Under Pressure: How Aluminum Extrusions Are Made

At any given time I’m likely to have multiple projects in-flight, by which of course I mean in various stages of neglect. My current big project is one where I finally feel like I have a chance to use some materials with real hacker street cred, like T-slot extruded aluminum profiles. We’ve all seen the stuff, the "Industrial Erector Set" as 80/20 likes to call their version of it. And we’ve all seen the cool projects made with it, from CNC machines to trade show displays, and in these pandemic times, even occasionally as sneeze guards in retail shops.

Aluminum T-slot profiles are wonderful to work with — strong, lightweight, easily connected with a wide range of fasteners, and infinitely configurable and reconfigurable as needs change. It's not cheap by any means, but when you factor in the fabrication time saved, it may well be a net benefit to spec the stuff for a project. Still, with the projected hit to my wallet, I’ve been looking for more affordable alternatives.

My exploration led me into the bewilderingly rich world of aluminum extrusions. Even excluding mundane items like beer and soda cans, you’re probably surrounded by extruded aluminum products right now. Everything from computer heatsinks to window frames to the parts that make up screen doors are made from extruded aluminum. So how exactly is this ubiquitous stuff made?

The basic process for extruding aluminum is, outwardly, as simple to understand as the extrusion process used by a 3D-printer: heat material and force it through a die with the desired shape and size. But when PLA is replaced by giant aluminum log, and a Bowden cable and stepper motor by an enormous hydraulic ram, the details quickly cloud the simplicity of the underlying concept.

Die design is perhaps the most critical part of the extrusion process. Dies have to withstand tremendous forces at high temperatures, and must maintain their dimensional stability while doing so. Extrusion dies start life as round bars of tool steel up to a meter or more in diameter but typically around 30 cm. Dies are usually fairly thin in profile relative to their diameter, since the longer the path the aluminum takes as it passes through the die, the greater the friction it experiences. More friction means more force, which means bigger presses, more wear on the dies, and generally higher costs.

Dies are generally created by specialty manufacturers that employ skilled die design engineers and machinists. The process of turning a design into a die usually starts with roughing out the blank on a CNC lathe, then proceeds to a sequence of CNC milling operations. Electric discharge machining (EDM) is used extensively to get the fine detail needed to provide a smooth finish, and to achieve the precise geometry needed to control the flow of the aluminum through the die.

Most extrusions will have one or more hollow chambers, like the lumen of a pipe or, in the case of our 80/20 profiles, the negative space of the T-slots and the central bore. The die has to create those features, which require parts of the die to "float" in the incoming flow of softened metal. Diemakers accomplish this by suspending these features on arms that bridge the space in the upstream part of the die. The shape and surface finish of these arms has to be carefully designed so that the metal flows around them and joins together to create a smooth, continuous stream of material without voids, which could lead to weakness in the finished product.

Careful consideration of the hydrodynamic forces exerted by and upon the flowing metal are also important to die design. While the exit side of the die is pretty much exactly the size and shape of the finished extrusion, the entrance side is anything but. By some estimates, half of the energy used while extruding aluminum goes into overcoming friction between the metal and the die, so everything that can be done to reduce those forces is like money in the bank. The entrance of the die has to be designed to direct the incoming metal as smoothly and easily as possible into the final shape, which is part of the reason die designers include very generous draft angles over the width of the die.

There are a number of different ways to approach the extrusion process, each with its own pros and cons. Direct extrusion is basically what you’re familiar with in 3D printing, or if you’ve ever used one of those squeezy things in a Play-Doh set: a slug of softened material is pressed against a die, which then flows through the die to assume its final shape. Indirect extrusion turns that around, forcing the die to move relative to the material. Both approaches have their pros and cons, and both result in extrusions with different metallurgical properties.

In either process, a large log of aluminum, called a billet, is heated in either a gas furnace or by induction. The temperature varies with the specific alloy and the complexity of the die, but it's important to note that the billet is not melted, just softened. The die and much of the hydraulic press are also heated, to prevent thermal stresses from breaking anything in the machinery and to prevent the aluminum from cooling too soon and sticking to the die.

Aluminum extrusion presses generally have a horizontal orientation, with a massive hydraulic ram facing the die across a narrow gap. The preheated billet is placed into the gap, and the hydraulic ram starts to press it into the die (or, in indirect extrusion, moves the die over the material). The softened metal begins to flow into the spaces of the die, around the arms, and narrowing down into the final shape as it exits the die.

The growing extrusion exits the press and is almost immediately quenched with either air or, more commonly, a water bath. The quenching process is important, because as the extrusion exits the die it is still soft and liable to be deformed. Quenching also sets the crystal structure of the metals in the alloy, giving the finished extrusion its desired metallurgical properties.

But even with quenching, the extrusions that come out of the die onto the long outfeed tables are far from complete. The tremendous forces exerted during extrusion coupled with the thermal stresses of quenching inevitably warps and twists the profiles. This is corrected with a stretching operation, where extrusions are literally picked up and stretched out the long way with hydraulic tools. This restores the profile to its intended shape; the few percent change in the length of the profile necessarily changes the profile dimensions slightly, a fact which has to be accounted for by the die designers.

Curiously, fresh extrusions need to be aged somewhat at elevated temperatures before reaching their final specified strength. This is accomplished in large aging towers over a period of hours to days, depending on the alloy. Aged extrusions are then cut to length, possibly have a finish applied — clear or dyed anodized finishes are very popular for 80/20 extrusions as they protect the aluminum from oxidation — and packaged for shipping.

Given the amount of material that goes into an aluminum extrusion, and the investment needed to run the massive machines that do the job, it's easy to understand why 80/20 profiles cost what they do. So now maybe I’ll just bite the bullet and order what I need.

Featured images: F&L Industrial Solutions, Inc.