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.