The preparation of the flan or planchet has varied over the years. In ancient times, the flan was heated before striking because the metal that the coin dies were made out of was not as hard as dies today, and the dies would have worn faster and broken sooner had the flan not been heated to a high temperature to soften it.
Today's dies are made from hardened steel, and the presses use many thousands of pounds of pressure to strike coins (varying according to the size of the coin and the complexity and relief of the design). In addition, today's coins have much lower relief than ancient coins. Because of this, the planchet no longer needs to be heated immediately before striking, although it is annealed by heating and quick cooling which softens the coin.
Preparation of the modern planchet involves several steps. First, the metal (or metals in the case of clad or multilayered coins) is rolled out into a large roll or sheet of the correct thickness. This process is often done by third parties, not by the mint itself. These flat rolls or sheets of metal are then punched out into round blanks that are a little larger than the coin being struck. The blanks are then subjected to an annealing process that softens the metal through heating to approximately 1400 degrees farenheit and are then quickly quenched in water. They are then washed to remove residue from the annealing process and dried. The blanks then go through an upsetting mill that raises the rim on the edge of the coin.
Finally, the planchet is struck. After striking, it becomes a coin and is no longer a planchet. Occasionally, a planchet will escape the mint without having been struck. This is a blank planchet error, and is usually worth a few dollars for modern coins. Occasionally, blank planchets can be rare and valuable, such is the case for Morgan dollar blank planchets, although authentication is highly recommended for such pieces as they would be fairly easy to counterfeit.
For more details on the coin production process, see coin dies.
External Sites
http://www.winsociety.org/newsletter/news_archive/rhart.html
http://www.usmint.gov/faqs/circulating_coins/index.cfm?flash=yes&action=coins
http://www.coinworld.com/NewCollector/MintingProcess.asp