The claims are true and incomplete

Marketing lists about extrusion tend to make the same five claims: fewer process steps, reusable material, labour savings, precision, and light weight with corrosion resistance. None of them is false. The problem is that each one is stated without the condition that makes it apply, and the condition is where the engineering decision lives.

The most consequential example is weight, because it is the claim buyers repeat most often and the one that most often fails to survive contact with a real design. So it is worth working through properly.

Advantage one: mass, and the correction that changes the answer

Aluminium has a density of about 2.70 grams per cubic centimetre against roughly 7.85 for steel. That is the source of the familiar claim: for the same section, aluminium is about two thirds lighter. It is correct, and it is the wrong comparison for most machine frames.

Aluminium’s elastic modulus is around 70 gigapascals against roughly 210 for steel. Stiffness scales with the modulus, so an aluminium member of the same shape and size deflects about three times as much as a steel one under the same load. Buyers who compare weight for the same section are comparing a stiff part with a flexible one.

The honest comparison is weight for the same stiffness. Because the density ratio and the modulus ratio are almost the same number, the specific stiffness of the two materials, which is the modulus divided by the density, comes out within a few per cent of each other. Scale a steel section geometrically until its aluminium equivalent has the same stiffness, and the aluminium version lands at roughly eighty per cent of the steel mass rather than the thirty-five per cent the density ratio implies.

That still leaves a genuine advantage, and there is a second one that matters more. Extrusion can put material where it is needed, in deep hollow sections and ribs, at a cost that no steel fabrication process can match. So a well-designed aluminium extrusion does beat a steel section of the same stiffness, and most of the gain comes from the shape rather than from the material.

Where mass genuinely governs, which means anything that is lifted, moved, shipped or accelerated, the advantage is large and immediate. Where stiffness governs a long span, the advantage largely disappears.

Advantage two: fewer process steps and less labour, with two conditions

The process claim is sound. An aluminium frame is designed, cut and drilled, and assembled. A welded steel frame is cut, welded, ground, and then usually painted or galvanised. The extrusion route removes welding, grinding and surface finishing, and it removes the distortion that welding introduces, which is a real and often underrated benefit because a welded frame needs straightening after fabrication.

The first condition is that the design must not need a machined face or a precise hole pattern. Extrusion gives a constant cross-section; it does not give a flat, square end, a flat face, or a hole in a defined position. Those are machining operations, they cost money, and a design that needs many of them loses much of the process advantage.

The second condition is volume. Welded steel has no connector hardware at all: the joint is the material. A bolted aluminium frame has brackets, nuts and fasteners at every corner, and the connector count rises as the section gets smaller. On a high-volume, never-changed product, that hardware bill can exceed the labour it saves. The bolt-together frame wins on labour and loses on parts.

One further point belongs here, and it is rarely mentioned: welding aluminium is not the same as welding steel. A welded aluminium joint loses strength in the heat-affected zone unless the assembly is re-heat-treated, which is impractical for a large frame. That is a strong argument for bolted construction on aluminium, and it is a material fact rather than a preference.

Advantage three: precision, and precision of what

The precision claim needs to be separated into two very different things, because suppliers often quote the better one.

An extrusion holds a cross-sectional profile to a tolerance along its whole length. That is genuinely valuable: every cut length matches, and a slot runs straight for six metres. It is also what makes the modular system work.

An extrusion does not give a machined flat face, a square cut end, or an accurately positioned drilled hole. Those come from a machining operation with its own tolerance, and they are usually what actually determines whether a frame assembles squarely. On a large frame the accumulated error from cut length tolerance is what makes a rectangle into a parallelogram, and no amount of profile precision removes it.

So the useful question on a precision claim is: precise in which direction, and to what figure, and how is it verified. A profile tolerance and a cut-length tolerance are different numbers and only one of them appears in the sales literature.

Advantage four: corrosion resistance without a coating, conditionally

The natural oxide layer is a genuine advantage: it forms immediately, it reforms if damaged, and it is the reason aluminium does not rust. It is also thin, easily damaged, and attacked at both ends of the pH scale.

The practical version of this claim is that aluminium needs no coating, which is true in a dry, neutral indoor environment and false in most others. Alkaline cleaners and fresh concrete attack it. Chlorides pit it. A cut end has no coating at all, whatever the profile was finished with.

So the advantage is real but it converts into a coating specification as soon as the environment is anything other than benign. That is not a weakness of aluminium; it is a cost line that belongs in the comparison from the start rather than being discovered later.

Advantage five: reusability, the one that compounds

The fifth advantage is the least quantified and often the most valuable: an aluminium frame comes apart and goes back together as something else. A welded steel frame does not.

This is not a sustainability slogan, it is a cash flow argument. Where a production line is expected to be reconfigured, the option to rebuild rather than replace has a value that is easy to underestimate at the quotation stage and easy to recognise three years later. It also changes the nature of the risk: a change in the process becomes a rebuild rather than a capital project.

The condition attached to it is that the frame has to be designed for disassembly. Concealed connectors that cannot be reached, joints that were torqued to a figure that damages the slot, and locking elements that cannot be re-used all reduce the option to zero. A frame that is technically reconfigurable and practically not is the worst of both worlds.

Where aluminium extrusion loses outright

Being clear about this is more useful than another list of benefits, because these are the cases where the material should not be chosen at all.

The break-even, and what to send with an enquiry

The comparison that decides most projects is installed cost over the life of the assembly, not cost per kilogram. That means material, machining, connector hardware, assembly labour, finishing, and the expected number of reconfigurations. In a stable, high-volume product with a long service life and no changes, welded steel usually wins. In a low-volume, frequently changed environment, aluminium extrusion usually wins by a margin that surprises people, because the reconfiguration cost of steel is not a repair, it is a replacement.