R20 or R40 — does corner radius matter on sign blanks?
What the radius actually protects, how to choose between the two common sizes, the exact material it costs, and why radii and mounting holes leave the press in the same pass.
Why do blank corners fail first?
Corner radius is not about stiffness — it is about the two places a blank actually fails in service: the corner itself, which is where stacking knocks, film lifting and tears start, and the punch line, which is why we form the radius with the mounting holes in one pass on a three-head rotary press. R20 is our default for blanks up to about 1200 mm; R40 for larger panels and film-wrapped edges. The material you give up is trivial: an R40 corner removes 30 g of aluminium from a 2.0 mm 3003 panel — about 0.5% of its weight.
A square corner concentrates everything: it is the point that catches on the stack below when panels are slid on and off pallets, the edge the forklift clamp meets first, and the spot where a reflective film edge starts to lift after seasons of temperature cycling. A nick at a 90° corner also propagates — the sheet flexes in handling and wind, and a small cut becomes a tear along the grain. Rounding the corner removes the catch point and drops the stress concentration, which is why traffic blanks have carried radii as trade practice for decades even though no single clause mandates one.
R20 or R40 — which should you specify?
Both are formed on the same three-head rotary punch in a single pass, so the choice is a drawing decision, not a cost decision. The working rules we apply by default:
| Panel | Default radius | Why |
|---|---|---|
| Up to 1200 mm side | R20 | Standard traffic-blank practice; adequate for hand-stacked pallets. |
| Over 1200 mm side | R40 | Larger panels see more edge handling; the bigger radius resists nicks. |
| Film-wrapped edges | R40 | Gentler film path at the corner; edge sealing still recommended. |
| Drawing specifies | Per drawing | Any radius quoted against the drawing; the punch program is set from it. |
These are fabrication defaults, not normative requirements — your road authority or end-client drawing always takes precedence.
How much material does a radius remove?
Easy to check: one rounded corner removes an area of (4 − π) r², and there are four corners. Multiply by 5.46 kg/m² — the weight of 1 mm of 3003-H14 per square metre — and by the thickness, and you have the exact weight given up. For a 2.0 mm 3003 blank:
| Radius | Area per corner | Area per panel (4 corners) | Weight removed |
|---|---|---|---|
| R20 | 3.43 cm² | 13.7 cm² | 7.5 g |
| R40 | 13.7 cm² | 54.9 cm² | 30.0 g |
Check: (4 − π) × 0.04² = 0.00137 m² per corner at R40; × 4 × 5.46 kg/m² × 2.0 mm = 30 g. The panel weighs 7.86 kg bare, so R40 gives up about 0.4% of the blank.
Can radii and mounting holes be punched together?
Yes — and this is the part that saves you a fabrication step. The corner radius and the mounting holes are one CNC program on the same press pass: the rotary heads form the radius while the hole stations place the bolt centres. There is no second handling, no re-squaring, and no risk of the holes drifting relative to the corners, which matters when the blank must drop onto a channel or a pre-drilled post pattern downstream.
For buyers this means a radius should never be “value-engineered out” to save cost — it costs nothing once the program is written. If you are unsure which radius your channel system expects, send the channel section or the fixing drawing and we will match it.
State the radius if it is not R20 or R40, and put hole centres on the drawing the same way your fixing system expects them — from the corner edge or from hole centre to hole centre. Both are read directly into the same punch program, so the drawing convention is the only thing that can go wrong.
Finished-sign requirements for fixed vertical road traffic signs — into which the blank, channel and sheeting assembly must hold up — are specified in EN 12899-1, “Fixed, vertical road traffic signs — Part 1: Fixed signs”, published by CEN; retroreflective sheeting classes are specified in ASTM D4956 from ASTM International. Neither standard prescribes a corner radius; the R20/R40 values discussed are fabrication practice.
Sources: CEN, Brussels · ASTM International, West Conshohocken, PA. Cited 1 November 2026.
Questions on this note
Is there a standard corner radius for traffic sign blanks?
No single standard mandates one. EN 12899 covers the performance of the finished sign assembly, and ASTM D4956 covers the retroreflective sheeting, but corner radius is trade practice — R20 and R40 being the common choices. If your authority drawing specifies a radius, we punch to it.
Does the corner radius affect how the reflective film is applied?
Only gently. Film is applied after fabrication and lays over the radius the same as over a square corner, but the radius keeps the film edge away from the zone that takes handling damage — which is where lifted film usually starts. Edge sealing is still recommended on cut-to-shape letters and edges.
Can you punch a radius other than R20 or R40?
Yes. The radius is a program parameter on the rotary head, so any radius on your drawing is quoted against the drawing and set up the same way. There is no tooling surcharge for a non-standard radius on production quantities.
Does a rounded corner weaken the mounting?
No. The blank mounts through the bolt holes, not through the corners, and the holes are punched in the same pass. If anything the radius helps: it removes the stress-raising corner nearest the sheet edge, so handling nicks have fewer places to start a tear.
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