Hydraulics for die cushions

Hydraulics for die cushions
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March 2017

 

Automotive manufacturers design some very complex components, which means that the supplier industry needs very sophisticated machines. One of these is the multiple-die press from ANDRITZ Kaiser GmbH. A key function is performed by a die cushion with hydraulics from Rexroth.

Hundreds of sheet metal parts are installed in cars. The standard method of shaping them is deep drawing. But since vehicle developers are coming up with increasingly complex geometries and using new materials, many suppliers are reaching the limits of what they can achieve with traditional production methods. In some cases, the trend towards increasingly intricate shapes is leading to high levels of rejects in the industry. This is exactly what the American supplier Tenneco wanted to avoid in the production of new shell mufflers for the intake and exhaust system. Modern mufflers are not only designed to help meet the legal noise limits, but also to selectively influence the acoustics inside the car – quiet and comfortable or sporty with a full sound depending on customer preferences. This is possible using complex designs with a filigree flow and sound transmission. In addition, developers use what are known as sandwich plates – two connected sheets separated by a thin layer of air that has a sound insulating effect. The external geometry is designed in such a way that it is tightly fitted onto the underbody of the vehicle and takes up minimal space. Therefore, very precise forming processes are required to produce the components using the cold forming method.

Tenneco turned to ANDRITZ Kaiser, an expert in pressing and forming methods, for a new multiple-die press. “The key features of this plant are the 180 millimeter draw depth and the demanding geometrical requirements for the components”, explains Wolfgang Wiedenmann, deputy sales manager at ANDRITZ Kaiser. It also handles relatively large parts, with sheets up to dimensions of 800 by 400 millimeters.

Perfect interaction

The principle of the deep drawing method is extremely simple. A press ram pushes the upper die into the sheet to be processed, which is positioned on the lower die, thus giving it the required shape. However, it looks a bit more complicated in practice. To ensure that no folds and cracks occur, there must not be any uncontrolled movement of the sheet – also known as a blank. A key component that prevents this is the die cushion. On the one hand, it secures the blanks using the sheet holders and on the other hand it ensures precise tracking of the sheet during forming. A further task of the die cushion is to eject the finished component and create space for the next one. The machine component is controlled and regulated by a hydraulic solution.

The major challenge was to integrate a servo-hydraulic die cushion into a mechanical servo press. The advantages of servo presses is that the ram movements can be programmed very precisely. As the die cushion is the counterpart to the ram during forming, its movements must match exactly. As a result, ANDRITZ Kaiser opted for ready-to-install servo hydraulics from Rexroth. “The die cushion now enables us to follow the ram movement with axis coordination”, says Wiedenmann. To achieve this, three die cushion blocks from Rexroth are flanged directly onto the cylinder and move the cushion up and down. A further block acts as an ejector. Two axial piston pumps from Rexroth build up the necessary pressure to keep the sheet under control. The freely programmable hydraulic motion control from Rexroth now enables ANDRITZ Kaiser’s customers to produce components that were not previously possible – up to 30 per minute. To ensure that the high number of cycles does not come at the expense of safety, the Rexroth drive solution provides functional safety technology for the upward and downward movement.

Fast productive

With new plants, it sometimes takes up to a year before all fine tuning takes effect and new processes have become established. By contrast, the new machine from ANDRITZ Kaiser was delivering high quality parts after just one day, thanks to the flexible control options. This was possible because the forces and positions of individual axes in the axis combination can be controlled precisely and with high repeat accuracy. Thanks to the rigidity and precision of the overall system, the end customer has managed to increase their productivity by 30 percent and reduce the number of rejected parts by up to 20 percent. The plant concept is also intended to provide scope for new designs and components. The servo hydraulics solution was the result of a close engineering cooperation between Tenneco, ANDRITZ Kaiser, and Bosch Rexroth. “The results are excellent and our customer is delighted”, says Wiedenmann.


The drawing process

The drawn part (3) is positioned on the sheet holders (4). The press ram (1) lowers and clamps the sheet between the upper die (2) and the sheet holder. The die cushion (7) builds up the necessary pressure. The press ram draws with upper die the sheet over the lower die. The die cushion controls continuously the draw force. The controlled flow of the sheet in the die is ensured by force control of the sheet holders. When the deep drawing process is complete, the press ram returns to its initial position. The finished drawing part can be ejected.

 
The drawing process