Nylon. For a gear that will run rather than sit in a display, PA6-CF is the first choice on a capable machine and unfilled PA12 or PA6 the fallback.
Weight the selector for a gear brief — strength heavily, heat moderately, ease lightly, on an enclosed machine with a 300 °C hotend and a hardened nozzle — and it returns PPA, PA6-CF, PA6-GF and polycarbonate. The realistic pick from that list is PA6-CF.
Gears fail differently from brackets
Tensile strength is close to irrelevant here. The failure modes that matter are:
- Tooth wear from millions of sliding contacts, which is an abrasion property
- Heat build-up from friction, which softens the tooth flank and accelerates the wear
- Shock loading at engagement, which is impact resistance rather than tensile strength
- Creep under sustained torque, which slowly changes the tooth profile
Nylon is chosen because it addresses the first two directly: it is abrasion-resistant and self-lubricating, which is precisely the property list a gear tooth wants.
Why PA6-CF over plain PA6
Dimensional stability. Plain PA6 contracts 0.8 to 2% and absorbs enough water from the air to swell a finished part measurably. On a gear, changing dimensions means changing backlash and centre distance, which is the difference between a mechanism and a noise.
PA6-CF contracts only 0.2 to 0.6% and holds its size far better. It also raises the heat deflection band to 120 to 190 °C, which matters once friction starts warming the teeth.
The costs are a hardened nozzle, an enclosed machine with a 270 to 300 °C hotend, 8 to 12 hours of drying at 80 °C, and $60 to $120 a kilogram.
What to use if that is out of reach
PETG makes a usable gear for light, intermittent duty. It is tougher than PLA and holds 68 to 80 °C, though it will wear and creep under continuous load.
PLA is fine for a prototype and for demonstrating that the geometry meshes. It is not a running gear: it sags near 55 °C and its teeth chip rather than deform.
Weight your own criteria and machine limits in the material selector.