Aluminium Square Tube Extrusion Dies: Standard Sizes and Custom Tooling

How a square tube extrusion die works

A square tube is a hollow profile, so it cannot be made on the flat, single-plate die used for angles or flat bar. It needs a two-part hollow die: a mandrel section that forms the inside of the tube and a die cap that forms the outside. The gap between them is the tube wall.

In a porthole die, the mandrel is held by bridges, and the hot billet (roughly 450 – 500 °C for 6xxx alloys) is forced through several ports around it. The separate metal streams meet again in a welding chamber behind the bridges and, under high pressure and temperature, bond into a continuous wall before leaving through the bearing. The result is a tube with longitudinal seam welds, usually one behind each bridge, that are solid-state bonded and accepted for structural service under ASTM B221 and EN 755.

The bearing, the short land that the metal passes over as it exits, controls flow speed. The die maker lengthens or shortens bearings around the profile so that thick and thin areas exit at the same speed; that balance decides whether the tube comes out straight, square and with an even wall. Getting it right on a new die often takes one or more trial pressings and corrections, which is why a proven existing die is valuable.

Existing die or custom die?

A practical rule: if a standard size within a millimetre or two of your target meets the structural and assembly requirements, use it. Custom tooling is justified when the profile must carry features a plain square tube cannot, such as screw ports, snap-fit grooves, internal webs or a non-standard wall, or when annual volume is high enough that the die cost becomes negligible per metre.

Widely tooled sharp-corner square tube sizes

Sharp-corner square tube has a very small outside corner radius and is the usual choice for architectural framing, furniture and display systems. The sizes below have tooling widely available in metric and inch series. Weights are calculated from the nominal sharp-corner section at 2.70 g/cm³ and are slightly above actual weights for tubes with small radii.

Larger square tube sizes and radius-corner sections

Above about 100 mm, square tube is usually extruded with radiused corners, which improve metal flow, reduce stress concentration and extend die life. These sizes need larger presses and are typical of structural, transport and solar work. Weights are again calculated for sharp corners as an upper estimate.

Square sections above 200 mm, and heavy walls above about 12 mm, are generally made as custom profiles. The practical limit is the circumscribing circle diameter (CCD) the press container can accept: a 200 × 200 mm tube already needs a CCD of about 283 mm, so large squares are produced only on large-diameter presses.

Which alloys run on square tube dies

The same die can usually run several 6xxx alloys, but harder alloys extrude more slowly and may need a heavier wall to fill the die. Confirm the alloy at quotation so the right press and die are allocated.

Porthole (seam-welded) tube is the normal product for 6xxx alloys. 2xxx, 5xxx and 7xxx alloys do not weld reliably in the die chamber and are therefore supplied as seamless tube, extruded over a mandrel or drawn, under ASTM B221/B241 or EN 755-7.

Tolerances and standards for square tube

Width tolerance depends on size and is tighter at the corners than at mid-face, because the faces of a thin hollow can bow slightly. Wall thickness is controlled both as an average and at any point. The exact limits come from the ordering standard; the drawing can tighten them, which may require a dedicated die.

• Mid-face tolerances generally do not apply to O temper or very thin walls, where faces may be slightly concave or convex
• Corner radius tolerance is specified separately from width
• Straightness and twist are measured per metre and over the full length
• Each lot is certified to EN 10204 3.1 with chemistry, mechanical results and dimensional check

Designing a custom square tube die

1. Send a drawingOutside size, wall, corner radii (inside and outside), any internal or external features, alloy, temper and finish.
2. Design reviewThe profile is checked for wall uniformity, tongue ratios on features and press suitability. Even walls and generous radii extrude faster and more accurately.
3. Die manufactureThe die is machined from hot-work tool steel (typically H13), heat-treated and nitrided, then bearings are finished.
4. Trial pressingFirst samples are measured against the drawing; bearings are corrected if needed and samples are sent for approval.
5. ProductionAfter approval the die is used for production orders, and the same tooling is reused for repeat orders.

Related tooling guides and square tube products

How to order square tube on an existing die

1. Pick the sizeChoose from the tables above, or send your target size and we suggest the nearest tooled section.
2. Confirm the specificationAlloy, temper, standard, length, quantity, finish and any cutting or machining.
3. QuoteWe confirm the die, tolerances, certification and delivery terms (EXW, FOB, CIF or DAP).
4. Production and inspectionExtrusion, ageing, finishing and inspection, with an EN 10204 3.1 certificate.
5. Packing and shippingInterleaved bundles in wooden crates or on pallets, shipped by sea or air worldwide.

Square tube extrusion dies: frequently asked questions