3 Fundamentals of press design

494 Solid forming (Forging)
Table 6.7.2: Increase of the allowable internal pressure p i zul. or reduction of the outside diameter d a
through single and double shrink rings
Characteristic value Press container
without shrink ring with single shrink ring with double shrink ring
pi zul. [N/mm2 ] 690 1,160 1,395
da [mm] with di = 20 mm 100 40 36.4
rings, can be reduced by about 60% (Table 6.7.2). In multiple-station
forming machines this results in a substantial reduction of die diameters,
the distances between stations, the dimensions of the die holder
and thus also the machine bed size.
Carbide cannot be subject to tensile stresses. Therefore, when using
dies or inserts from carbide, the extrusion container must be prestressed
in such a way that no tangential tensile stress occurs under
internal pressure. Accordingly, the interference dimensions are considerably
higher than those used for tool steels. Guidelines also have been
established for the calculation of axial pre-stress levels; in the thin, ringshaped
lateral split, surface pressures of 700 to 1,000 N/mm 2 are assumed
as guideline values.
In extrusion dies, the actual stress conditions differ from the ideal
assumptions made here, however. The extrusion containers have a
finite length, a limited and possibly eccentric pressure area, no uniform
internal pressure exists, and the containers have often off-center die
openings. Realistic conditions in the configuration of shrink fit assemblies
can be determined by using discrete approximation techniques,
for example the finite element method (FEM). For practice-oriented
application, various calculations have been conducted by making a
parametric study using FEM. These results have been made accessible
for practical application in the form of Nomograms (VDI 3176, ICFG
Doc. 5/82) and corresponding computer programs.
Punch design
Extrusion punches are subjected to an axial force with an average compressive
stress. They are calculated as follows:
FSt
pSt
=
A
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
St