by Tammy Schroeder
Stretch forming is a process in which s metal extrusion is kept under constant tension, while stretched and bent simultaneously over a die to create contoured parts. The variety of shapes and cross-sections that can be stretch-formed is almost unlimited. Window systems, skylights, storefronts, signs, flashings, curtain walls, walkway enclosures, and hand railings can be accurately and precisely formed to the desired shapes.
In most cases the level of precision is so high, 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 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. This method of curving eliminates several conventional fabrication and welding steps.
The basic stretch forming machine has two arms or carriage beams that hold multiple-positioning gripping jaws. The jaws are attached to hydraulic tension cylinders that provide the stretch of the extrusion. The arms swing by rotating on large machined pins with bearings, allowing the extrusion to wrap around and against the forming die. This produces perfectly contoured products, while limiting or even eliminating wrinkling inside the arc.
Close and consistent tolerances, along with lack of surface marring, distortion, or ripples, and surface misalignment of complex profiles, are the benefits inherent in stretch forming. The process yields a smooth and even surface.
Structural vs. non-structural application
Aluminum has proven to be a suitable material for load-bearing structures for more than 100 years. However, the application of the parts being curved dictates the process used.
After being pushed through an extrusion press, extrusions are cut and placed into a tempering oven to harden and give them structural integrity. When they are fully hardened to a T5 of T6 temper, they are difficult to curve. If the parts to be curved have been fully tempered, they will need to be annealed before curving. To do this, the part is placed in a large oven and heated to a to a peak temperature of 399 to 427 C (750 to 800 F) for two to three hours, dependent on the extrusions size, weight and wall thickness. Annealing makes the extrusion soft again—enabling it to be curved. Once an extrusion is annealed, it cannot cost-effectively be re-tempered. In applications where the parts are expected to carry a structural load or have another structural application, annealing is generally not an acceptable practice.
For structural or load-bearing applications, the best practice is to have extrusions tempered to a soft state 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 temper 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.
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