Specifying aluminum stretch forming

by Jennifer Wilson | August 28, 2015 11:52 am

[1]

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.

[2]

 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.

Painted or anodized finishes
Similar to the curving process, the end application of the part will determine the best practice for how curved parts should be finished. If an extrusion has been painted or anodized, and has been tempered to a T5 or T6 hardness before being curved, the parts will need to be annealed. The high heat associated with the annealing process will likely cause painted finishes to burn and anodized finishes to discolor or craze. For this reason, when parts require annealing it is best to finish them after the curving process has been completed.

[3]

For extrusions tempered to a T1, T4, or T52 hardness, parts can be finished before curving. However, some marring or slight damage to the finished surface should be expected due to the parts being stretched across a hard die surface during the curving process. Depending on the tightness of the radius, anodic coatings may also craze or discolor as a result of being curved. If the T1 or T4 tempered extrusions require oven-aged tempering after curving, the high heat will likely damage the coating.

Regardless of the effect the curving process has on the finish, nearly all manufacturer and applicator warranties are voided when extrusions or brake metal are finished prior to curving. To obtain the best finish quality and keep parts fully warranted, it is best to finish after curving—regardless of the temper of the extrusion.

For parts that require a poured thermal break, it is best practice to have extrusions poured and thermally broken after curving. This helps avoid cracking of the thermal break, and the manufacturers’ warranties stay intact.

To ensure the optimal quality and convenience, some finishers provide a single-source solution where stretch forming, thermal improvement and finishing are synchronized and retain the full warranty. When available, utilizing the finisher’s trucks also can reduce material handling and packaging. Working with a single source not only eases the burden of multiple contacts, it also reduces opportunity for errors and damage, while saving costs and time.

[4]Tammy Schroeder, LEED Green Associate, is a senior marketing specialist at Linetec. She has more than a decade of experience working with architectural aluminum products and finishes. She also develops the company’s American Institute of Architects/Continuing Education System (AIA/CES) programs, and serves on the American Architectural Manufacturers Association’s (AAMA’s) Tactical Marketing Plan Group and Value Proposition Work Group committees. Schroeder can be reached at tammy.schroeder@linetec.com[5].

 

 

Source URL: https://www.constructionspecifier.com/specifying-aluminum-stretch-forming/

---