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Tube hydroforming for
expanded design options
Knowing the
capabilities,
limitations, and
misconceptions
is key to
understanding
hydroforming s
potential
By Paul Tauzer
MUCH of the attention on hydroform- handles for medical instruments, door are: How much can the cross section
ing concerns automotive components and hardware, table and chair legs, rifle change? How tight can the corner radius
focuses on weight savings, strength scopes, and numerous sporting good be? How much will the wall thin? The
improvements, and reducing the number components. Choosing hydroforming for answer is always,  It depends. Some
of parts. While all of these are advantages these applications has less to do with limitations are driven by properties of the
gained through hydroforming, a less com- weight savings and more to do with product material, some by the hydro-
monly discussed reason to hydroform reducing the number of processing steps forming system constraints, and others
components is more design options. In to produce interesting and aesthetically by specific design features of the final
some cases, hydroforming is the only pleasing shapes. product.
method that can generate a unique geom- Hydroformed component features can
etry in a single process or as the final step include changing cross sections with MATERIAL REQUIREMENTS
in a combination of processes. varying corner radii and curved sections. Engineers and part designers must consid-
Hydroforming is useful for manufac- The process can produce smooth transi- er material properties such as yield
turing many products, including faucets, tions between round and oval or rectan- strength, ultimate strength, and allowable
gular cross sections. The cross sections elongation in the design of a component.
S H OR T C U T S
can vary as the contour of the part Materials commonly formed are brass,
Hydroforming s benefits include sweeps around a changing centerline stainless steel, carbon steel, and alu-
weight savings, part reduction, and through a radius or changing spline. minum. These materials vary widely in
additional design options. Features can be produced with this formability characteristics. Heat-treating
process that are not easily obtained by operations provide more latitude in con-
Learn hydroforming limitations. any other method. trolling the material s temper and grain
Design limitations of hydroforming size. Balance is necessary for parts that
Read about some strategies used to defy a simple explanation. Three ques- require a particular cosmetic appear-
overcome some of the limitations. tions that tend to arise with all designs ance the material must be soft enough to
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Material Feed Material Feed
Side Ram Hydraulic Side Ram Hydraulic
Cylinder Cylinder
Forming Pressure
Material Formed Mainly
by Internal Pressure
Material Formed by
Combined Pressure and
Material Feed
Figure 1
The effect of axial feeding is greatest near the tube ends, where it assists the internal forming pressure to form part features. The effect of axial
feeding diminishes with distance the farther a feature is from the end, the less axial feeding assists in forming it.
Final Formed Part
Tube Bent on Hydraulic Tube Bent Tube Loaded Into
Bender With Compound Mold Cavity
Radius Spline
Figure 2
Hydroforming usually requires at least two steps: bending the tubular blank so it fits into the mold cavity, and expand-
ing it by hydroforming.
Location of
Cross Section of
Maximum Thinning
Mold Cavity
Round Starting
Blank
Figure 3
A common misconception is that hydroformed
parts have a uniform wall thickness. For near-
ly every hydroformed shape, the amount of
Forming Progression
wall thinning varies. The most material thin-
ning occurs in the tightest corners.
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form and have a tight enough grain struc- WANT MORE INFORMATION? thickness, this simply isn t true when
ture to prevent an orange peel surface " If you need more information about hydroforming any other geometry. In these
condition. evaluating lubricants for hydroforming, cases, forming is influenced by locking fea-
In some cases, hydroforming a com- see  Analyzing tubes, lubes, dies, and tures in the geometry and the relationship
ponent is done in multiple steps with friction at www.thefabricator.com/ between material lubrication and mold cav-
intermediate annealing processes Hydroforming/Hydroforming_Article.c ity friction. Tighter corners are filled
between each step. This increases the fm?ID=281. though localized stretching and thinning of
total elongation of the material from " For an in-depth look at how axial material (see Figure 3).
starting blank to the final formed part. feeding affects thickness and strain dis- Most hydroforming systems consist of a
The expansion during each forming step tribution, see  Examining the effects of hydraulic press, a high-pressure pumping sys-
can be controlled by using separate push assist on the formability of alu- tem, and side hydraulic ram cylinders. The
molds with incremental increases in the minum tubes at www.thefab hydraulic press opens and closes a split mold
size of the forming cavity. Another, less ricator.com/Hydroforming/Hydroformi cavity to allow insertion of a blank and
consistent and less costly method is use of ng_Article.cfm?ID=641. removal of a formed part. It also holds the
a single forming cavity. In the initial " Interested in a book on hydroforming? mold halves together as the blank is pressur-
forming step, the forming pressure is lim- See Fundamentals of Hydroforming by ized. The pumping system provides the
ited and the cycle is stopped before filling Harjinder Singh. Go to the Fabricators & internal forming pressure. The ram cylinders
the mold cavity. After annealing, the Manufacturers Assocation, International® s seal the ends of the tube and can feed material
blank is formed to completion in the bookstore at www.fmanet.org/FMAstore/ into the cavity by pushing against the ends of
same cavity. index.cfm and search for hydroforming. the blank.
The pressure the pumping system pro-
EQUIPMENT CAPABILITIES The geometry that can be formed is duces typically is the limiting factor that
AND LIMITATIONS first limited by the ability to fit a blank determines the tightest radius the system
One aspect of the design that has a direct into a mold cavity (see Figure 2). The can form. A smaller radius requires more
impact on both the extent of formability closer the blank fits into the mold, the forming pressure. In some cases, on
of a feature and the amount of thinning less forming is required. The ability to larger parts a pressure below the maximum
the material will undergo is the location control the dimensional characteristics of pump pressure causes the mold to open. In
of the feature in relation to the part s the blank, including diameter and wall this case, the limiting factor is the press ton-
end. Features near the end of the forming thickness, or to produce a blank drawn nage. The cross-sectional area of the
cavity can be more easily influenced by to a nonround shape, improves the range formed part, as viewed from above the
axial feeding of the material with side of products that can be hydroformed. In mold cavity, multiplied by the pressure used
rams than can features at the center of some cases, blanks are partially reduced to form the blank, must be balanced against
the forming cavity (see Figure 1). As the or expanded, dented, or bent in single or the available tonnage of the hydraulic press
feature location moves away from the multiple operations. Being able to bend system to keep the mold closed.
end, the material has a higher tendency to complex or spline geometry increases the
lock in the mold cavity. The result is variety of items that can be produced. Paul Tauzer is engineering manager for
a decreased capability to feed material One common misconception regard- H & H Tube & Manufacturing Co., 108
during the forming process. Features in ing hydroforming is that it results in Garfield St., Vanderbilt, MI 49795, 989-983-
this area are produced strictly by stretch uniform material thinning. While 2800, fax [phone], www.h-htube.com.
and are more dependent on the material expanding a round tube to make a larger
properties. round shape does result in uniform wall
Reprinted with permission from theJune 2006 issue of TPJ - Tube & Pipe Journal®, copyright 2006 by FMA Communications Inc., Rockford,
Illinois, www.thefabricator.com.
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