1997-02-24

Advances and Trends in Sheet Metal Forming Processes 970436

In order to reduce fuel consumption light weight vehicles are in development. For this HSLA- steel, bakehardening- steels, stainless-steels and aluminum will be used for the body in white of personal cars.
To form inner automotive parts of high dimensional accuracy out of steel with high strength or out of aluminum and to form automotive parts of high surface quality as well as of high dimensional accuracy a reproducible forming process is needed. For low volume cars tools and dies have to be as cheep as possible. The press has to be able to take over die functions and over the stroke steerable blankholder forces. Tools and dies for high volume parts can use expensive tools and dies like dies with nitrogen cylinders.
In the future these cylinders will have the possibility to steer the pressure and so in turn the blankholder force over the stroke. See paper „Force- Stroke- Curve of Gas Springs” (State-of-the-Art Stamping) held on the same conference. Mechanical single acting presses with CNC controllable hydraulic cushion systems will replace mechanical double acting presses. For dies up to 1500mm x 1500mm CNC controllable 4 point hydraulic cushion systems will be installed in new presses as well as replacement in existing presses instead of pneumatic systems and instead of double acting presses.
For larger dies multipoint hydraulic cushion systems will be installed in presses as well as in transfer- presses. There are two different possibilities to build up such cushion systems. One possibility is a cushion system builded up by a lot of hydraulic cylinders, each cylinder equipped with a direct connected servo valve or proportional valve. On the cushion plate mechanical height adjustable cushion pins support the lower binder of the die. So the blankholder force can be steered over the stroke by the 4 point hydraulic system.
The pressure between the blank and the binders can be adjusted for specific areas by the height of the cushion pins and by the pressure in the hydraulic cylinders.
In the near future it seems to be possible to run closed loop blankholder forces by measuring the friction forces and the wrinkle height of the blank between the binders with special sensors following desired friction forces and maximum allowable wrinkle heights curves over the stroke. (SAE Paper 960824). Furthermore pulsating blankholder forces seem to be helpful in draw operations specially for difficult to deep drawn parts. Trends and goals for the future show that we will come to a reproducible forming process with closed loop pulsating BHF- systems to meet the task of drawing difficult to form parts out of HSLA-steel, bake hardening-steel, stainless- steel and aluminum with high dimensional quality and excellent surface.

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