Stretch forming relies on tensile and bending forces to shape the aluminum extrusion profiles into simple or complex, wrinkle-free curves. The aluminum profile(such as an aluminum plate) is kept under constant tension, while wrapping it around a form, or die, to produce the desired arc. Stretch forming turns a straight aluminum extrusion into a vital component of contemporary structures: a hotel skylight, a contoured sign, a mall archway or a skyscraper’s glass and metal facing.
Heat and press
Most metals can be stretch formed, although aluminum is by far the most common. In simplest terms, stretch forming is a way of bending metal into a desired shape—usually curved—by heat treating it and then pressing it over a die.
Tips 1#: How to stretch-forming the Aluminum structural and non-structural？
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.
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 or 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 peak temperature of 399 to 427 C (750 to 800 F) for two to three hours, dependent on the extrusion’s 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 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.
Tips 2#: How to stretch-forming the Painted or anodized finishes
Parts require annealing
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.
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.
Parts that require a poured thermal break
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.
Tips 3#: 4 Steps for Stretch Forming Process
- Loading: Both ends of the part are inserted in gripper jaws, themselves mounted on the swing arm.
- Pre-Stretching: The part is stretched to its yield point.
- Wrapping: The part is wrapped around the form die while the stretch force is continuously maintained.
- Release: When the wrapping is completed, the stretch force is released and the gripper jaws are opened.