In the most basic example of impression die forging, which accounts for the majority of forging production, two dies are brought together and the workpiece undergoes plastic deformation until its enlarged sides touch the die side walls. Then, some material begins to flow outside the die impression, forming flash. The flash cools rapidly and presents increased resistance to deformation, effectively becoming a part of the tool. This builds pressure inside the bulk of the workpiece, aiding material flow into unfilled impressions.
Impression die forgings may be produced on a horizontal forging machine (upsetter) in a process referred to as upsetting. In upsetting, stock is held between a fixed and moving die while a horizontal ram provides the pressure to forge the stock. After each ramstroke, the multiple-impression dies can open to permit transfer of stock from one cavity to another.
Fig. 5. Upsetting.
A form of impression die forging, closed die forging does not depend on flash formation to achieve complete filling of the die. Material is deformed in a cavity that allows little or no escape of excess material, thus placing greater demands on die design.
For impression die forging, forging dies become more important, and operator skill level is less critical in press forging operations. The press forging sequence is usually block and finish, sometimes with a preform, pierce, or trim operation. The piece is usually hit only once in each die cavity.