The original definition of forging is the "mechanical deformation of metal when heated to a plastic state". However, in recent years it has come to include similar operations done in the cold or room temperature state as well. A forging press is used to form car wheels, bushings, gears and other parts. It is basically the mechanical forming by squeezing metal between dies in a press. Press forgings use a slow squeezing action of a press, to transfer a great amount of compressive force to the work piece. Unlike an open-die forging where multiple blows transfer the compressive energy to the outside of the product, press forging transfers the force uniformly to the bulk of the material. This results in uniform material properties and is necessary for large weight forgings. This process is similar to kneading, where a slow continuous pressure is applied to the area to be forged. The pressure will extend deep into the material and can be completed either cold or hot. A cold press forging is used on a thin, annealed material, and a hot press forging is done on large work such as armor plating, locomotives and heavy machinery. Press forging is more economical than hammer forging and closer tolerances can be obtained. A greater proportion of the work done is transmitted to the work piece, differing from that of the hammer forging operation, where much of the work is absorbed by the machine and foundation. The amount of time the dies are in contact with the work piece is measured in seconds. The press forging operation can be done either cold or hot.
The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete work piece. Drop-hammer forging usually only deforms the surfaces of the work piece in contact with the hammer and anvil; the interior of the work piece will stay relatively undeformed. Another advantage to the process includes the knowledge of the new parts strain rate. We specifically know what kind of strain can be put on the part, because the compression rate of the press forging operation is controlled. There are a few disadvantages to this process, most stemming from the workpiece being in contact with the dies for such an extended period of time. The operation is a time consuming process due to the amount of steps and how long each of them take. The workpiece will cool faster because the dies are in contact with workpiece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the workpiece cools it becomes stronger and less ductile, which may induce cracking if deformation continues. Therefore heated dies are usually used to reduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. The workpiece may also need to be reheated. When done in high productivity, press forging is more economical than hammer forging. The operation also creates closer tolerances.
