3 Fundamentals of press design

6 Solid forming (Forging)
6.5 Force and work requirement
Determination of the force and work requirement is used as a basis for
selection of the forming machine and the structural design of the forming
dies. In practice, calculation of these characteristics simultaneously
produces values of process-related parameters such as the overall degree
of true strain, flow stress, forming pressure, compressive stresses on the
inside wall of the extrusion container, etc. (cf. Sect. 2.2). The process
limits and the respective case of application of the forming method
must also be taken into consideration here.
The formulae provided below are based on only one method of calculation
and make no claim of completeness. The flow stress levels are
average values obtained from the flow curves of VDI 3200 Sheets 2 and
3. To calculate the process variables concerning machine selection, the
upper limit of a flow curve should be selected. However, in making calculations
regarding die configuration, the lower limit of a flow curve
should be selected. Table 6.5.1 provides an overall view of the process
limits for the main extrusion processes.
6.5.1 Forward rod extrusion
The forward rod extrusion method transforms a billet into a solid part
with a reduced cross section (cf. Fig. 6.1.1). The die opening which
determines the shape of the extruded parts is formed solely by the press
container. As a result of the wall friction which occurs – also during the
ejection process – the ratio of height to diameter of the initial billet
h 0/d 0 [–] should be no greater than 5 to 10 (Fig. 6.5.1). If the required
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998