Nearly every other filled-versus-unfilled comparison on this site is about whether the fibre is worth the money. This one is different. Unfilled PA6 has the largest shrinkage of any common filament, and on a desktop machine that is not a quality issue — it is the reason parts lift off the plate and end up in the bin.
Carbon fibre does not improve PA6 so much as convert it into something you can actually finish a print in.
What the fibre does to contraction
PA6-CF's stored shrinkage range is 0.2 to 0.6%, against unfilled PA6's 0.8 to 2.0. The stored note refuses to round that to one tidy fraction: at the bottom of both ranges the filled grade moves a quarter as much, at the top three-tenths as much. Either way, that reduction is the entire point of the material.
Follow it onto a 150 mm part at the worst end of each range. The filled version needs a 0.9054 mm correction — the kind of number PETG asks for. The unfilled version needs 3.0612 mm, more than three times as much, and it finishes at 147 mm if you do nothing. A part that finishes at 147 mm when you asked for 150 is not out of tolerance, it is a different part. Whichever end of PA6's 0.8-to-2.0 band your chamber leaves you at, the shrinkage calculator turns it into the dimension to draw.
The compensation is the smaller half of the story. Contraction that large generates force while the part is still stuck to the plate, and that force is what curls a 150 mm unfilled nylon part off the bed before it reaches full height. Reducing shrinkage reduces the force, which is why the filled grade stays down on geometry the unfilled one abandons.
And it is a genuinely different material mechanically
Unlike most fibre-filled grades, where the tensile improvement is marginal, PA6-CF is a real step up:
| PA6 | PA6-CF | |
|---|---|---|
| Tensile | 50–80 MPa | 80–130 MPa |
| Deflection | 60–100 °C | 120–190 °C |
| Shrinkage | 0.8–2.0% | 0.2–0.6% |
| Density | 1.12–1.15 g/cm³ | 1.14–1.24 g/cm³ |
| Nozzle | 250–280 °C | 270–300 °C |
| Price | $35–70 /kg | $60–120 /kg |
That deflection row is the one to notice. Unfilled PA6 overlaps PETG at the bottom of its range; PA6-CF sits in territory nothing else on a desktop machine reaches short of PPA. This is the material that makes printed under-bonnet brackets and jigs for hot processes a realistic idea.
Weight the selector towards strength and heat on a capable enclosed machine and PA6-CF ranks second of the eligible materials, behind only PPA and ahead of polycarbonate — see PC against nylon for where that comparison lands.
What it costs you
A 300 °C hotend. The stored weakness list names machines without one. At 270–300 °C this is above what many enclosed printers manage, and the top of the range is where layer bonding on a thick part lives.
A hardened nozzle, and it will still wear. Carbon fibre destroys brass. Hardened steel lasts, ruby and carbide last longer.
The spool has to stay dry while it prints, not merely start dry. Both grades want 80 °C for 8 to 12 hours before printing, but the filled one's stored note goes further and specifies printing from a heated dry box. Over ten or twelve hours a spool dried this morning is wet again by the evening, and the last layers of the part are the compromised ones.
Money. Roughly double per kilogram, and the same model comes out slightly heavier because the fibre raises density — so the cost per part rises a little faster than the spool price suggests.
Impact resistance. The usual fibre trade. PA6-CF is stiffer and stronger in tension and less forgiving of a sharp knock than unfilled nylon, which is one of the toughest materials here.
Where unfilled PA6 is still the right choice
It has a genuine niche and it is not simply the budget option.
Living hinges and flexures. The fibre stiffens the matrix and shortens fatigue life in bending. If the part's job is to flex thousands of times, unfilled nylon is the better polymer.
Small parts. Shrinkage is a percentage, so on a 20 or 30 mm part the absolute movement is small and the warping force is low. Unfilled PA6 prints small parts perfectly well.
Anything sliding against metal. Unfilled nylon is self-lubricating; exposed carbon fibre at a wearing surface is abrasive and will work against the counterface.
Cost-sensitive work in quantity, where the geometry is forgiving and half the filament price is worth the extra care.
What to have in place before the first print
Filled nylon punishes an incomplete setup more than any other material here. The checklist is short and none of it is optional:
- A hotend rated to 300 °C, verified rather than assumed.
- A hardened steel, ruby or carbide nozzle at 0.4 mm or wider.
- A chamber that stays warm, actively heated if the part is large.
- Garolite or a polyamide-specific build sheet.
- A dryer that reaches 80 °C, and a heated box to print from.
- A slicer profile built from scratch, not adapted from a PETG one.
Glass fibre, if carbon is not the answer
There is a third grade worth knowing about, because the choice of fibre changes the result as much as the choice to use one.
PA6-GF carries glass rather than carbon: tensile of 70–110 MPa, a shrinkage range of 0.3 to 0.8%, and a density of 1.20–1.35 g/cm³ because glass is heavier than carbon. It reduces shrinkage less than carbon does and it costs slightly less, but it keeps more of the base polymer's impact resistance, which is exactly the property carbon takes away.
The stored weakness list also names its real cost: glass is harder on a nozzle than carbon is. If a part needs stiffness and has to survive being dropped, glass fibre is the better compromise; if it needs maximum stiffness and dimensional stability, carbon is.
The practical rule
If the nylon part is larger than a fist, or flat, or has to hold a dimension, print PA6-CF and budget for the nozzle. If it is small, or hinges, or slides, print unfilled PA6 and enjoy the toughness.
And if you are still deciding whether nylon is the right family at all, the moisture argument in nylon against PETG comes before either of these — a nylon part that absorbs water in service moves whether or not there is fibre in it.