The datasheet is a starting point
Reinforced nylon datasheets quote tensile figures from injection-moulded test coupons. A printed part is not an injection-moulded coupon. It is anisotropic, it has internal voids, and its properties depend on how dry the filament was when it went through the nozzle.
Moisture is the whole game
Nylon absorbs water from the air fast enough to matter within hours. Wet filament prints with visible steam, poor layer bonding and a surface that looks bubbled. We dry every spool immediately before printing and keep it dry during the print. This single change did more for part strength than any settings work.
Chopped fibre stiffens, it does not toughen
The fibres are typically 100 to 150 microns long. They dramatically increase stiffness and dimensional stability, and they reduce warping. They do not make the part tougher. A carbon-filled part is often more brittle than the unfilled base polymer. If your failure mode is impact rather than deflection, unfilled nylon may serve better.
It destroys brass nozzles
Chopped carbon is abrasive. A brass nozzle visibly wears within a couple of kilograms and the extrusion width drifts as it does. Hardened steel or ruby is not optional.
What two hundred hours showed
On the planetary gearbox we ran a 27:1 stage continuously and measured backlash weekly. Growth stayed under 0.3 degrees. The wear that did appear was on the tooth flanks of the sun gear, exactly where contact pressure is highest, and annealing after printing measurably slowed it.
When we recommend it
Structural brackets, drone frames, robot arm segments, end-use tooling and anything dimensionally fussy that lives near heat. For a decorative part or a first concept model, it is an expensive way to buy stiffness nobody needs.