The filament list is long and most of the choice comes down to four questions: how hot does it get, does it need to be tough, does it live outdoors, and what does it touch. Answering those eliminates most of the list quickly and usually leaves two candidates whose difference is printability rather than performance.
The everyday materials
PLA is stiff, easy, cheap and softens in a warm car, which makes it excellent for shape checks and poor for anything thermal. PETG is tougher, prints nearly as easily, resists chemicals better and is the sensible default for a functional part on a basic machine. Both print without an enclosure and neither is suitable outdoors for long.
The heat and outdoor materials
ABS survives more heat and warps as it prints, so it needs an enclosure. ASA prints like ABS with far better ultraviolet stability, which makes it the outdoor answer. Polycarbonate is very strong and needs a hot enclosure and thorough drying. Each of these is a step up in capability and a step up in what the machine must be.
Nylon and the filled grades
Nylon is tough, fatigue resistant and hygroscopic, so it must be dried and it grows as it absorbs moisture. Glass and carbon filled grades of nylon and other polymers are stiffer, warp less and abrade nozzles, so a hardened nozzle is required. Filled grades are the usual answer for shop floor fixtures where stiffness matters more than impact resistance.
Choosing on the four questions
Hot: ABS, ASA, polycarbonate or a filled nylon. Tough: PETG, nylon or polycarbonate. Outdoors: ASA, or ultraviolet stabilised grades. Chemical exposure: check the specific chemical rather than the family, since polycarbonate in particular is attacked by things that leave PETG untouched. Everything else, and every shape check, is PLA.
Questions people ask about fdm materials
Which filament should I use for a functional part?
PETG for a general purpose tough part, nylon or a filled nylon where fatigue or stiffness matters, polycarbonate where strength and heat both matter.
Which filament is best outdoors?
ASA. It prints like ABS and resists ultraviolet degradation far better, whereas PLA, PETG and unmodified ABS all degrade outdoors.
Do filled filaments need special hardware?
Yes, a hardened nozzle at minimum, because the fibres abrade brass quickly. A hardened feed path helps on machines that run them often.
Why does my nylon part change size?
It absorbs moisture from the air and grows, becoming tougher and less stiff as it does. Dry the filament before printing and condition parts where dimensions matter.