An ABS helmet shell is a stiffness component. Its job is to spread a point impact over a large area of foam, then deform without puncturing, and the gauge and the rim detail decide whether it does that.
A skate brand in California formed 2 mm shells for 20,000 helmets. Field returns clustered at the rear rim, where the trim line met the adjustable fit system and the shell had no local thickening.
ABS helmet shell gauge and geometry
- Skate and bike: 2 to 2.5 mm
- Motorcycle and equestrian: 3 mm
- Vents: 8 to 12 mm slots, radiused ends
- Rim: rolled 2 mm or locally thickened
Vent slots are stress raisers. A slot with a square end concentrates load at the corner and cracks propagate from there; an 8 mm end radius distributes it. This is a tool detail, and it costs nothing to design in.
Bonding, trimming and the standard
The shell-to-liner bond is what makes the system work. A shell sitting 3 mm off the EPS deflects on impact before the foam engages, which reads as a soft helmet and fails a drop test that the same shell passes when bonded tight.
Trim tolerance decides fit and comfort. A shell trimmed 2 mm long at the ear cut rubs the wearer; 2 mm short leaves a gap that looks like a defect. CNC trimming against a fixture holds 0.5 mm and is the only reliable route at volume.
The standard is the specification. EN 1078, CPSC or ECE 22.06 each define impact energies and penetration tests, and the shell thickness is derived from those tests, not from a catalogue. Buyers should state the standard before the tool is cut.
For the material side, see ABS sheet weight for per-shell costing, and ABS cut to size for the trimming route.
Send your shell drawing, standard and volume to oversales@rpplexiglass.com for a formed shell quote.
Frequently Asked Questions
What thickness ABS for a helmet shell?
2 to 2.5 mm for skate and bike shells, 3 mm for motorcycle and equestrian use. Stiffness and penetration resistance set the gauge, and the standard names the test.
Why do helmet shells crack at the rim?
A sharp trimmed edge with no rolled rim. The rim is where a shell takes a glancing impact and where the trim line concentrates stress; a rolled or thickened rim removes the weak point.
How is the shell bonded to the liner?
Usually a contact adhesive or a pressure-sensitive tape between the shell and the EPS liner, with the shell formed to match the liner geometry. A gap of more than 2 mm between shell and liner lets the shell deflect before the foam works.