed on bonding pads in order to enhance adhesion, these bumps are then flattened by the coining process. Unlike typical coining applications where the goal is to create a patterned surface on the metal, in this case the goal of coining is to create a smooth, flat surface.
