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Technological & Economical Aspects Page 1:

Dr.-Ing. Alfons Böhm, Siempelkamp Pressen Systeme, Krefeld
Anthony Abbey,
Siempelkamp Pahnke Engineering, Springfield


1.) Part design determines production costs: Example engine cradle
2.) How to reduce the number of production steps: Examples
Hydroforming processes and die design for high volume production
Cost studies for various part
ALLFORM Double Ram Press Systems (DRS) for high volume production
Achievable reliability and quality: Some selected aspects

1. Part Design determines production costs: Example engine cradle

Hydroforming is a comparatively new technology in its application in automotive mass production. Nevertheless an old rule is still valid: The production costs are mainly designated by the part design.
The number of applications of hydroforming processes in automotive industry especially in the field of chassis and suspension systems is growing rapidly. One of the most often applications are engine cradles (Fig. 2.1). These parts are typically produced beginning with the production steps bending, pre-forming and hydroforming. It is relative simple to decide, whether bending or hydroforming is necessary, but it is more complex to decide, whether pre-forming is necessary or not and which type of pre-forming process should be used. In the following a few general methods of pre-forming and their necessity requirements are discussed.

Benteler / Germany

Fig 2.1: Two examples produced by Hydroforming (before / after assembly)

To load the bended part into the hydroform die, all square sections of the bended tubes have to be tighter than the opening of the hydroform die. Considering the positioning tolerance of handling systems like robots or feeder systems and the bending process usually means that the gap between outer tube surface and hydroform die should be between 0,5 to 1,5 mm.
Only in the end area of the tube is the outer shape of the tube similar to the die to guarantee sealing and the possibilty of applying axial forces in the end are. Very often the necessary tube circumference and thereby the tube diameter itself has to be adjusted to the square section with the maximum expansion in the hydroform process. Three different cases can be distinguished:

Click on image for enlargement.
CASE A / Fig. 2.2.1/2:
Faultless loading of the bent tube into the hydroform die; sufficient circumferential gap around tube for safe loading. Normally in this case after bending the tube can be hydroformed without intermediate processes. This is the most economic way to produce engine cradles with hydroforming technology. During closure of the hydroform die, a slight bending in second plane of the pre-bent tube is possible.

CASE B / Fig. 2.2.1/2 Click on image for enlargement.
Case B / Fig: 2.2.2 Click on image for enlargement.
CASE B / Fig. 2.2.1/2:
After bending, loading of the pre-bent tube is not possible, because the tube width in the split plane of the hydroform die is wider than the opening of the die contour. The tube is not bent in this area. A pre-forming operation is necessary to flatten the tube in the described area. If the described area is not bent before, a simple flattening procedure is all that is necessary. Sometimes the flattening operation can be integrated in the hydroform die with so-called pusher units which are activated by special hydraulic or wedge slide units.

CASE C / Fig. 2.2.1/2 Click on image for enlargement.
Case C / Fig: 2.2.2. Click on image for enlargement.
CASE C / Fig. 2.2.1/2:
After bending a loading of the pre-bent tube is not possible for above mentioned reason but the contour of the hydroform die is tighter than the tube in a pre-bent section or in a few other areas.

In this case a flattening procedure is sometimes not possible; this leads to a pre-forming operation in a die with tapered sides; the tube can be either flattened or drawn within such an operation. The operation allows a pre-distribution of the material in a many different ways to guarantee the optimum wall thickness distribution in the hydroform process an to achieve a high variety of different square section geometries as shown in Fig. 2.3.

Bending -> Pre-forming -> Hydroforming

Click on image for enlargement.
Fig 2.3: Selected square sections of an engine cradle (steel, tube diameter 60 mm)

As an advantage in comparison to case B is that there are more possibilities to change the contour of the pre-bent tube but as a disadvantage a press operation including handling and lubrication is necessary. In comparison to case B the die contact friction is relatively high which leads to press forces in a range of about 500 to 3000 kN depending mainly on the die design, tube material and dimension.
A computer simulation shown in Fig. 2.4 shows the principle design of such a tapered pre-form die.

Click on image for enlargement.
Fig 2.4: Simulation (FEA) of tapered die pre-forming followed by hydroforming.

The first pre-forming (bending) as a detail image.

An other option to avoid the requirement for the described preforming methods is the use of oval starting tube material, the economy of such a method mainly depends on the costs of the tube material.

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With permission of Siempelkamp Pressen Systeme / Germany.
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Last update: Januar 12, 2000
Letzte Änderung: 12 Januar 2000

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