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HYDROFORMING:

Technological & Economical Aspects Page 3:

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

Contents

1.) Part design determines production costs: Example engine cradle
2.) How to reduce the number of production steps: Examples
3.)
Hydroforming processes and die design for high volume production
4.)
Cost studies for various part
5.)
ALLFORM Double Ram Press Systems (DRS) for high volume production
6.)
Achievable reliability and quality: Some selected aspects

3. Hydroforming processes and die design for high volume production

The tool is still the key element of every cold and warm forming process and machinery and therefore an important aspect of the

  • Availabilty of the whole system,

  • Production safety and

  • Part quality

A common comment heard within the industry is: For hydroforming you just need a good machine and a simple tool which is mainly made by two die halves similar to the outer shape of the component and some axial cylinders. Unfortunately this is wrong and you should know before you start using hydroforming in mass production. The mentioned "one die = two halves" - procedure might work for some prototyping samples but is far away from mass production application. A few aspects of this point of view will be discussed.

Die Design in general
Fig. 3.5 shows a square section view of an example of a mass production hydroform die with insert technology. In general, to achieve the best possible hydroforming part quality, the whole die system should be as strong as possible. This includes a rigid guiding of the upper and lower tool halves (1/2) :

  • Pillars (3) are pre-guide the upper and lower die during closure.

  • During hydroforming the upper and lower die are guided by guiding plates (4) and the contour of the wear resistant inserts (5) all along the length of the part. Seperate guidance of the press system itself is not necessary or contrary to this.

  • Pressure and matching plates (6) allows the optimisation of the stiffness behavior of large hydroform dies under loads within a range of more than 20.000 kN especially if the press ram is not loaded in centre or non-symmetrical.

  • Wear restistant (7) inserts allows the replacement of areas with higher wear without excessive downtime or appreciable machining costs. The insert size depends on the coating method used. For safety reasons the inserts (7) and pressure plates (8) should be available as spare parts which means that a complete spare tool is not required. Every inserts causes split planes within the die which usually causes no reduction of part quality if the inserts are designed and machined according to the state of the art. However extra split planes over and above this ideal can generate split line marks on the part surface.

Fig 3.5: Selected basic elements for a mass production Fig 3.5: Selected basic elements for a mass production hydroform die with inserts (square section)

Axial Forming Cylinder Units

One general difference of hydroform tools in comparison to other forming tools is the requirement of axial forming cylinders. In Fig. 3.6 the Allform-method of connecting theses cylinders to the hydroform tool is shown in comparison to the so-called "plate design fixture". The advantages of the sketched system B) are:

  • Reduced size of platen of the press; axial forming cylinders can be outside of press platen area as shown in picture Fig. 3.6. This means under normal circumstances reduced capital investment for a smaller press system.

  • No deflection of punch in tube end axis under axial load; this can happen to system A) especially if the upper/lower tool heights are different which happens frequently when there are units in the lower die (hydraulic piercing units) which are not required in the upper. Therefore the wear of end inserts and punches are reduced.

  • Reduced tool costs because system is quiet simple to fix (two threads in die only).

  • Axial-forming cylinders can be fixed on upper or lower die (fixing in the upper die can sometimes make loading easier for robot or feeder units).

  • Compact tool, easy handling of tool during tool change and handling.

The mass production proven Allform axial forming cylinder system is available up to forces of 50.000 kN. It is equipped and delivered with a small hydraulic unit to fix the system to the tool with the correct torque.

Fig 3.6 A: Fixture of Axial Cylinders A) Plate Design Fig 3.6 A: Fixture of Axial Cylinders A) Plate Design

Fig 3.6 B: Fixture of Axial Cylinders B) ALLFORM Design Fig 3.6 B: Fixture of Axial Cylinders B) ALLFORM Design

Piercing Units

Hydropiercing, which means piercing within the hydroform process, requiring integrated piercing units within in the hydroform tool, has several advantages:

  • Excellent positioning of the holes relative to the outer shape of the part i.e. for further positioning of the parts during welding and assembly

  • Reduction of necessary production steps

In general hydropiercing is possible piercing outward with means of internal pressure or inward by means of a piercing unit. In most applications the piercing is done into the part by piercing units by using a hydraulic cylinder.

