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Art of Aluminum Stretch Forming: Basics, Principle, Process

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The process of stretch forming is more of an art than a science. It takes years of experience to become a skilled craftsperson that can stretch from consistent, successful, curved aluminum components for architectural projects.

What is Stretch Forming?

Aluminum Stretch Forming
Aluminum Stretch Forming

Stretch forming is a metal bending process in which a lineal shape, such as an aluminum extrusion, is stretched and bent simultaneously over a form, called a die. Each form is built to the required radius.

Stretch forming was invented during the 1940s with the rise of the aerospace industry for the curving of aluminum aircraft parts to reduce weight, and thereby, fuel consumption.

Stretch Forming in Industrial Buildings

Today, Stretch Forming is used extensively in the aerospace, marine, and shipbuilding industry, automotive industry, rail cars, and industrial buildings. Architects and designers pursued new opportunities to create curved facades and building components, it opened to possibilities of curved facades, architectural details, and roof lines previously not possible.

Now, the large curved glass facades are now possible, as the process is a perfect match for the curving of large curtain-wall systems.

Stretch Forming Principle

stretch bending machine
stretch bending machine

Stretch Forming involves placing an extrusion along a rounded, fixed bending die which is secured with clamps on each end. The machine will then rotate the clamped ends together, bending the extrusion to angles up to 180°. The extrusion takes its shape when it is bent around the bending die to reach its desired angle. Stretch forming is commonly used with components that require a larger bend radius.

Stretch forming is commonly used for parts with a larger bend radius, as the minimum bend radius is generally two to three times greater than other forming/bending methods.

Read More: The best guide to the Stretch Forming Process in 2022

Stretch Forming Capabilities

The variety of shapes and cross-sections that can be stretch-formed is almost unlimited. It is commonly used in the curving of aluminum sheets, extruded profiles, stainless sheets, bent or rolled profiles, structural shapes, various steel alloys, aluminum, stainless steel, brass, bronze, and copper.

Stretch-forming capabilities typically include portions of circles, including half-circles and eyebrows, ellipses, and arched shapes. These shapes can be formed with straight leg sections at one or both ends of the curve. It allows the curving of true radius contours, ellipses, compound bends, and helical bends. This method of curving eliminates several conventional fabrication and welding steps.

For Industrial Buildings

From muntin bars and panning for windows, cladding, and spirals for handrails, large mullions for building envelopes to serpentine shapes for canopies.

Stretch forming allows architects, designers, and builders to realize forms as graceful as they are sturdy and functional.

The variety of shapes and cross-sections that can be stretch-formed is almost unlimited: The window systems(aluminum window frames), skylights, storefronts, signs, flashings, curtain walls, walkway enclosures, and hand railings can be accurately and precisely formed to the desired shapes.

Read More: 5 basics of the aluminum window frame bending process

Stretch Forming Process for Bending Aluminum

In most cases, the stretch-formed aluminum component’s curvature is so highly precise that even intricate multi-components and snap-together curtain wall components can be formed from metal without loss of section properties or original design function.

Stretch Forming the Aluminum Profile Without Breaking It

To achieve this level of precision, the basic stretch-forming machine has two arms or carriage beams that hold multiple-positioning gripping jaws. Both ends of the extrusion are inserted into the gripper jaws and stretched to their yield point. The jaws are attached to hydraulic tension cylinders that stretch the extrusion. The arms swing by rotating on large, machined pins with bearings that allow the extrusion to wrap around and against the form. This produces perfectly contoured products while limiting or even eliminating wrinkling inside the arc. When the wrapping is completed, the stretch force is released and the gripper jaws are opened.

Different from the section bending process of the profile bending machine, stretch forming maintains close and consistent tolerances with excellent repeatability and alignments of complex profiles and compound curves. There should be no visible surface marring, distortion, or ripples. These benefits were inherent in the stretch-forming process yielding a smooth and even surface. Each component must meet the project’s specifications and warranty conditions.

Read More: How to bending aluminum without cracking it

How to Stretch-Forming the Structural or Load-bearing Applications

For structural or load-bearing applications, the best practice is to have extrusions tempered to a soft state of T1, T4 or to a T52 state. Material tempered to a T1 or T4 can be bent without annealing and can be tempered after the curving process to a T5 or T6 which is typical in structural applications. T52 is a very stable temper and can be curved without annealing, and it maintains its properties after curving without the need for additional tempering.