Forging shapes metal by compressing it between tools or dies. The workpiece remains solid while plastic flow redistributes material.

In open-die forging, simple tools compress the workpiece without enclosing it completely. The material can spread laterally, and the operator or process sequence controls the developing shape.

In closed-die forging, shaped die cavities constrain flow toward a more specific geometry. Excess material may enter a thin region called flash. Flash wastes material, but its high resistance to further flow can help build pressure that fills the die cavity.

Friction changes deformation strongly. In simple compression of a cylinder, friction at the die faces resists outward sliding, so the specimen can bulge into a barrel shape rather than deform homogeneously.

Forging load rises with material flow stress, contact area and friction. Sharp corners, thin ribs and abrupt changes can be difficult to fill because material must flow farther through increasingly constrained regions.

Forging can also improve performance by controlling material flow and reducing some defect types associated with cast feedstock. However, poor temperature, excessive strain or unsuitable geometry can produce laps, underfill or cracking.

Good forging design therefore couples part geometry, die geometry, deformation sequence, temperature and material flow rather than treating the die cavity as a passive mould.