Aluminium extrusions: a profile design guide

The die decides the profile

An aluminium extrusion is a constant cross-section pushed through a steel die. That sentence contains both the opportunity and the limit. The opportunity is that a profile with integral flanges, slots and cooling fins costs about the same per metre as a simple rectangle once the die is made. The limit is that nothing can change along the length: no pockets, no taper and no local boss unless a second operation adds it.

For a solid profile the metal flows through a flat die with an opening shaped like the part. For a hollow profile with an enclosed void, a porthole die splits the metal around a mandrel and welds it back together inside the die. The weld line is a real feature of the part, and it sits where the streams rejoin. After quenching, the profile is stretched to straighten it and artificially aged to the specified temper.

The alloy and temper are part of the design, not an afterthought. 6063 is the common choice for electrical and architectural profiles because it extrudes at speed and takes a good anodised finish. 6082 and 6005A are stronger and harder to press, so they need thicker walls and a slower route.

Wall thickness and metal flow

Wall thickness is the first number on an extrusion drawing, and uniformity matters more than the absolute value. Metal flows fastest where the die opening is thick and slowest where it is thin. A profile with a 4 mm wall next to a 1.2 mm wall fills unevenly, and the thin wall may come out under size, with a poor surface or with a visible streak where the flow changed speed.

For 6063, a wall of about 1.2 to 2 mm is routine and 0.8 mm is possible on a small profile with a well-made die. These are illustrative starting points, not limits. The real limit depends on the alloy, the circumscribing circle, the number of voids and the finish required.

Design the wall as a set of similar values. Where a change is unavoidable, taper it rather than step it, and put the transition where it will not be seen. A sudden change creates a dead zone in the metal flow, which shows as a streak on the anodised face and as a soft spot in the dimensional check.

Hollow sections, tongues and aspect ratio

A hollow profile is described by the number of voids and the tongue ratio. The tongue is the part of the die that forms the void, and it is supported at one end only. A long, thin tongue deflects under the pressure of the metal, so the void comes out off centre, the wall thins on one side, or the die breaks. A tongue ratio above about 3:1 is difficult, and above 5:1 it becomes a specialist job with a shorter die life.

Keep voids symmetrical and keep the wall around each void even. Two voids side by side are easier than one wide void with a thin dividing web, because the web has to support two metal streams at once.

The circumscribing circle is the smallest circle that contains the profile. It sets the press size, the die cost and often the minimum order. A profile inside a 100 mm circle can run on a smaller press than one that needs 250 mm, and the difference in tooling and freight is significant. If the design allows, split a wide profile into two extrusions that are joined later.

Corners, flats and slots

External corners are cheap. Internal corners need a radius, because the die cannot hold a sharp inside corner and because a sharp corner concentrates stress. A fillet of about half the wall thickness is a practical starting point, and a larger fillet at a loaded corner is better for fatigue.

A flat face is not automatically flat. A wide, thin face between two walls bows as it cools, and the bow appears as a profile tolerance failure rather than a flatness failure. Add a rib, reduce the face width, or accept a straighter tolerance and pay for more stretching. A decorative face should be identified, because the surface on the bearing side is not the same as the surface on the other side.

Slots and grooves are easy to extrude when they run the full length and are open to the outside. A closed slot is a hollow and follows the tongue rules. A slot with a narrow opening and a wide base is an undercut, and it needs a die with a removable insert or a machining operation after extrusion. A screw groove for a self-tapping screw is common, but the wall behind it must be thick enough to carry the thread and the splitting load.

Tolerances across the profile

An extrusion tolerance is not one number. Wall thickness, contour, twist, straightness and cut length each have their own value, and they interact. EN 755-9 groups the standard values for aluminium profiles. A drawing that says only general tolerances leaves the supplier to apply the table, which is usually the right starting point, but the critical faces still need their own call.

Secondary operations change the rules

An extrusion is rarely the finished part. It is cut, machined, punched, bent or anodised, and each operation has a consequence for the design.

• Machining: a long profile is flexible and rings under a cutter. Support it along its length, or machine short blanks rather than full lengths.
• Bending: a hollow profile collapses unless it is filled with a mandrel or a low-melt alloy. The minimum bend radius depends on the wall and the void shape, not only on the alloy.
• Punching: a thin wall distorts around a hole. Keep the hole size and the edge distance sensible, or drill and ream after extrusion.
• Anodising: the layer is hard, decorative and insulating. It changes the surface dimension slightly, and it must be masked at any earth contact.
• Fastening: a self-tapping screw into a single wall strips the thread. Use a boss, a thicker wall or a thread-forming screw designed for aluminium.

What to send for a profile quote

Send the profile drawing at 1:1 with the alloy and temper, the circumscribing circle, the wall thickness range, the number of voids, the critical faces, the cut length and the quantity in metres. Add the finish, the colour if anodised, and the secondary operations. The 1:1 view matters because a profile is often quoted from the shape rather than a dimension table.

Say which faces are cosmetic and which are functional. A die line on a hidden face is normal, while the same line on a front panel is a reject. Say whether the part will be anodised, powder coated or left mill finish, because the finish changes the allowable surface.

A profile drawing with no alloy and no temper is a sketch. 6063-T5 and 6063-T6 have different strength and straightness behaviour, and the extruder will choose one if the drawing does not.