Extrusion tolerances explained — what EN 12020 actually controls

A standard is a default, not a specification

The first thing to understand is that quoting a standard is not the same as quoting a number. EN 12020 is the European reference for precision extruded profiles in the 6000-series alloys, and it sets out permitted deviations by profile size and feature. It is a sensible default. It is not a statement that your particular section will be held to any one figure.

That matters because tolerance bands widen as sections get larger and more complex. A small solid bar and a large hollow architectural mullion are not held to the same deviation, even though both are 'to EN 12020'. If your drawing has one dimension that genuinely must be tighter than the standard default, saying 'EN 12020' does not get you there. You have to call that dimension out.

So the practical reading of 'to EN 12020' is: *unless the drawing says otherwise, standard permitted deviations apply.* Everything tighter than that is a specification you write, not a standard you cite.

Cross-section tolerances are the ones everyone checks

These are the intuitive ones — wall thickness, overall width and height, the position of a groove or screw port. They are what a buyer measures with a caliper when the sample lands on the desk, and they are usually fine, because they are what the die and the press are set up to control directly.

Wall thickness deserves one note. Thin walls are harder to hold than thick ones, and a thin wall on a wide flat face is harder again, because the metal has further to travel and more opportunity to vary. We extrude from 0.7 mm, but a 0.7 mm wall on a small section and a 0.7 mm wall spanning a wide face are not the same manufacturing problem. If your design leans on a thin wall over a wide span, flag it at enquiry rather than discovering the constraint at first article.

Form tolerances are the ones that cost money

Straightness, twist, flatness, angularity — these describe the shape of the bar along its length rather than the shape of its cross-section. They are far less often specified by buyers and far more often the reason a delivery causes trouble.

The reason is simple: a cross-section error is visible in one measurement, but a form error only shows up when you try to assemble. A mullion that is dimensionally perfect but bowed over six metres will fight you on site. A T-slot beam with twist will not sit square in a frame no matter how accurate the slot is. A heat sink base that is not flat will not make thermal contact, whatever the fin geometry says.

Form tolerances also scale badly with length. The same bar is easier to hold straight at 3 m than at 12 m — which is why, if your application is length-sensitive, the length you order at is part of the tolerance conversation, not separate from it.

If you take one thing from this article: **on long profiles, ask about straightness and twist, not just section dimensions.**

What happens after extrusion also moves the numbers

A profile is not finished when it leaves the press. It is stretched, cut, aged, sometimes machined, and usually finished — and several of those steps can move a dimension.

Anodizing builds a layer that grows both outward and into the metal, so an anodized profile is not dimensionally identical to the same profile mill finish. On most sections this is irrelevant. On a close-fitting slot, a bearing surface, or a mating pair of profiles, it is not — and it is worth stating on the drawing which dimension is critical *after* finishing.

Powder coating adds film thickness on top of the surface, which matters in the same places for the same reason. Machining after finishing, conversely, exposes bare aluminium at every cut edge — a trade-off worth thinking about deliberately rather than by default.

None of this is exotic. It is just the difference between specifying a profile and specifying a finished part.

What to write in the enquiry

State the standard, then state the exceptions. 'EN 12020 general, except dimension A at ±0.05 mm' is a clear, quotable instruction. 'Tight tolerances please' is not.

Mark which dimensions are functional. On most sections only two or three genuinely matter — the ones that mate, seal, or carry load. Marking those tells the supplier where to concentrate, and it usually costs less than tightening everything.

Say whether the critical dimension is before or after finishing. This one line prevents a whole category of dispute.

Give the cut length you actually need, and say if straightness over that length is functional.

And ask for the inspection report. Dimensional checks are part of our standard QC and results are available on request — asking for them costs nothing, and it keeps any supplier honest.

When to push back on a supplier

A supplier who cannot tell you which tolerances they hold beyond citing the standard, or who treats a request for form tolerances as unusual, is telling you something about how they run their inspection.

Equally, a supplier who agrees to every tolerance you name without asking a single question about the section is also telling you something. Tolerance is a manufacturing conversation. If nobody pushes back on anything, nobody has looked at the drawing.

The useful answer sits in between: this dimension we can hold, this one is harder on a section this size and here is why, this one we would like to discuss because there may be a cheaper way to get the fit you need.

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