Stretch Forming

Stretch forming pulls long extrusions or tubes over a form die to create smooth, large-radius curves with minimal wrinkling and cross-section distortion.

Overview

Stretch forming plastically elongates a clamped extrusion, sheet, or tube while wrapping it around a precision form die. The material stays in tension throughout the bend, which controls wrinkling, flattening, and cross-section distortion better than most other large-radius bending methods. It excels on long, uniform profiles where you need smooth, repeatable contours and tight shape control over several feet.

You should consider stretch forming for aerospace-style parts: long fuselage frames, stringers, window frames, and large-radius tubular structures that must match a master contour. The tradeoffs: you need dedicated hard tooling, large equipment, and enough volume or critical geometry to justify setup and die cost. Small radii, sharp reverse bends, and highly localized features are poor fits. Expect good surface finish and consistent geometry, but not ultra-tight machining-level tolerances over very long spans.

Common Materials

  • Aluminum 2024
  • Aluminum 6061
  • Aluminum 7075
  • Titanium Ti-6Al-4V
  • Stainless steel 304

Tolerances

±0.010" to ±0.030" on formed profile, depending on length and cross-section

Applications

  • Aircraft fuselage frames and stringers
  • Wing and tail surface spars and ribs from extrusions
  • Aircraft cabin window and door frames
  • Large-radius tubular seat and monument frames
  • Rail car and bus roof bows from aluminum extrusions
  • Curved architectural mullions, frames, and handrails

When to Choose Stretch Forming

Choose stretch forming for long, constant-cross-section extrusions or tubes that need smooth, large-radius curves with high contour accuracy and minimal distortion. It fits medium to high volumes or critical-contour parts where you can justify custom form dies. Best for ductile alloys and aerospace-style geometries where the entire profile must match a controlled aerodynamic or aesthetic surface.

vs Mandrel Bending

Pick stretch forming when you need very smooth, large-radius curves over long lengths with minimal twist and excellent contour match to a form die. Mandrel bending is better for tighter radii in smaller tubes where internal support is needed, but it does not control long, complex contours as well as a full-length stretch-form setup.

vs Rotary Draw Bending

Use stretch forming when the part must follow a precise, often multi-plane contour over several feet, such as aircraft frames and stringers. Rotary draw bending is efficient for shorter bends and tighter radii, but it can show more ovality and local distortion on large sweeps than a properly tensioned stretch-formed part.

vs Compression Bending

Choose stretch forming when cross-section stability and surface quality are critical and you can’t accept wrinkles or flattening on the inside of the bend. Compression bending is fine for less critical structural shapes, but it loads much of the section in compression, which increases the risk of buckling on thin-walled or aerospace-grade extrusions.

vs Roll Bending

Select stretch forming when you need tight, repeatable contour control tied to a hard die, especially for matched sets of aerospace parts. Roll bending is more flexible for variable radii and very large arcs, but offers less precise, die-defined geometry and can require more iterative adjustment to hit exact contours.

vs CNC Tube Bending

Go with stretch forming for long, constant-profile parts that must match a master surface, such as frame members that follow fuselage curvature. CNC tube bending handles complex multi-radius paths and multiple short bends along a tube, but it does not inherently keep the entire span under tension like stretch forming, so distortion control on large sweeps is not as strong.

Design Considerations

  • Design with generous bend radii relative to wall thickness and alloy; consult forming limits for your specific temper before locking geometry
  • Keep cross-sections uniform through the bend region; abrupt thickness or feature changes near the bend increase risk of cracking or distortion
  • Reserve straight, obstruction-free grip lengths at both ends for clamp blocks and specify any non-grip regions clearly on the print
  • Dimension and tolerance to functional datums and contour, not arbitrary points; specify profile or spline tolerances along the formed arc
  • Provide full 3D CAD of the as-formed shape and clearly state whether the model is pre- or post-springback to avoid tooling interpretation errors
  • Call out acceptable surface marking and cosmetic zones; clamp and die contact will leave some marking unless you pay for special tooling and protection