3 Fundamentals of press design

190 Sheet metal forming and blanking
contoured blank), the more suited is the operation for using the Active
Hydro-Mec method.
Operating principle
The Active Hydro-Mec method involves forming with an operating
medium (oil-water emulsion), in which a large-size blank is clamped all
around with a watertight seal between the female die and the blank
holder – in the form of a water container (Fig. 4.2.12/2). At this stage, a
defined, part-specific gap exists between the clamped blank and the
punch. As soon as the blankholding force has built up, the emulsion is
actively introduced into the water container by means of a pressure
intensifier and a defined pressure is built up (depending on the part
between 20 and 30 bar). This pressure causes a controlled bulging of the
blank which is pre-stretched over what is to become the entire surface
of the component until it comes to rest against the punch in the center
of the punch surface. This controlled expansion of the sheet metal
results in work hardening of the workpiece which leads to a substantial
improvement in buckling strength of the formed part.
After the pre-stretching process, the whole system – blank holder,
blank and female die – is pressed upwards against the stationary punch.
This process corresponds to the counter-drawing principle (Fig. 4.2.4)
in which the emulsion is displaced. During the process, the pressure
level can be kept constant or can be regulated to increase or decrease
progressively or degressively. After completion of the drawing process,
the emulsion pressure is actively built up (to a maximum of 600 bar) in
order to ensure optimum application and formation of the blank
against the contour of the punch.
Press
The press developed for the Active Hydro-Mec-process is a doubleaction
straight-sided press design with Hydro-Mec pressure intensifier
and a quick die changing system. Figure 4.2.13 provides a schematic
view of the press.
The press force of max. 50,000 kN is applied from below by eight
cylinders located in the press bed. Six blank holder cylinders in the
press crown apply a blankholding pressure of max. 16,000 kN during
the pre-stretching process. Six powered mechanical stops can be adjusted
for each individual part to determine the limit of the draw depth.
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998