aluminum extrusion

The Process Behind The Making Of Aluminum Extrusions

Whether you are looking at planes, ships, trains, heatsinks, your house (including your balcony doors and your windows), it is all made with the help of extruded aluminum. Aluminum extrusions are strong, lightweight, and can be connected with a considerable array of fasteners. 

Extruded aluminum is also infinitely customizable and re-customizable, whatever the needed change may be, making them great to work with. Relatively speaking, it is not a cheap endeavor, but when you save time in production – it might as well make your organization a significant net profit on your next projects.

So, what is the story behind the making of these beautiful metalworks that find usage in so many areas?

Casting The Die 

Basically, the process of aluminum extrusion is relatively simple to understand. It can be likened to squeezing toothpaste out of a tube. Of course, the process is much more complicated, but those are the underlying basics. Probably the most crucial component of the extrusion process is the die design. 

Following the previous analogy, the die would be the circular opening at the end of the toothpaste tube. Dies must endure enormous forces applied at high temperatures and must preserve their structural integrity while doing so.

Photo from GrabCAD

Extrusion dies can be up to a meter or more in diameter but are usually about 30 cm. Dies are typically relatively thin in profile. This is because the longer the aluminum’s route takes as it moves through the die, the higher the friction it encounters. More friction means more force. More force means larger presses, more wear on the dies, and higher general costs.

Specialist manufacturers typically produce the dies. These companies hire professional engineers and machinists that have specialized in their die design craft. To transform a template into a die, they begin with roughing out the blank on a CNC lathe. After that, they are subjected to a series of CNC milling operations. 

Next, engineers use electric discharge machining (EDM) on the dies. This is done to obtain the sufficient detail required to provide a smooth finish and achieve the precise geometry necessary to regulate the aluminum flow through the die.

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Most extrusions would have one or more hollow chambers, much like the case of a pipe’s lumen or T-slots’ negative space and the central bore in 80/20 profiles. When softened metal flows in, the die has to build specific features that enable sections of the die to “float.” This is accomplished by suspending these features in the upstream part of the die on arms that bridge the space. A crucial part of the process is carefully designing the form and surface finish of these limbs so that the metal flows around them and blends together to create a smooth, continuous material stream without voids. This is done to ensure no weaknesses are present in the finished product.

It is fundamental to consider the hydrodynamic forces applied to the flowing metal in the die design. Although the finished extrusion’s size and shape are mostly identical to the die’s exit side, it is not the case with the entrance side. 

According to some reports, nearly half of the energy used when extruding aluminum is attributed to overcoming the friction between the metal and the die. This means that it’s highly essential to do whatever possible to reduce these forces.

The die entrance must be built to guide the incoming metal into its final form as smoothly and efficiently as possible. That’s why die designers are very liberal when setting the angles over the die width.

Squeezing It Out 

There are various ways to approach the extrusion process, each with its pros and cons. Direct extrusion is probably what you know if you ever used a Play-Doh set. A slug of softened material is pressed against a die, and then it flows to assume its final form. 

There is also indirect extrusion, which forces the die to move relative to the material. While both approaches have their ups and downs, extrusions are done with both of them. Each method results in different extruded metalworks with distinct metallurgical properties.

Nevertheless, both approaches include heating a large aluminum log (called a billet) by induction or in a gas furnace. The temperature varies depending on the specific alloy and the die. 

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Photo from Medium

Whatever the required temperature is, it is imperative to have in mind that this process doesn’t result in the billet’s melting. It only needs to be softened. Heating is also applied to the die and most parts of the hydraulic press so that thermal stresses wouldn’t destroy components in the machinery and prevent the aluminum from cooling down too soon and sticking to the die. 

The preheated aluminum log is then pressed into the die by a hydraulic press, after which the softened metal flows into the spaces of the die and around its limbs, exiting the die shaped into the final form. 

After leaving the die, it is crucial to quench the metal with water (or air), so it doesn’t get deformed. This process sets the crystal structure of the metals to achieve the desired metallurgical properties.

The Finish 

Once quenching is done, the profiles that came out of the die are still not finished because the enormous forces and thermal stresses applied to the aluminum will inescapably twist it. To correct this, aluminum extrusion service providers apply the so-called stretching technique. This is achieved with hydraulic tools that bring the profile back to a straight shape. 

The next step is aging. This technique is required due to the specific process, so it gives the profiles the needed strength. Giant aging towers are used to store the metalworks for some time (it can be hours or days, depending on the alloy). Subsequently, the profiles are cut into the prescribed length, finished with anodization to increase corrosion resistance.

Photo from The Fabricator

Final Thoughts 

As we’ve laid out the process, we can see how much machinery and expertise goes into the aluminum profiles’ extrusion. So, it is quite understandable that it has a particular price tag. But as many industries continue to rely on this technology, it is advisable not to evade it if your project can benefit from it.

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