Glass fiber filament guide
Glass fiber filaments are ordinary plastics, usually PETG, nylon, ABS, ASA or PLA, filled with short chopped glass fibers. The fibers make parts stiffer and more dimensionally stable, and they reduce warping. They do the same job as carbon fiber fills, with a different set of tradeoffs, and every one of them needs a hardened nozzle.
What the glass fibers do
Chopped glass fibers stiffen the base plastic, so parts flex less under load and hold their shape better as they warm up. They also reduce shrinkage while the part cools, which means less warping and more accurate dimensions, especially with materials like nylon and ABS that normally warp badly.
What they do not do is turn a weak plastic into a strong one. The fill mainly changes stiffness and stability; the base material still decides chemical resistance and how the part behaves outdoors, and it sets most of the heat limit. In PETG and ABS the fibers add only a modest amount of heat resistance, so a PETG-GF part still has roughly PETG's heat limits. Nylon is the exception: glass fiber raises its softening point under load by a large margin, which is one reason PA-GF suits hot jobs.
Glass fiber vs carbon fiber
Carbon fiber fills generally make parts stiffer and slightly lighter, since carbon fiber is less dense than glass, and they give a dark matte finish. Glass fiber fills are noticeably less stiff but often tougher under impact, and they are generally cheaper. Glass fiber is also electrically insulating, while carbon fiber fills can conduct slightly, which matters for parts near electronics. Glass filled parts can be made in more colors, since the fibers are not black.
If you want the stiffest part for the least weight, carbon fiber usually wins. If you want a stable, stiff part that handles knocks and costs less, glass fiber is a strong choice. The carbon fiber filament guide covers the other side.
Printing it
The one thing not to get wrong
Glass fibers are abrasive and will wear out a standard brass nozzle, sometimes within a single spool. Use a hardened steel or other wear resistant nozzle, and check your extruder gears are rated for abrasive filament.
Otherwise, print a glass filled filament much like its base material: PETG-GF with PETG style settings, PA-GF like nylon, ABS-GF like ABS in an enclosure. Many fiber filled filaments print better with a slightly larger nozzle, such as 0.6mm, which also reduces the chance of clogs. Nylon based grades still need to be kept very dry.
When to choose glass fiber
Choose a glass filled filament for stiff, dimensionally stable functional parts: jigs and fixtures, brackets, mounts and housings, especially where you want less warping than the plain material gives. PETG-GF is the easiest starting point on an open printer; nylon and ABS based grades suit tougher or hotter jobs if you can print them.
Skip it for cosmetic prints, flexible parts or anything where you do not want to swap nozzles. For most everyday parts, the plain version of the same material is cheaper and easier.
Compare Glass Fiber Prices
Compare live glass fiber listings by brand, weight and price per kg. PETG-GF has the most choice; nylon and ABS grades are listed on their own pages.