March 19, 2001

               

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March 2001 Forming and Fabricating Vol. 8 No. 3  

Hydroforming's potential cannot be realized unless the 
entire process is well controlled.  

Controlling Tube 

Hydroforming  

By Tom Driggers, President and CEO, 

Interlaken Technology Corp., Eden Prairie, MN  

Tube hydroforming has been developing rapidly 

in the US, driven primarily by the automotive industry. 
The need for stronger, lighter, and more precise parts 
that can be produced in volume at reasonable cost has 
driven the process' adoption.  

Many parts that required extensive fabrication 

can now be manufactured to near net shape in a single 
operation. Weight is often reduced, component strength 
enhanced, and higher accuracy obtained. But while many 
companies have begun to adopt this technology, it has 

not been without a certain amount of pain.  

Some early adopters found themselves 

struggling to develop processes that run reliably, finding 
that variations in materials, fluids, lubricants, and tube 
preparation, produced unpredictable results.  

Typical Hydroforming Press Controls  

Presses developed for hydroforming have come 

from traditional press manufacturers. Controls for these 
presses have evolved from traditional stamping press 
controls. Stamping has, for the most part, required high 
forces and high speeds, but due to the process' nature, 
controls have typically been open loop and have simply 
controlled time sequencing in a series of steps.  

A typical tube hydroforming sequence might be:  

1. Close the press  

2. Move feed actuators to position docking rods  

3. Inject high-pressure fluid into the tube, while 

feed actuators apply force to tube ends.  

These hydroforming processes consist of 

operation sequences that are typically determined by trial 
and error. Finding a process that works, regardless of 
differences in material properties and part preparation, 
can be daunting. Programmable logic controllers are 
adequate in performing these sequential operations, but 
as manufacturers begin to use the hydroforming process, 
they often find that a simple, time-based sequence of 
steps cannot be made to work reliably. In this example, if 
the docking rods don't seal perfectly in the tube ends and 
pressure doesn't build at the expected rate, the push 
from the feed actuators might buckle the tube wall. High 
scrap rates, coupled with low production rates, might 
make the process impractical.  

Closed-Loop Controls  

The manufacturer might discover in this 

scenario that he could produce parts more reliably by 
implementing the following process:  

1. Close the press  

2. Move feed actuators to position docking rods  

3. Switch to load control on the docking rods to 

maintain a specific force  

4. Inject high-pressure fluid into the tube  

5. Control feed actuators to maintain a force 

equal to the docking force, and increase that force as a 
function of the pressure increase in the tube  

In this scenario, the force on the tube ends 

increases only at the rate of the pressure inside, reducing 
the likelihood of buckling the tube wall. In order to 
implement this process, the press must be closed-loop 
controlled and feed actuators must be controlled as a 
function of the pressure profile in the tube. This process 
is impossible to define with an open-loop control system 
that manages only time-based operating sequences.  

An alternative is to drive the feed actuators at a 

specified rate and control internal pressure based on feed 
actuator loads. Maintaining smooth feed actuator motion 
in the presence of friction requires closed-loop feed 
actuator motion control.  

In another scenario, the process might call for 

backing or bucking actuators to react to localized tube 
deformation during forming. The process might call for 
one of these actuators to maintain a constant force and a 
second to move as a function of feed actuator motion.  

This would require these additional steps:  

6. Move the backing actuators to contact the 

tube in position control  

7. Switch one backing actuator to load control 

and maintain a constant force on the tube wall  

8. Move the second backing actuator as a 

function of the feed actuator position  

These additional actions call for a multi-channel, 

closed-loop control system that can be programmed to 
handle events, as well as time dependencies. It must also 
have the ability to "gear" the motion of one or more 
channels to the actions of other channels. It must also 
accommodate dynamic feedback switching during the 

process. It must provide a high level of sophisticated 
control to be useful for manufacturing operations and 
have a practical operator interface that allows 
straightforward programming of these complex 
operations.  

Interlaken Technology has developed such a 

control system and software for use on its servo-hydraulic 
presses for materials and metal formability testing 
applications. The requirements for controlling hydraulic 
presses for materials testing and metal formability testing 
are very similar to hydroforming, namely rigorous real-
time, multi-channel, closed-loop control of high-force 
hydraulic systems. In addition, testing system users have 
always demanded great flexibility in programming these 
systems, as well as real-time process variable display 
and process data recording. To accomplish this, a 
Windows application was developed that provides a 
graphical interface for developing program sequences, as 
well as a real-time graphical display of the machine's 
motions. Process data is recorded and time stamped for 
analysis and use in statistical process control 
applications. This system is currently employed in several 
hydroforming applications and is offered to press 
manufacturers and systems integrators on an OEM basis. 

While the vast majority of parts being 

manufactured today are steel parts for the automotive 

industry, there is a great deal of interest in this process in 
the aircraft industry. The same advantages that 
hydroforming brings to the automotive industry are even 
more important in the aircraft industry where weight 
savings and part quality are seen as significant 
advantages. In addition to work with aluminum, research 
is being conducted with composites and other materials.  

More sophisticated hydroforming process 

control is starting to allow solutions to be developed for 
more complex hydroformed parts using a broader range 
of materials. This is control that can accommodate 
unavoidable variations in material properties, as well as 
variations in tube preparation, allowing higher quality 
parts and productivity and lower scrap rates.