In open-die forging, a hammer strikes and deforms the workpiece, which is placed
on a stationary anvil. Open-die forging gets its name from the fact that the dies
(the surfaces that are in contact with the workpiece) do not enclose the workpiece,
allowing it to flow except where contacted by the dies. Therefore the operator,
or a robot, needs to orient and position the workpiece to get the desired shape.
The dies are usually flat in shape, but some have a specially shaped surface for
specialized operations. For example, a die may have a round, concave, or convex
surface or be a tool to form holes or be a cut-off tool.
In close die forging, the 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 workpiece, causing the metal to flow and fill the die cavities. The hammer
is generally in contact with the workpiece 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.