Closed Die Forging

Closed die forging is also called impression-die forging. In closed die forging process, work metal is placed in a die resembling a mold, which is attached to the anvil. Usually the hammer die is shaped as well. The hammer is then dropped on the work piece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the work piece on the scale of milliseconds. Depending on the size and complexity of the part the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities forming what is referred to as flash. The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging the flash is removed by trimming.

In commercial closed die forging the work piece is usually moved through a series of cavities in a die to get from an ingot to the final form. The first impression is used to distribute the metal into the rough shape in accordance to the needs of later cavities; this impression is called an edging, fullering, or bending impression. The following cavities are called blocking cavities, in which the piece is working into a shape that more closely resembles the final product. These stages usually impart the work piece with generous bends and large fillets. The final shape is forged in a final or finisher impression cavity. If there is only a short run of parts to be done it may be more economical for the die to lack a final impression cavity and instead machine the final features.

Advantages of closed die forging
Hot forging prevents work hardening associated with cold forming
Large Parts can be produced with closed die forging
Closed die forging can produce near net shapes and in some cases blind cores that will require only a small amount of machining or finishing
Economic for large runs of parts
Forging provides superior mechanical properties over castings due to the internal grain structure formation in forged parts