Extrusion Die Design Basics for Buyers: Wall Thickness, Corners and Tolerances

When a buyer sends a drawing for a custom aluminium profile, the first thing an extrusion engineer looks at is not the price. It is whether that section can actually be pushed through a die and come out straight, sharp and inside tolerance. Plenty of drawings that look perfect in CAD turn out to be expensive, slow, or simply not extrudable. The rules below decide which category your profile falls into.

What a die actually is

An extrusion die is a disc of hot-work tool steel (usually H13) cut as the negative of your section. Billet heated to roughly 450-500 °C is forced through it at high pressure and emerges as your profile.

Two families cover most work:

• Solid dies for open sections - angles, channels, tubes with a seam, heat sink bases, trim profiles.
• Porthole (bridge) dies for hollow sections. Metal is split into streams around a mandrel, then re-joins under pressure. That re-join leaves a weld seam running along the profile. It is structurally sound, but it is a visible line and it can show after anodizing, so where that seam sits matters.

Each profile needs its own die, and the die is a one-off cost paid once. That is why a design change after tooling is cut is far more expensive than a design change before.

Rule 1: wall thickness sets the price and the risk

Wall thickness is the single biggest driver of die cost, press pressure, scrap rate and surface quality. Thin walls need higher pressure, wear the die faster and break more often. Uneven walls are worse than thin walls: when a thick leg sits next to a thin one, metal flows at different speeds and the section twists, bends, or ripples on the thin side.

Practical guidance for common alloys:

A 0.6 mm wall in 6061 across 120 mm usually means a refusal, a heavy premium, or a die that fails after a few hundred kilos.

Rule 2: corners and radii

Sharp internal corners are a die killer. The metal has to turn a hard angle at high temperature and pressure, which concentrates stress in the die and creates a weak point in the profile. The rule of thumb: internal radius at least half the wall thickness, and never below 0.5 mm. External corners can be sharper, but a small chamfer or radius also protects the anodized layer from chipping during handling and packing.

For visible or anodized profiles, keep radii consistent along the whole section. An inconsistent radius reads as a defect to a customer's eye, even when it is inside tolerance.

Rule 3: tongue ratio and features that cannot be extruded

A tongue is a thin feature standing up inside the die, such as a narrow rib or the gap between two close-together legs. The deeper and narrower the tongue, the more likely it is to break. As a working limit, keep tongue depth to no more than about 3:1 against its width in 6063, and closer to 2:1 in 6061 or 6005.

Two more limits worth knowing before you finalise a drawing:

• No undercuts. Metal must flow in one direction. A shape that locks the die in place has to be redesigned as two profiles, or extruded and then machined.
• Hollow sections need room for the mandrel. Very small internal cavities, or cavities with thin webs, are either impossible or need a much more expensive die.

Rule 4: don't tolerance everything tight

Standard dimensional tolerance tables (EN 755-9, GB/T 5237, ANSI H35.2) allow more variation than most designers expect, and the allowance grows as the section gets bigger. A common mistake is to put a tight tolerance on every dimension on the drawing. That does not make the part better - it makes the quote higher and the reject rate worse.

Do this instead:

• Mark the three to eight dimensions that actually matter and leave the rest on the standard class.
• Remember that machining after extrusion is usually cheaper than forcing a tighter die tolerance, especially on a dimension that also has to fit an assembly.
• Specify straightness, twist and bow separately. They are measured differently and a profile can be perfect in section and still unusable because it is twisted over 3 metres.

Rule 5: alloy and press size decide who can run it

6063 is the workhorse: it extrudes fast, takes anodizing well and holds good surface finish. 6061 and 6005 are stronger but need higher pressure, thicker walls and more die maintenance. Press size matters too. A large circumscribed circle or a wide, thin section needs a big press with a big billet. Our plant runs eight presses from 660 T to 4000 T with billet diameters of 90-254 mm and a maximum circumscribed circle of roughly 320 mm, in 6063, 6061 and 6005. If your section exceeds what a supplier's largest press can take, no amount of die design will fix it.

What die design costs you in time

A new die is not a same-week item. The sequence is: die cut, trial run, sample measurement, die correction, re-trial - and one to three correction rounds is normal on a new section. Only after the sample is approved does production scheduling start.

That is why we quote extrusion lead time separately from tooling. Once a die is proven, billet-only orders run in about 5-7 days, orders with anodizing or powder coating around 12 days, and orders that also need CNC machining 15-18 days. Tooling time sits on top of that.

Checklist to send with a new profile drawing

• 2D drawing with dimensions and tolerances, plus a 3D STEP file if you have one
• Alloy and temper (6063-T5 is the default for most architectural and heat sink work)
• Finish: anodizing thickness and colour, or powder coat colour and gloss
• The dimensions that are critical, and what they mate with
• Visible faces, and where a weld seam is acceptable
• Estimated annual volume and first-order quantity - it changes whether a die is worth building
• Any packing, cutting length or assembly requirement

Get a quote on your section

Send us your drawing and we will tell you straight away whether it extrudes cleanly, where the die risk sits, and what it will cost to tool and run. SYHARVEST extrudes custom profiles in 6063, 6061 and 6005 with in-house anodizing and powder coating, works to an ISO 9001:2015 quality system, and will flag design changes before a die is cut rather than after. Contact us with your drawing and we will come back with a quotation, and with any design changes worth making before the die is cut.