Key takeaways
- Fibre raises flexural modulus, not impact strength. Stiff and tough are different properties.
- Fibre loading decides, not the letters on the label. A lightly filled grade trades tensile strength for dimensional stability, not for stiffness.
- The matrix sets the heat ceiling, not the fibre. PLA-CF still softens at 54 °C.
- Glass fibre is harder on nozzles than carbon — treat the nozzle as a wear part.
- The biggest under-advertised benefit: fibre cuts shrinkage and warping significantly.
The letters CF sell spools, and that is the problem. Plenty of people buy PLA-CF believing it is stronger than PLA, snap the first part by hand, and conclude they were sold a lie. Nobody was lied to — the material did exactly what it was designed to do, and that is not what the buyer assumed.
What the fibre does inside the material
Fibre in filament is chopped, roughly 50 to 150 microns long after compounding, at 10–20 % by weight. It sits loose in the polymer matrix rather than woven into a fabric like true composite. As the melt passes through the nozzle, shear aligns most of the fibre along the extrusion direction.
Three consequences follow, and all three are measurable on the data sheet:
- Modulus climbs — but only when the fibre loading is high enough. Fibre is tens of times stiffer than the matrix, so under bending it carries most of the load. That shows up in heavily filled grades: Polymaker publishes over 8 600 MPa for PA6-CF20, at 20 % carbon fibre. It does not show up in lightly filled ones: from the same manufacturer, PLA-CF measures 3 281 MPa tensile modulus against 3 427 MPa for unfilled PolyLite PLA — no gain at all. The letters on the label do not tell you the loading.
- Tensile strength usually falls. Every fibre end is a stress concentration. Polymaker's PLA-CF reaches 31.2 MPa against 52.3 MPa for PolyLite PLA — about 40 % lower. PETG runs the other way: Fiberon PETG-rCF08 reaches 59.8 MPa against 50.8 MPa for PolyLite PETG, because the matrix is tougher and the loading is chosen differently.
- Shrinkage drops. Fibre does not shrink as the part cools, so it holds the part back. This is why filled grades come off the plate flatter — on large parts it is often the biggest practical benefit.
Fibre buys you stiffness. It pays with toughness, money, and nozzle life.
Carbon or glass
The two fibres do the same job differently, and the decision usually comes down to budget and service temperature rather than pure mechanical performance.
| Carbon fibre (CF) | Glass fibre (GF) | |
|---|---|---|
| Stiffness gained | Higher per percent of weight | Lower, needs a higher loading |
| Density | Lighter — lighter part at the same volume | Heavier |
| Heat resistance of the composite | Good | Better — glass takes more heat |
| Nozzle wear | Significant | Noticeably worse |
| Electrically conductive | Yes — not for insulating parts | No |
| Relative cost | Higher | Lower |
| Surface | Matte black, even | Matte grey, coarser grain |
That last-but-two row matters if the part sits near electronics: carbon fibre conducts. A PA-CF housing is not an insulator, and neither is the dust from sanding it.
Four filled grades, and what each is for
| Polymaker grade | Matrix | Tensile | Impact (notched Charpy) | HDT 0.45 MPa |
|---|---|---|---|---|
| PLA-CF | PLA | 31.2 MPa | 5.5 kJ/m² | 54 °C |
| Fiberon PETG-rCF08 | PETG | 59.8 MPa | 4.0 kJ/m² | 68.6 °C |
| Fiberon PA6-CF20 | PA6 | 109.3 MPa | 11.0 kJ/m² | 215 °C |
| Fiberon PA6-GF25 | PA6 | 80.1 MPa | 10.0 kJ/m² | 191 °C |
Pick by the job
- Indoor jigs and fixtures that must not flex
- PLA-CF — cheapest, flattest, prints on an open frame
- Brackets that live outdoors or in a car
- PETG-CF — keeps enough toughness and needs no enclosure
- End-use mechanical parts, gears, bushings
- PA-CF — needs a 50 °C chamber and an active dry box
- Parts that run hot, near a motor
- PA6-GF — around 190 °C HDT at 25 % glass, 0.6 mm hardened nozzle
What printing composite actually costs
Spool price is the visible part and usually not the largest one. The other three:
- The nozzle. Brass wears open after a few hundred grams of filled filament, and the symptom is not sudden failure — it is a surface that degrades and extrusion widths that creep until you start blaming the printer. A hardened steel or ruby tip is a requirement, not a recommendation.
- The filament path. PTFE liners, extruder gears and even textured plates wear faster. With glass fibre, treat both the nozzle and the drive gears as consumables.
- Failure rate. Dry filled filament is easy filament; wet filled filament gives pitted surfaces and weak layers. On a nylon matrix, drying is not optional. The same logic applies to reading any grade's numbers — see how to read a filament data sheet.
When not to use a filled grade
- Living hinges and snap fits. They survive on elongation, which is exactly what fibre removes. Unfilled PA12 is the right answer here.
- Impact-loaded parts. Fibre lowers impact energy. A part that gets dropped wants PETG, PA12 or PC, not CF.
- Watertight parts. Fibre creates micro-channels along the extrusion direction. Filled parts leak far more readily than unfilled parts at the same wall thickness.
- When the real problem is orientation. A part loaded across its layers fails whatever material you use. Read print settings for strength before you upgrade material.
If a part needs both stiffness and heat, the right move is to change the matrix — the carbon fibre nylon guide covers what that involves. Print settings and mechanical figures for every grade sit on its Material Passport.
Frequently asked
Is PLA-CF stronger than plain PLA?
Not stronger. On Polymaker's published figures, PLA-CF reaches 31.2 MPa tensile against 52.3 MPa for PolyLite PLA, and its tensile modulus is slightly lower too (3 281 against 3 427 MPa). What it actually buys is an even matte surface, lower shrinkage and a flatter part. If the part must not deflect, it is the better one.
Do I need a special nozzle for carbon fibre filament?
Yes. Brass wears open after a few hundred grams, and the first symptom is a slowly degrading surface and widening extrusions rather than an obvious failure. Use hardened steel from 0.4 mm upward for carbon, and 0.6 mm for glass fibre, whose particles are coarser and more abrasive.
Is glass fibre or carbon fibre better?
It depends on the goal. Carbon gives more stiffness per percent of weight and a lighter part, but costs more and conducts electricity. Glass is cheaper, takes more heat and does not conduct, at the price of weight and nozzle life. For parts that run hot, PA6-GF is often the more sensible choice over PA-CF.
Can filled filament be printed on an open-frame printer?
PLA-CF yes, because the PLA matrix needs no chamber heat. PETG-CF usually yes for moderate part sizes. PA-CF and PA6-GF no: they need a 50–55 °C chamber and an active dry box, and skipping either gives you warped or delaminated parts.
Why do carbon fibre parts leak more easily?
Because the fibre aligns with the extrusion direction and creates micro-channels running along each bead. For parts that must seal, use an unfilled grade, add wall loops, or accept a sealing step after printing.