Coin minting, or “coining,” refers to the process of manufacturing coins with a stamping technique. It is the most common method for coin manufacture today and differs from an older process of coin manufacture called cast coinage, which involves pouring melted metal into a mold.
Coin minting involves the stamping of two coin dies—one for the obverse and one for the reverse—onto blank pieces of metal. Each die contains an inverse version of the image that is to be stamped onto the side of the coin. These dies are generally made from hardened steel and can produce many hundreds of thousands of coins before they need to be replaced. When it is time for the dies to be used in coin production, they are “married” (paired in such a way that one die will produce the obverse of the coin and the other will produce the reverse) and then positioned into a minting press. These dies will remain in the press for the duration of the production run, and there may be several production runs per year in order to produce the needed amount of coins.
Coin minting dies in a press are struck onto small, blank round metal discs called planchets. These planchets, or blanks, are cut from sheets of metal of the very thickness of the coin being produced. Often it is third parties and not the mint itself that produce these metal sheets. The planchets are made to be slightly larger than the coin being struck and then are subjected to an annealing process, which involves heating and then slowly air cooling the planchets in order to make them more malleable. Then the planchets pass through an upsetting mill that raises the rim on the edge of the future coin.
Once the planchets are ready, they pass through the coin minting machine, and each planchet is struck simultaneously by the obverse and reverse die in order to create the coin. There is also a collar positioned between the dies to maintain the shape and thickness of the coin during striking. In the case of coins that have a “third edge,” be it ridges along the edge of the coin or a written engraving along this edge, a special collar essentially acts as a third die. This collar is positioned in such a way that when the planchet is struck, the coin expands into the collar to create the engraving along the edge.
Because coins in coin minting are cold struck, meaning the planchets are not heated before striking, minting presses must use many thousands of pounds of force in striking. The main reasons behind not heating planchets before striking are twofold: one, the process involved would make for much more expensive coin production; and two, the coins that are struck today have much less relief than coins of the past and thus do not need to be heated beforehand. These presses are capable of producing about 120 coins per minute, though this rapid coining does cause wear on the dies—especially with the use of harder metals such as nickel alloys and copper.