Glass-filled and carbon-filled polyamide 6 look identical on a specification sheet until you read the numbers carefully, and they behave differently in ways that decide real projects. The reinforcement is the only variable, and glass fibre differs from carbon fibre in four respects that all matter.
Glass is thicker, less stiff and more forgiving
A glass fibre has a lower modulus than a carbon one and a larger diameter, so it stiffens the matrix less and it does not fracture as readily. That single difference propagates through the whole property set.
- Stiffness is lower. Tensile lands at 70 to 110 MPa against the carbon grade's 80 to 130, and the modulus gap is wider than the tensile gap.
- Toughness is better. Glass-filled parts retain more elongation before failure and take an impact that would crack the carbon version. If a part gets knocked as well as loaded, this is the one.
- Warping control is weaker. Shrinkage falls to 0.3%–0.8% against carbon's 0.2%–0.6%, both a large improvement on unfilled PA6's 0.8%–2.0%, but glass restrains the polymer less effectively.
- Heat resistance is marginally lower, at 110 to 190 °C against 120 to 190.
That is a coherent trade rather than a worse material: you give up some stiffness and dimensional control to get impact resistance back.
It does not conduct, and sometimes that is the whole reason
Carbon fibre is electrically conductive. A carbon-filled print is not a conductor in any useful sense, but it is not a reliable insulator either, and its surface resistivity is low enough to matter around exposed contacts, high-voltage sections and sensitive electronics.
Glass fibre is an insulator, full stop. For connector bodies, terminal blocks, motor mounts near windings, battery holders and enclosures that carry live parts, that is not a nuance — it is the deciding property, and no amount of mechanical advantage on the carbon side outweighs it.
It is heavier, and the difference shows up on the invoice
Glass is denser than carbon fibre, so a glass-filled grade runs 1.20 to 1.35 g/cm³ against carbon's 1.14 to 1.24. The same part therefore weighs more.
Take a component that consumes 300 g in a 1.19 g/cm³ carbon-filled grade. At 1.28 in the glass-filled equivalent, the identical geometry is 7.56% heavier, and where the carbon version drew 104.81 m off the spool the glass one draws 97.44 m for its 300 g — 7.37 m less. Over a production run that is a real material cost, and if your slicer profile carries the carbon density it is a real quoting error. Set the figure you actually bought and check it in the length and weight converter.
Glass is harder on a nozzle than carbon
This surprises people, because carbon fibre has the fearsome reputation. Glass is harder and its fibres are stiffer, so they scour rather than polish. A hardened steel nozzle is the minimum; for sustained production, tungsten carbide or a ruby tip is the sensible investment, and even hardened steel wears measurably.
The same applies to everything else the filament passes through. Drive gears, PTFE couplings, filament guides and the extruder path all wear faster than with carbon-filled material, which is worth budgeting for rather than discovering.
Running it, and the dust it makes
The machine requirements are indistinguishable from the carbon-filled grade: a hotend reaching 270 to 300 °C, a bed at 70 to 100 °C, a heated chamber, and a Garolite or polyamide-specific plate. Drying likewise sits at 80 °C for 8 to 12 hours with the job fed from a heated dry box, since the matrix underneath the glass is ordinary polyamide 6 and has lost none of its appetite for water.
One practical difference from the carbon grade: glass-filled material shows its fibres at the surface more readily, so prints come out with a slightly rough, sparkly finish rather than the deep matte black of a carbon composite. Sanding it produces glass dust, which is worth a mask and a vacuum rather than a shrug.
What PA6-GF is genuinely bad at
- Surface finish, per above. It is a functional material and it looks like one.
- Nozzle and extruder life, worse than carbon-filled.
- Stiffness, if that is the property you actually needed.
- Fine detail and small nozzles. A 0.6 mm nozzle is the practical starting point.
- Cost. $55 to $110 per kilogram, a little below carbon-filled but far above anything a hobby machine usually runs.
- Interlayer strength. As with every short-fibre composite, none of the reinforcement crosses between layers. Orient the part so the load runs in-plane.
- Availability. Fewer brands stock glass-filled polyamide than carbon-filled, and colour choice is usually one: a pale grey-white that the fibre itself dictates.
Where to choose it over the carbon version
Anything electrical. Anything that takes impacts — tool housings, guards, mounts on vibrating machinery, parts on vehicles that get stones thrown at them. Parts where you would rather have a component that bends and survives than one that is stiffer and cracks.
Choose PA6-CF instead when deflection under load is the failure you are designing against, when the part must be as light as possible, or when the flattest possible large part is the goal.