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The three basic categories of forging are open die, impression die, and closed die.

Open die forging

The simplest form of open-die forging involves placing a solid cylindrical workpiece between two flat dies and reducing its height by compressing it. The die surfaces be shaped therefore forming the ends of the cylindrical workpiece while compressing it.

Solid cylindrical billet deformed between two flat dies. Uniform deformation of the billet without friction. Deformation with friction. Note barreling of the billet caused by friction forces at the billet-die interface.

Impression die forging

The workpiece is forced to conform to the shape of the die cavities while it is being compressed between the closing dies. The closing of the die cavities occurs at high striking forces. Some of the material flow radially outward and forms a flash. The formation of this flash prevents further material from flowing in the radial direction in the flash gap. As the length-to-thickness ratio of the flash is high, it is subjected to high pressure that result in high frictional resistance to material flow. Further, in the case of hot forging, the flash cools faster than the bulk of the wokrpiece. This increases its resistance to deformation compared to the bulk. Thus the flash plays a significant role in helping filling the die cavities.

Closed die forging

In closed-die-forging, no flash is formed and the workpiece is completely surrounded by the dies. Therefore, proper control of the volume of material is essential to obtain a forging of desired dimensions. Undersized blanks in closd-dieforging prevent the complete filling of the die, while oversized blanks may cause premature die failure or jamming of the dies.