4.3 Impression-die and Closed-die Forging

In impression-die forging, the workpiece acquires the shape of the die cavities (impressions) while being forged between two shaped dies (Fig.4.1). Note that some of the material flows outward and forms a flash. The flash has a significant role in the flow of material in impression-die forging. The thin flash cools rapidly, and because of its frictional resistance, it subjects the material in the die cavity to high pressures, thereby encouraging the filling of the die cavity.[3]

Fig.4.1 Stages in impression-die forging of a solid round billet

The blank is placed on the lower die and, as the upper die begins to descend, the blank’s shape gradually changes, as is shown for the forging of a connecting rod in Fig.4.2a. Preforming processes, such as fullering and edging (Figs. 4.2b and c), are used to distribute the material into various regions of the blank, much as in shaping dough to make pastry. In fullering, material is distributed away from an area; in edging, it is gathered in to a localized area.[4] The part is then formed into the rough shape of a connecting rod by a process called blocking, using blocker dies. The final operation is the finishing of the forging in impression dies that give the forging its final shape. The examples shown in Figs. 4.1 and 4.2a are also referred to as closed-die forgings. However, in true closed-die or flashless forging, flash does not form and the workpiece completely fills the die cavity. Accurate control of the volume of material and proper die design are essential in order to obtain a closed-die forging of the desired dimensions and tolerances. Undersize blanks prevent the complete filling of the die cavity; conversely, oversize blanks generate excessive pressures and may cause dies to fail prematurely or to jam.

Fig. 4.2 (a) Stages in forging a connecting rod; (b) fullering operations; (c) edging operations