PETG-CF vs PA-CF: when chopped-fibre actually helps

Chopped-carbon-fibre filaments have gone from exotic to ordinary in a couple of years. Both PETG-CF and PA-CF show up in drone frames, jigs and brackets across our floor — but they behave differently enough that picking the wrong one wastes either money or a week of reprints. This is the comparison we wish we’d had on day one.

What “CF” actually changes

The carbon in these filaments is chopped fibre, not continuous strand. It won’t multiply a part’s raw tensile strength the way people expect. What it does, reliably, is raise stiffness — the part flexes less under the same load — and improve dimensional stability, so large flat parts warp and creep less.

It also changes how the filament prints and handles:

• Matte finish — layer lines hide better, parts look engineered rather than glossy.
• Abrasive — fibre wears brass nozzles fast; a hardened steel nozzle is mandatory.
• Less stringing — the fibre stiffens the melt, so bridges and overhangs clean up nicely.

PETG-CF: stiff, easy, matte

PETG-CF takes the forgiving PETG base and makes it stiffer and better-looking. It prints at roughly the same temperatures as plain PETG, doesn’t demand an enclosure, and holds up to about 75 °C in service. Tensile strength lands near 70 MPa — but the real win is rigidity and a clean matte surface.

It’s our default for visible structural parts: gimbal mounts, instrument brackets, anything where a customer will look at the part and expect it to feel solid.

PA-CF: the engineering option

PA-CF is carbon-filled nylon, and it’s a different class of material. It’s the strongest filament we stock in practice — tensile up to roughly 90 MPa — with excellent heat resistance (around 130 °C) and superb wear and fatigue behaviour. It’s what goes into field-replaceable, load-bearing parts.

The catch is moisture. Nylon is hygroscopic: leave PA-CF on the shelf and it soaks up water from the air, which then flashes to steam in the hot end and ruins both strength and surface. It needs a dry box before and during printing, and an enclosure to manage warp.

If the part is structural and lives in the field, PA-CF. If it’s structural and lives on a desk, PETG-CF. If it just needs to exist, neither.

The numbers, side by side

When plain PETG still wins

Plenty of parts don’t need carbon at all. If a bracket isn’t stiffness-critical, plain PETG is cheaper, prints on a standard brass nozzle and is tougher against impact than either CF grade — chopped fibre actually makes parts slightly more brittle. Don’t pay the CF premium for a part that a 3-wall PETG print would handle.

How we run them

Both materials go through a hardened 0.6 mm nozzle for speed and to survive the abrasion. PA-CF comes straight from a dryer to a heated, enclosed chamber and goes back into a sealed box between jobs. We log spool and supplier per batch, and every shipped part still gets a caliper-checked measurement report — fibre or not.

If you’re not sure which way to go, send the model and the use case. We’ll tell you honestly whether carbon earns its keep on your part — and quote both if it’s a close call.