Carbon Fibre Filament (CF/GF): Properties, Nozzle, Limits
are explained briefly when you hover over or tap them.
Filaments with “CF” or “GF” in the name contain short, chopped carbon or glass fibres. They are often regarded as the “strong” version of a material. That is only partly true: the fibres mainly make parts stiffer and more dimensionally accurate, not automatically stronger under load. And they wear out a normal nozzle in a short time.
What the fibres do
- More stiffness: Parts bend less under load. This is the main reason for CF filaments, for example for brackets, frames or jigs.
- Less warping: The fibres hinder shrinkage as the material cools. This is a big advantage especially with nylon. PA-CF is often much easier to print than pure PA.
- Better dimensional accuracy: Parts stay closer to the nominal size.
- Matt look: Layer lines are less noticeable, the surface looks refined and even.
- Less weight: Carbon is light, and parts become somewhat lighter at the same stiffness.
What the fibres do not do
The fibres in the filament are very short, mostly only fractions of a millimetre. This has little to do with the continuous fibres of a real carbon part, such as a bicycle frame.
Tests also show that, compared with the base material, the tensile strength of CF filaments often even drops, especially between the layers. The fibres do not bond with the plastic. If they lie exactly at the layer boundary, they weaken the bond rather than help. In tests, the loss was greater with PETG-CF than with PLA-CF.
In addition, many CF parts become more brittle. They are more likely to break instead of bending first.
In short: CF and GF are worthwhile if a part is to be stiff, dimensionally accurate and low-warp. If it has to withstand impacts or tension, the base material without fibres is sometimes the better choice. You can often get more strength more simply with more walls, see Walls and top layers.
Carbon or glass?
| Carbon fibre (CF) | Glass fibre (GF) | |
|---|---|---|
| Stiffness | very high | high |
| Toughness | rather brittle | tends to give way before it breaks |
| Weight | lighter | heavier |
| Colours | almost only black and dark tones | many colours possible |
| Price | higher | usually cheaper |
| Nozzle wear | heavy | heavy |
A hardened nozzle is a must
Carbon and glass fibres act like an abrasive in the nozzle. A normal brass nozzle visibly widens after comparatively little material. The consequences are inaccurate dimensions, under-extrusion and strings.
Manufacturers therefore explicitly require a hardened nozzle for CF and GF filaments, for example one made of hardened steel. Many current printers have such a nozzle from the factory, and on others it can be retrofitted cheaply. Check this in your printer’s data sheet. If it only says “brass” or “stainless steel”, the nozzle is not intended for CF and GF.
Good to know: The extruder can also wear if the drive gears are not hardened. With occasional use this is usually no problem, but with a lot of CF printing it is worth a look.
Nozzle size: 0.4 or 0.6 mm?
The fibres can catch in narrow nozzles and clog them. Many manufacturers give 0.4, 0.6 and 0.8 mm as suitable, and for fibre-reinforced nylon 0.6 mm is often explicitly recommended.
- 0.4 mm: finer details, but a higher risk of the nozzle clogging
- 0.6 mm: less clogging, faster printing, somewhat coarser details. Usually the better choice for functional parts.
- 0.2 mm: unsuitable for fibre-reinforced filaments
The common base materials
| Material | Typical nozzle | Typical bed | Use |
|---|---|---|---|
| PLA-CF | 210 to 240 °C | 45 to 65 °C | stiff parts, looks, open printer possible |
| PETG-CF | 240 to 270 °C | 60 to 80 °C | stiff, somewhat more heat-resistant than PLA-CF |
| PA-CF, PA-GF | 260 to 300 °C | 100 to 120 °C | technical parts, heat, wear |
The values come from manufacturers’ figures and vary by brand. The spool takes priority. The same rules as for nylon also apply to PA-CF: dry before every print, enclosed printer. Details in the article Printing nylon (PA).
Fibre-reinforced filaments should also be dry. Typical figures: PLA-CF like PLA, PETG-CF at around 65 °C, PA-CF at around 80 °C for 8 to 12 hours. More under Drying filament.
Health: dust from sanding and cutting
During printing itself, most fibres stay bound in the plastic. There are indications, however, that individual fibre fragments get into the air while printing. How much that is has hardly been measured so far.
What is certain: sanding, sawing, drilling or filing releases fibres. Fine fibres should not be inhaled and irritate skin and eyes.
- Sand wet where possible when post-processing
- wear a respirator mask while doing so, at least FFP2
- wipe up sanding dust damp, do not sweep or blow it dry
- printer with HEPA filter or good ventilation, activated carbon alone does not filter particles
- let the chamber cool briefly before opening it
General notes on fumes and particles are under Safety in 3D printing.
Continue with: the filament overview.