Designing a profile for extrusion: practical DFM rules

Why does design for extrusion matter?

Most of the cost and quality of an extrusion is decided in the die, and the die is decided by the cross-section. A change at the drawing stage costs very little; a change after the die is cut costs a lot. The rules below are a compact design for manufacture checklist. The ratios are rules of thumb from general practice, and the final limits come from your extruder.

Why should wall thickness be uniform?

Aluminium flows through a die opening more slowly where the wall is thin and faster where it is thick. If a section mixes very different thicknesses, metal leaves the thick part sooner than the thin part, and the profile bows, twists or drifts out of tolerance. As a working guide:

• Keep the ratio between adjacent wall thicknesses low, in many cases within about 2:1.
• Where thickness must change, use a taper or a radius rather than an abrupt step.
• Do not hang a very thin leg off a heavy mass.
• Thinning everywhere by the same amount is better than thinning one region.

The thinnest wall that can be extruded depends on alloy, section and circle size, so ask your extruder for the limit that applies.

What should you know about hollows and tongues?

Sections fall into three classes: solid, semi-hollow and hollow. A semi-hollow section has a wide void almost enclosed by a narrow gap. In the die the steel tongue that forms that void takes heavy loading, and as the ratio of void area to gap width squared rises, the tongue becomes fragile. So:

• Avoid narrow, deep slots. Open the gap wherever the function allows.
• Closing a semi-hollow into a true hollow, or splitting it into two profiles, can reduce die cost.
• On hollow profiles, keeping inner and outer surfaces roughly parallel evens out the flow.
• The position of the bridges that hold the core also fixes the position of the weld lines. For structural or visible parts, discuss it early.

How porthole dies work is described in the step-by-step extrusion summary.

Why do you need radii?

Sharp internal corners concentrate stress in the die and can crack it. They also disturb the metal flow, which can leave a line on the surface or a corner that does not fill. A common rule is a radius of at least about 0.5 mm on inside and outside corners, with larger radii generally better. On profiles that will be coated, a radius also stops the coating from thinning out at the edge, as the guide to powder coating aluminium profiles explains.

How does the circumscribing circle affect the design?

The circumscribing circle is the smallest circle that holds the whole section. The allowable circle depends on the press opening, tooling, alloy and section geometry, so confirm the limit with the extruder. As a general rule the section should sit near the centre of the die rather than the edge. The larger the circle:

• the fewer presses and containers can run it,
• the larger and more expensive the die becomes,
• the harder it is to balance the metal flow.

The aim is to reduce the circle without giving up function. A long, thin section drives the circle up, and splitting it into two profiles is sometimes the more economical route. Balancing the section so that its centre of mass sits near the centre of the circle also helps flow.

How does cross-section area change cost?

Extruded profile is commonly priced by weight, and weight per metre follows directly from cross-section area. Unnecessary wall thickness increases material consumption. Press load and achievable speed depend on the extrusion ratio, geometry, alloy, temperature and die design. The extrusion ratio is the container cross-sectional area divided by the total metal cross-sectional area of all simultaneous die exits, and when the section becomes very small the ratio rises and the process becomes harder. Most profiles have a sweet spot: the thinnest, most balanced wall that still does the job.

How do you add functions to the section?

One strength of extrusion is that functions can be built in: screw ports, snap-fits, gasket grooves, guide rails, cable channels, cooling fins. When adding them:

1. Keep balance. Every new feature must obey the wall thickness rule.
2. Know the tolerance. For precision mating surfaces, check which standard and limits apply in the guide to extrusion tolerances.
3. Think about marks. Move anything that could leave a line or mark to a surface that is not visible.
4. Reduce asymmetry. Symmetrical sections flow more evenly and come out straighter.

Which mistakes are common?

• Joining one heavy region to thin legs.
• Leaving every corner sharp and rounding only the visible ones.
• Keeping a deep, narrow channel in a single-piece section when it is not needed.
• Writing unnecessarily tight general tolerances. It is more economical to specify numerical exceptions individually on the dimensions whose function requires them.
• Forgetting that the inside of a channel is hard to coat.

How do alloy and temper change the design?

Higher-strength alloys call for thicker walls, larger radii and simpler sections. Thin, complex shapes press more easily in 6060 and 6063. The comparison of 6060, 6063, 6005A and 6082 sets out the general differences.

What should you check before sending the drawing?

Before you send a drawing, run the checklist: are the walls balanced, do corners carry radii, are the gaps open enough, has the circle been kept small, and have the dimensions that function requires and the visible surfaces been identified? A drawing that answers yes to these questions usually needs fewer changes at first review.

Send the drawing through the quote request page, and put any design questions in the same message.