Closed-die forging, involves the work piece being placed between two dies containing the impression of the forging shape to be produced. The dies are brought together and the work piece is plastically deformed until the sides come in contact with the walls of the die. As the deformation continues, a small amount of material begins to flow outside the die impression, forming flash. The thin flash cools rapidly, creating a pressure increase inside the work piece.
The increased pressure assists the flow of material into the unfilled portion of the impression. The majority of the forgings produced are done using closed die forging. After the forging process is completed the flash is cut off in another press with a trimming die.