Chopped carbon fibre in PETG is one of the few filled filaments where the marketing and the reality point in genuinely different directions. It is sold on strength and bought, by people who have used it, for how much better behaved it is.
The verdict: PETG-CF for large flat parts, outdoor structures and anything where warping or surface finish has been the problem. Plain PETG for anything transparent, anything that takes impacts, and anything printed on a brass nozzle you want to keep.
The number that actually changes
Shrinkage. PETG's stored range is 0.2 to 0.6%; PETG-CF's is 0.1 to 0.4%. The fibres physically restrain the polymer as it contracts, and the effect is largest along the extrusion direction.
Take a 300 mm panel — the kind of part where this decision gets made. At the top of plain PETG's range it finishes at 298.2 mm, needing a 1.8109 mm correction. At the top of PETG-CF's, it finishes at 298.8 mm and needs 1.2048 mm. Six-tenths of a millimetre across a 300 mm panel is inside most people's tolerance for a bracket and outside it for anything that bolts to an extrusion, so the question the shrinkage calculator is really answering is what your panel has to mate with.
Six tenths of a millimetre on a 300 mm part is not, by itself, dramatic. What matters is what the reduced contraction does during the print: less force pulling the corners up, so a large flat PETG-CF part stays on the plate where the plain one lifts. The stored note for the filled grade says it directly — it warps far less than plain PETG, and that is often the real reason people buy it.
The three quality-of-life improvements
It barely strings. Stringing is plain PETG's most notorious habit. The fibres raise the melt's viscosity and shorten the ooze, and a PETG-CF print off an untuned profile is usually cleaner than a plain PETG print off a tuned one.
It releases from the plate properly. PETG's worst behaviour is bonding to smooth PEI hard enough to take chunks out of it. The filled grade's matte surface releases far more readily — the stored note calls it better behaved than plain PETG for exactly this reason. That alone is worth something to anyone who has damaged a build sheet.
The matte finish hides everything. Layer lines, seams and small first-layer flaws all but disappear into the texture. For functional parts that people will look at, it is a genuinely better-looking material.
What you give up
Transparency, completely. If the part needs to be see-through, this comparison is already over.
Impact resistance. The stored weakness list names impact loading, and this is the trade at the heart of every fibre-filled material: fibres stiffen the matrix and give cracks somewhere to start. A PETG-CF clip is stiffer and snaps sooner than a plain PETG one. If the part flexes, springs or takes knocks, buy the unfilled grade.
Brass nozzles. Chopped carbon fibre destroys them, and not slowly. A hardened steel nozzle is a requirement rather than a suggestion, and 0.4 mm is the practical minimum diameter — choosing a nozzle size covers what that means for your machine.
Some heat, some money. The filled grade runs at 240–265 °C against 230–250, and its bed range extends to 90 °C. Price is $30–60 per kilogram against $18–30.
Where the strength claim stands
Honestly: mostly sideways. Stored tensile figures are 45–65 MPa for PETG-CF against 45–55 for plain PETG — a real but modest improvement at the top end, and nothing like the transformation the packaging implies.
Stiffness is where the fibre genuinely earns its name. A PETG-CF part deflects less under the same load, which for a bracket, a mount or a jig is often what "stronger" actually means to the person asking. But if the requirement is not breaking, the unfilled grade is the tougher material and the filled one is a downgrade.
Its deflection range also nudges up, 70–85 °C against 68–80, which is not enough to change what the part can be used for.
Both still have to be dry
Neither material forgives moisture, and the filled grade is worse: the stored schedule is 65 °C for 6 to 8 hours against 4 to 6 for plain PETG, because both the polyester and the fibre hold water. A damp PETG-CF spool prints with the same popping and voids as damp PETG, with the added irritation that the matte surface hides the evidence until the part is weak.
The overhang problem gets slightly worse
One consequence of the higher melt viscosity is worth flagging because it surprises people who bought the filled grade expecting an all-round improvement.
Plain PETG already droops on unsupported spans more than PLA does. PETG-CF's stiffer melt does not fix that, and the fibres interfere with the way a bridging strand stretches taut between anchors, so long bridges come out at least as poorly. Plan on support, or on a slower bridging profile, for either material — when supports are worth it applies unchanged.
Fine detail also softens. The fibre sets a floor on how small a feature can be resolved, so text, thin ribs and sharp internal corners all come out slightly blunter than in the unfilled grade. On a bracket that is irrelevant; on a part with engraved markings it is not.
Choosing by part
- Large flat panel, plate or enclosure face — PETG-CF, for the warping alone.
- Outdoor structural part — PETG-CF; both are rated good outdoors and the filled one holds its shape better.
- Anything transparent — plain PETG, or step across to PCTG if it also gets flexed.
- Clips, hinges, springs, anything impacted — plain PETG.
- Printer parts and tool mounts — PETG-CF, which is what its stored use list is full of.
- You only own a brass nozzle — plain PETG, until you do not.