The aluminium extrusion process, step by step

What does an extrusion press actually do?

Extrusion shapes metal by forcing it through an opening of fixed cross-section. For aluminium the work is done hot. A cylindrical log of alloy, called a billet, is heated until it is soft, loaded into the press and pushed by a hydraulic ram through a hardened steel die. Whatever shape the die opening has, the profile has. This is why a single die can deliver a window frame, a heat sink or a machine rail in lengths of many metres.

How does a billet become a profile?

1. Billet preparation. The billet is cut to a length that suits the press container and heated in a furnace. For 6xxx alloys the temperature typically lies between 400 and 500 °C.
2. Die preparation. The die is made of hardened tool steel and is preheated before it goes into the press. A cold die disturbs metal flow and marks the surface.
3. Extrusion. The billet is loaded into the container. The ram applies its force to the billet through a dummy block, pushing the metal through the die, and it exits as a continuous profile onto the run-out table.
4. Quench. Directly after the die the profile is cooled by forced air or a water mist, or by water for alloys and sections that need a faster rate. The cooling rate sets the strength available after ageing.
5. Stretching. The cooled length is gripped at both ends and stretched by a controlled amount. This straightens the section and relieves residual stress.
6. Sawing to length. On the run-out line the stretched profile is sawn to standard commercial lengths.
7. Ageing. Artificial ageing in an oven is used where the specified temper requires it; T4 develops through natural ageing at room temperature. Time and temperature depend on the alloy and on the temper being produced. The article on tempers T4, T5, T6 and T66 covers this in detail.
8. Powder coating and packing. Where required the profile is powder coated, then packed to protect the surface. The guide to powder coating aluminium profiles describes that step.

How are hollow profiles made?

A solid profile needs only a plate die with an opening. A hollow profile needs a core to form the void, and the core must be held in place somewhere. A porthole die solves this: the metal splits around bridges that carry the core, flows into a welding chamber and rejoins under pressure and heat before it leaves the die. The rejoined metal forms a solid-state weld along the length of the profile. For most uses the weld line is irrelevant, but in highly stressed parts it is wise to know where it runs. Design rules for hollows are collected in designing profiles for extrusion.

What is the difference between direct and indirect extrusion?

In direct extrusion the ram and the billet move in the same direction, and the billet slides against the container wall. That friction adds to the press load. In indirect extrusion the die sits on a hollow ram and moves against a stationary billet, so there is no billet-to-container friction and the load is lower. The hollow ram limits the profile size, though. Most commercial profile production uses direct extrusion.

Why design with extrusion?

The die lets a designer put functions into one section: screw ports, snap-fits, gasket grooves, cooling fins. That reduces the number of parts that must be joined or machined afterwards. Aluminium is light and recyclable, and the process produces little scrap. The tooling cost is spread over the volume produced, so extrusion is economical for constant-section parts in medium to high volumes.

What decides the quality of the result?

Three variables dominate.

• The die. Dimensional accuracy, surface and output speed depend on die design and on how well the die is maintained.
• Temperature. Billet, container and die temperatures govern metal flow and surface. A billet that is too cold raises the press load, and one that is too hot can tear the surface.
• Exit speed. Friction heats the metal as speed rises. Thin walls and complex sections therefore run slower than simple solids.

The extrusion ratio, which is the container cross-sectional area divided by the total metal cross-sectional area of all simultaneous die exits, adds a fourth consideration. A very low ratio gives too little working of the metal, and a very high ratio loads the press and the die heavily.

Which alloys are used?

The 6xxx family, with magnesium and silicon as the main additions, dominates. These alloys extrude well, gain strength by heat treatment, resist corrosion and take surface treatments. The comparison of 6060, 6063, 6005A and 6082 sets out how they differ.

How is a profile verified after extrusion?

In general practice the first sample is checked for dimensions and surface quality. Dimensions are measured with callipers, gauges or measuring machines against the applicable tolerances, and the surface is inspected for scratches, depressions and die lines. Hardness testing can be used for process monitoring, while yield strength, tensile strength and elongation are verified by the tensile test that the relevant standard specifies. The guide to extrusion tolerances explains which dimensional limits may apply.

Where can I see it moving?

Open What is aluminium extrusion? for an interactive press with its parts and eight stages. This article is the written summary, and the interactive page shows the flow inside the press.

What should an enquiry include?

For your profile, prepare the section drawing (PDF, DWG, DXF or STEP), your alloy and temper preference, finish requirements and the annual volume. This lets the quotation be prepared accurately and quickly.

If you have a profile in mind, send the drawing and the annual volume through the quote request page; the minimum order is 300 kg per profile.