Closed-die drop forging is also called ‘impression-die forging’. It’s a manufacturing process where a metal workpiece is positioned in a die that’s attached to an anvil, under a hammer. The hammer die is often also shaped to give the forged part shape on the top and bottom.
The hammer is then dropped onto the workpiece, providing a powerful blow. As the metal disperses, it fills the open spaces of the die to reshape the workpiece. This process may involve a single powerful blow by the hammer or the hammer being dropped multiple times in quick succession.
Any excess material that was between the two dies is forced out through the die cavities. This excess material is known as the ‘flash’ and it is discarded after the forging is complete.
Similarly to upset forging, closed-die forging may involve a workpiece moving between multiple drop forging dies to go from the raw material to the finished product. The first stage is used to form the rough metal into the rough shape of the product. Allowing for the needs of later stages.
The first impression is called a ‘fullering’ ‘edging’ or ‘bending’ impression. Subsequent stages are called ‘blocking’ cavities in which the workpiece is moulded into a shape that more closely resembles the final product. Finally, the last stage is forging the part using a ‘finisher’ impression to give the workpiece its final shape.
Initially, closed-die forging has a high start-up cost. This is due to the time, intricacy and production of the dies that are required as part of the process. However, once these costs have been absorbed, the recurring costs for each part are relatively low making closed-die forged parts more economical with greater volumes.