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4 Q&A about the aluminum sheet stretch forming process

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stretch forming process

What Aluminum Profile Thickness is Best for Stretch Forming?

Aluminum sheet thickness is important to stretch forming success. The wrong gauge of aluminum can cause failures that can range from torn edges to popping and cracking.

Why Does Aluminum Profile Thickness Matter?

stretch forming machine
stretch forming machine

Aluminum is a relatively soft material and, therefore, very malleable. Whether you are using a press brake, aluminum bender, or a stretch forming press, success depends on your ability to control the shape of your product without damaging its structure. The best way to control your product’s shape during stretch forming is by working with the right aluminum thickness.

If you choose to thin an aluminum alloy, your part may tear or crack as you stretch it over the die. If you choose too thick an alloy, your part may be difficult to work with, resulting in inconsistent quality across your production run.

In the stretch forming industry, choosing an aluminum sheet gauge is one of the most important decisions. When selecting aluminum sheet gauges, some factors include the material’s thickness, strength, and durability.

As the thickness increases, so does the minimum radius.
The minimum radius also increases with harder material.
The trend is obvious: The harder and thicker the plate is, the greater the minimum bend radius. For 0.5-in.-thick 7050 aluminum, the minimum bend radius may be specified as much as 9.5 times the material thickness.

The Fabricator offers certain key tables and general rules which are helpful for understanding the limits to bendability for specific aluminum alloys.

Does the Aluminum Sheet Have to Be Preheated Before Stretch Forming?

The most common reason for preheating is to avoid surface tears and fractures during forming: This is because the metal at the inside bend radius will undergo more deformation than the outside radius and therefore overstretch, causing tearing and cracking.

If you bend anything harder than 5054 aluminum, you will need to anneal it by heating along the bend line. If you don’t, such hard aluminum will crack and break during forming.

Aluminum melts between 865 and 1,240 degrees F, so you obviously can’t heat it as much as steel. In some ways aluminum heats, bends, and recrystallizes the way steel does, and in other ways it responds very differently. When heated, aluminum tends to have a little more springback. You might achieve the desired bend angle and radius, but as soon as it cools, it springs back slightly more.

Bending Basics: The fundamentals of heavy bending

Pros

  • Preheating softens the aluminum so that it is easier to form.
  • An even temperature across the entire length of the material helps prevent part distortion due to uneven heating or cooling during forming.
  • Preheating can reduce cracking or tearing in some materials by reducing spring back after forming or reducing internal stresses.

Cons

  • An aluminum sheet is not a good thermal conductor, so the internal temperature of the sheet may be quite different from that of the external surface. The use of induction heating technology may result in higher material temperatures near the surface than at the core.
  • The outer surface of the aluminum may be oxidized, and therefore heat will not readily penetrate the material. In some cases, it may be necessary to remove this oxide layer with a suitable cleaning agent or mechanical sanding before healing can begin. This does not apply to aluminum alloys that form a thin oxide film adherent to the material surface.
  • When aluminum is heated, it undergoes a volume expansion of about 2 percent, but it only contracts by about 0.2 percent when it cools.

Can Aluminum Sheet Be Machined After Stretch Forming?

Yes. An aluminum sheet can be machined after stretch forming, but we recommend that you allow the part to stabilize for 1-2 weeks to relieve stresses in the material.

The machining requirements will depend on your design specifications, but you should remember that lower alloy aluminum has better machining characteristics than higher alloy aluminum.

What Kind of Bend Radius and Minimum Flange Height Can I Get with Stretch Forming?

Any 1:1 ratio of bend radius to stock thickness can be formed.

Routinely form parts with a 1/2″ bend radius and hold 0.005″-0.010″ minimum flange height on 0.064″ aluminum material. If the part geometry is such that it requires more tension than the standard 15%, can stretch up to 30% of the original material length.

Source: ETCN