The piercing itself follow the calibration stage of the hydroforming process; therefore the cycle time is slightly increased by about 1 to 1,5 seconds. In Fig. 3.7 three mass production proven methods are shown for hydropiercing: All methods have in common designs which avoid the separation of the slug after piercing to avoid any risk of tool damage caused by uncontrolled slugs within the die.

Method 1: Typical method i.e. for painting holes; slug connected for part lifetime.
Method 2: Typical method i.e. for index holes; slug connected for part lifetime.
Method 3: "Coining": Pre - cutting within the hydroform tool / process and full cut after hydroforming i.e. within part check oder handling system; the final cutting force to get rid of the slug is quite low because of the brittle connection of slug and part caused by extreme local hardening.

Fig 3.7: Mass production proven methods of hydropiercing

Fig 3.7: Mass production proven methods of hydropiercing

The necessary piercing force can be calculated by the sum of two parts:

  • Cutting force itself similar to other cutting procedures /4/.

  • Force to overcome internal pressure.

In Fig. 3.8 the necessary piercing unit force is shown for a circular hole with different diameters, assuming an UTS of 400 MPa, tube thickness of 2,5 mm and a piercing pressure of 160 MPa. It shows that typical piercing forces are in a range of 500 kN up to 2000 kN. The magnitude of this force can lead to the problem of piercing holes next to each other when piercing of multiple holes with one piercing unit is not possible (example Fig. 3.9).

Fig 3.8: Example: Necessary force of hydrulic piercing unit depending on hole diameter

Fig 3.8: Example: Necessary force of hydrulic piercing unit depending on hole diameter

Fig 3.9 Example of coined Click on image for enlargement.

Fig 3.9 (a) Example of "coined" (see Fig 3.7) holes within the hydroform process

Fig 3.9: Example of coined Click on image for enlargement.

Fig 3.9 (b) Example of "coined" (see Fig 3.7) holes within the hydroform process

As a solution SPS offers ultra-compact piercing units which also satisfy other principle requirements for hydroforming piercing units:

  • Compact design / high force, accordingly nearly free arrangement of piercing units in hydroform die.

  • Quick change of piercing punches.

  • Positive locking to make sure, punches are locked and in the right position.

  • Lifetime comparable to similar press hydraulic units.

As an example such a 500 kN - piercing unit is shown in Fig. 3.10. They are only used for mentioned reasons and not used in standard applications where less sophisticated solutions are more applicable.

Fig 3.10 Ultra-compact hydropiercing unit in die split plane Click on image for enlargement.

Fig 3.10: Ultra-compact hydropiercing unit in die split plane

It is also possible to pierce holes without slug; an example is shown in Fig 3.11

Fig 3.11: Hydropiercing / Example: Index hole

Fig 3.11: Hydropiercing / Example: Index hole

The necessary piercing pressure also depends on the necessary piercing quality; two aspects are shown in Fig. 3.12: The rollover height sA and the influence of the rollover area (Diameter De) are reduced when applying higher internal pressure during piercing. By increasing the piercing pressure the necessary force of the piercing unit increases; this leads not only to a bigger size of the piercing unit but also to higher clamping and press force. Normally the piercing pressure is lower than the final (and highest) calibration pressure for a part and the press system should decrease the internal pressure to the necessary piercing pressure automatically.

If the piercing pressure is higher than the calibration pressure the economy of hydropiercing has be checked carefully. This leads to the fact that hydropiercing can be in some cases uneconomic and each single application should be checked for viability.

Fig 3.12: Selected quality aspect of hydropiercing and influence of internal pressure during piercing

Fig 3.12: Selected quality aspect of hydropiercing and influence of internal pressure during piercing

Fig 3.12: Selected quality aspect of hydropiercing and influence of internal pressure during piercing

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With permission of Siempelkamp Pressen Systeme / Germany.
All rights are reserved.

   

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Last update: Januar 12, 2000
Letzte Änderung: 12 Januar 2000

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