How an aluminium profile is made: extrusion

The idea: like a tube of toothpaste

Extrusion works like squeezing a tube of toothpaste: the material comes out in the shape of the opening. In the press, the aluminium billet is heated to about 450 to 500 °C (it softens but stays solid) and a ram pushes it against a steel die. The aluminium comes out the other side with the exact shape of the cross-section, in one long piece.

Step by step

1. Billet cutting and heating. The alloy billets, usually 6063 or 6060, are cut to length and heated in a furnace.
2. Extrusion. The press pushes the billet through the die, which is also preheated. Press force is measured in tonnes: from a few hundred to several thousand.
3. Quenching at the press exit. The profile is cooled as soon as it comes out, with air or water. This cooling is key to its mechanical properties.
4. Stretching. Once the profile has cooled, a stretcher pulls it from both ends to straighten it and relieve stresses.
5. Cutting. It is cut to commercial length: in Argentina, 6-metre lengths.
6. Ageing. The profiles spend several hours in an oven at about 170 to 200 °C. This is how they gain strength and end up in T5 or T6 temper.
7. Finishing. They are left in mill finish or go on to anodising or powder coating.

Solid dies and bridge dies

The die is the part that gives the profile its shape, and it is made to suit each cross-section:

• Solid die: for open profiles such as angles, flat bars or U-channels. It is a steel disc with the outline cut through it.
• Bridge die (or porthole die): for profiles with closed hollows, such as tubes. The aluminium splits into several streams around a mandrel and welds back together inside the die, under pressure and heat.

That is why a custom profile needs its own die: it is the up-front investment in any new profile.

What limits what can be extruded

• The circumscribing circle: the smallest circle that contains the whole cross-section. It has to fit in the press container, so it determines which press is needed.
• Wall thickness: very thin or very uneven walls make it hard for the metal to flow evenly.
• Tongues and hollows: long, narrow projecting parts of the die can break under the pressure.