3 Fundamentals of press design

Force and work requirement
The minimum achievable wall thicknesses for steel are around 1 mm,
for aluminium around 0.1mm. Extremely thin walls can be produced
by a subsequent ironing process following backward cup extrusion. For
steel, the minimum cup bottom thickness is 1 to 2 mm, for NF metals
0.1 to 0.3 mm. The bottom thickness should always be greater than the
wall thickness (underfilling). In the case of thick-walled cups which are
additionally expected to satisfy high concentricity requirements, it is
advisable to carry out a centering operation prior to cup extrusion.
Centering is a process very similar to backward cup extrusion, although
with only a small deformation stroke. Centering is generally performed
using guided punches.
6.5.4 Reducing (open die forward extrusion)
The principle of reducing is similar to that used for forward rod extrusion
as far as the die design is concerned (cf. Fig. 6.1.2). However, in
contrast to forward rod extrusion, the billet material is not pressed
against the container wall. Thus, only a limited degree of deformation
is possible without buckling. During the forming process, the material
must neither be deformed (upset) in the container nor buckle. In the
case of starting billets with h 0/d 0 > 10, reductions in cross section can
only be achieved using this process. Larger amounts of deformation
require several successive reducing operations. The achievable level of
deformation depends on the material, the preliminary state of strain
hardening and the die opening angle. An overall level of deformation
(calculated from the first to the n th reducing operation) of 50 % should
not be exceeded due to the formation of central bursts (chevrons).
Reducing or open die forward rod extrusion is a process which is not
used in practical warm forming operations, as the achievable degree of
deformation is too low due to significantly reduced flow stress in warm
forming.
With a die opening angle of approx. 20°, it is possible to achieve a
maximum level of deformation of � = 0.3. If the material used has
already been strain hardened, for example as a result of a previous
reducing operation, a somewhat higher degree of deformation is possible.
However, with an increasing die opening angle, the achievable level
of deformation drops substantially.
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
475