Aluminium Section Guide: Shapes, Alloys, Tolerances and Supplier Checks

A window fabricator once rejected 1.8 tonnes of aluminium section because the outer frame wall measured 1.08 mm against a drawing callout of 1.2 mm. The alloy was correct, the anodising was even, the price was competitive, and the batch still failed incoming inspection. An aluminium section is judged against the drawing, not against the description.

An aluminium section, also called an aluminium profile or extrusion, is a length of aluminium pushed hot through a steel die until it takes a fixed cross-section: solid, hollow or semi-hollow. Sizes range from a 10 mm window bead to a 400 mm battery pack frame. When two suppliers quote the same profile at prices that differ by 20 percent, they are almost never quoting the same thing. Alloy, temper, wall thickness tolerance, straightness, surface finish and packaging all move the number.

How an Aluminium Section Is Actually Produced

Most quality problems in extruded sections are created in the first 48 hours of production, long before anyone discusses anodising or packing.

1. Billet casting and homogenising. The billet is cast to a controlled chemistry and heat-treated so alloying elements distribute evenly. Skipping homogenising shows up later as streaks and colour variation after anodising.
2. Die preheating. The H13 steel die is brought to roughly 450 to 480 °C. A cold die tears the surface of the section and shortens die life.
3. Extrusion. The billet is heated to about 450 to 500 °C and pushed through the die at a speed that depends on wall thickness, alloy and how many cavities the die holds.
4. Quenching. Air or water cooling on the run-out table freezes the metallurgical structure. The cooling rate is one of the differences between a T5 and a T6 delivery.
5. Stretching and cutting. The section is stretched by roughly 0.5 to 1.5 percent to remove twist and bow, then cut to length within the agreed tolerance.
6. Ageing and surface treatment. Artificial ageing at about 175 to 200 °C for several hours raises yield strength. Anodising, powder coating or brushing follows when specified.

Die cost is where small orders hurt. A simple hollow die for a 40 mm window frame may cost a few hundred US dollars, while a large semi-hollow die for a battery pack frame with a tight tolerance band can run into several thousand, usually amortised across the first order. This is why a fully custom section is rarely economical below one to two tonnes, and why buyers with repeat programmes should confirm who owns the die.

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Alloy and Temper Decide Almost Everything

Alloy choice sets the ceiling on strength, corrosion resistance and weldability. Temper sets how much of that strength you actually receive.

Temper matters as much as alloy. T5 means cooled and artificially aged without additional cold work. T6 adds a solution treatment step and delivers higher strength with slightly lower ductility. A drawing that says "6063" without a temper is incomplete, and a supplier who accepts it will ship whatever is easiest to produce that week.

For a side-by-side view of how strength, shape and surface finish trade off against each other, this comparison of aluminium extrusion profiles is a practical reference before you finalise a drawing.

Comparing Common Section Types and Their Best Fit

The sections below account for most industrial purchasing volume. The risk column is where disputes usually start.

Note: tolerance bands are commonly stated against EN 755-3 or GB/T 6892. The entries in the risk column are the points most often raised in incoming inspection reports.

Hidden Cost Drivers Most Buyers Miss

Die amortisation dominates a first order of two or three tonnes, then fades once the tooling has been paid for. After that, tolerance becomes the largest lever. Requesting plus or minus 0.05 mm on a decorative trim where plus or minus 0.15 mm would work can add roughly a quarter to the unit cost and buy nothing the customer will ever see. The reverse is also true: saving 0.1 mm of wall thickness to shave material cost is the fastest way to lose straightness on a long section.

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Where Aluminium Sections Are Used

These shares reflect typical extrusion consumption rather than any single factory's order book. The practical point is that construction still absorbs the largest volume, which is why architectural tolerances and coating quality drive so much of the conversation, while the fastest growth sits in transport and energy. Photovoltaic racking and battery enclosures demand weldable alloys and load calculations that a window profile never has to face, so the same extrusion line serves two very different specifications.

Requirements for new energy aluminium extrusions are usually written around load cases, weld joints and corrosion class, which is a useful reminder that the section shape is only half of the specification.

A Sourcing Sequence That Reduces Rework

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Maintenance, Compliance and Long-Term Performance

• Clean anodised sections with a pH-neutral detergent and a soft cloth. Alkaline and acidic cleaners attack the oxide layer and leave a dull, patchy finish that cannot be polished back.
• Powder-coated profiles tolerate slightly stronger cleaning agents, but abrasive pads and solvent wipes are still the fastest route to a warranty claim on a visible facade.
• Hollow sections used outdoors need drain or weep holes. Trapped water inside a sealed cavity causes internal corrosion that inspection never catches until the section is cut open.
• Isolate aluminium from steel, stainless and copper with nylon washers or a coated separator. Direct contact in a wet environment turns the aluminium into the sacrificial anode.
• Allow for thermal movement at roughly 23 µm per metre per degree Celsius. A six-metre rail can grow by about 4 mm over a 30 °C swing, which is enough to bow a fixed assembly.
• Keep documentation per heat number: alloy and temper certificates, dimensional reports, coating thickness readings, and salt-spray results for outdoor work. Architectural and transport projects commonly reference EN 755 tolerances, REACH and RoHS restrictions, and Qualanod or Qualicoat for anodised finishes.

An aluminium section is a simple product sold into an unforgiving specification. The buyers who avoid rework are not the ones who negotiate the hardest on price per tonne; they are the ones who settle the alloy, temper, wall thickness and tolerance band first and then ask for a quote against that fixed definition. Every rejected batch traces back to one of those four numbers that was left vague on a drawing.