When aluminum is extruded through a die it’s supposed to come out straight. Die designers put a lot of effort into achieving that result because bent or twisted extrusion is of no use in many applications. There are occasions when curved or radiused aluminum extrusion is needed. Boat gunnels are one example, and trailer frame components are another. Clients sometimes ask if we can bend aluminum extrusion: our answer is usually some variant of, “It depends.”
There are a lot of challenges in bending aluminum extrusion. Here we’ll explain why it’s not an easy thing to do and introduce the five methods of doing it. If you’re a designer who wants to know what radii we can achieve on various sections you’ll find some of what you need here but we recommend you contact us for a more in-depth discussion.
Aluminum Properties and Composition
Aluminum is generally fairly soft and ductile. That makes it easy to work with but not terribly useful for most applications. To overcome this it’s alloyed with other elements. These aluminum alloys are produced in various grades, defined as “series”. There’s a 1000 series, a 2000 and so on, up to 7000.
Each series has a slightly different composition and different properties. In addition, the 2000, 6000, and 7000 series alloys are suitable for heat treatment. This increases their strength while reducing ductility.
Most aluminum extrusion is of the 6000 series. As well as being highly formable, this has good strength and corrosion resistance. Heat treatment makes the alloy harder to bend so it’s generally best done after forming the final shape.
Bending compresses the inner side and stretches the outside. When bending a thin strip in the direction of its thickness, (not its width), the difference in bend radius is small enough that the tensile and compressive forces aren’t great. As the work piece gets thicker the challenges increase.
As an example of the difficulty, consider bending an “I” beam of the type commonly used in trailers and architectural applications. To start with, we’ll bend it in the direction of the top and bottom flanges. In this case, the center web bends easily but as the inner and outer edges of flanges bend to different radii they experience more tension and compression. Tension thins the metal while compression can cause buckling.
Now, if we bend the “I” beam in the direction of the web, the problems multiply. Not only are top and bottom flanges forced to different radii, but so too are the top and bottom of the web.
The key to successful bending is controlling how each element of the extrusion moves. This is a lot easier when the extrusion is symmetrical, providing the axis of symmetry is aligned with the bend radius. Asymmetric profiles are extremely hard to bend without significant distortion.
Hollow section extrusion poses another problem: with nothing to support it there’s a tendency for metal under compression to buckle inwards. This can be overcome, to an extent, by filling the hollow interior with something that resists the buckling, but it makes bending a lot more complicated. Alternatively, thicken up the walls, incorporate generous internal radii and, if possible, add internal stiffeners.
Methods of Bending Aluminum Extrusion
Unlike bending metal sheet on a press brake, when bending extrusion the goal is to create curvature. To do this, the extruded section is wrapped around a die. Some over bending is usually needed because the extrusion will exhibit a degree of spring back. The five most common methods of bending extrusion are:
1. Ram/push bending
In this method, a semicircular ram is pushed against the extruded section. To either side pressure dies to support the section, pivoting as it wraps around the ram. The ram is shaped as a mirror image of the external section profile, as are the pressure dies. Multiple bends can be placed close together, although only one is made at a time. The tightest achievable bend radius is around four times the diameter of the section, (or the circumscribing circle.)
2. Rotary draw bending
This method can produce a very tight bend radii – around the same as the section diameter. The bending die is circular with a mirror of the section external profile over some 270° of the surface. The section is clamped against this die and held in place by a pressure die. The bending die rotates, wrapping the section around it. Precise bend angles are achievable by controlling the die rotation.
3. Three-roller bending
This is the method we use at Wiley. As with ram/push bending, the section is supported in two locations while a die pushes down in between. Where it differs is that rollers replace the ram and pressure dies. Shaped to match the section, these rotate, pulling the section through and creating a large radius. Roll bending can even produce complete circles. It’s often used for shaping large sections used in architectural or construction applications.
4. Compression bending
Here the extrusion is clamped against the side of a stationary ram which has the mirror profile around a 180° arc. A compression die then wraps the extrusion around the ram. The minimum radius is around three times the diameter of the extrusion or circumscribing circle. Compression bending is often performed on twin-head machines to produce forms with symmetrical bends at each end.
5. Stretch forming
This method is used for parts with a large bend radius. It entails clamping both ends of the extrusion to hold it in tension (the stretch.) A radiused die is then pushed against the mid-point of the extrusion to create the bend. The tension maintains control over overall length, there’s little surface marking and good bend angle repeatability.
If You Need to Bend Aluminum Extrusion, Talk to Us
When properly set up, an extrusion press produces straight lengths of extruded section. This is what many applications, such as machine frames, need. In some situations, though, it’s desirable to put a radius in the section. Aluminum extrusion from the 6000 series has good formability and, depending on geometry, can be radiused by a number of methods. However, it’s not easy. If you’re designing with extrusion and want to add bends, we suggest you contact us to discuss the specific details. It could save a lot of time and even improve your design.