When you order a custom metal bellows, you’re not just specifying a shape — you’re specifying a manufacturing process, even if you don’t realize it. The method used to form the convolutions of your bellows has direct consequences for wall thickness consistency, residual stress, fatigue life, surface finish, and the range of geometries and materials that can be produced.

Most customers don’t ask which forming process their bellows manufacturer uses. Most manufacturers only use one. Bellows Systems uses both hydroforming and mechanical forming — and has since acquiring Kopperman Industries in 1986, which brought with it decades of Kopperman-style hydroformed bellows expertise.

Understanding the difference helps you specify more accurately and evaluate manufacturers more effectively.

In mechanical punch forming, a tube of material is placed over a mandrel or die set and a series of punches form the convolutions by compressing the material radially. The tube is stepped through the press, forming one convolution at a time.

Roll forming uses a set of rollers to progressively form convolutions as the tube is advanced through the rolling station. It’s typically faster than punch forming for certain geometries.

• Handles large diameter bellows — mechanical forming is practical for very large bore sizes (up to 157″ in BSI’s range)
• More versatile for non-circular profiles — rectangular, oval, and custom cross-sections can be formed mechanically but not hydroformed
• Lower tooling cost for large diameters and custom shapes
• Faster for high-volume production of standard profiles
• Works well with thicker-wall material that won’t hydroform reliably

• Wall thickness variation — the forming process can thin the convolution crown and thicken the root, creating non-uniform stress distribution
• Higher residual stress in some geometries — the forming forces leave compressive or tensile residual stresses in the convolution walls
• More limited convolution geometry range — convolution pitch and height are constrained by the tooling

Hydroforming uses hydraulic pressure — typically water or oil — applied to the inside of a tube constrained by an external die set. The fluid pressure expands the tube material outward into the die cavities, forming the convolutions. Because pressure is applied uniformly across the entire tube surface, the material flows into the convolution geometry with much more uniform thickness distribution than punch forming can achieve.

BSI has been manufacturing Kopperman-style hydroformed bellows since 1986, when BSI acquired Kopperman Industries. The Kopperman method is known for producing bellows with exceptional convolution geometry consistency and superior fatigue life in high-cycle applications.

• Superior wall thickness uniformity — the most important factor for fatigue life in high-cycle applications
• Lower residual stress — hydraulic pressure is gentler on the material than mechanical force
• Better convolution geometry consistency lot-to-lot — die-controlled forming is highly repeatable
• Excellent for thin-wall, high-strength alloys that would crack or tear under mechanical forming
• Preferred for aerospace, defense, and precision OEM applications where fatigue life documentation is required

• Limited to circular cross-sections — cannot produce rectangular or oval bellows
• Tooling (die sets) is more expensive for custom sizes
• Practical bore size range is narrower than mechanical forming
• Higher per-unit cost for small quantities of custom sizes

In most cases, the manufacturer’s engineering team will select the forming process based on your application requirements. But if you’re comparing manufacturers, here’s a simple guide:

• Your application has high cycle life requirements (>50,000 cycles) and fatigue performance documentation is needed
• You’re using thin-wall exotic alloys (Inconel 625, Hastelloy, Titanium)
• The bore size is within the practical hydroforming range (typically under 24″–30″)
• The application is aerospace, defense, or precision OEM where consistency lot-to-lot is critical

• You need large-diameter bellows (above ~24″ bore, up to 157″)
• Your profile is non-circular: rectangular, oval, or custom cross-section
• Wall thickness is moderate to heavy and material formability favors mechanical methods
• Budget and volume favor the lower tooling cost of mechanical forming

The practical advantage of working with a manufacturer that has both capabilities — like Bellows Systems — is that the engineering team can select the optimal process for each application without being constrained by what equipment they happen to own. A manufacturer with only one process will always find a way to justify using it.

• Custom Fabrication Services — BSI’s manufacturing capabilities overview
• Multi Ply Bellows — high cycle life bellows using advanced forming methods
• Aerospace Bellows Applications — where hydroforming precision is most critical
• Design Gallery — examples of BSI manufactured bellows configurations