Printing quotes are often compared on price alone, which is a mistake: the cheap process is expensive if the part fails the fit check or arrives with a surface nobody wants to show a customer. SLA, SLS and FDM are not interchangeable, and each one is bad at something specific.
SLA: surface first
Stereolithography cures resin in thin layers, so it produces the smoothest surface of the three and holds fine detail well. It is the right choice for cosmetic shells, display windows, light guides and master patterns for silicone casting. The weakness is toughness: standard resins are brittle, and a thin clip will snap. Tough and PC-like resins improve this, but they are still not nylon. Support marks also have to be removed from a visible face, which is why orientation is agreed before the build.
SLS: toughness first
Selective laser sintering fuses nylon powder without support structures, so there are no support marks and no orientation penalty on the cosmetic face. PA12 is tough enough for snap fits, living hinges and brackets that get handled daily. The trade-off is surface: SLS has a fine, slightly grainy texture that reads as matte rather than smooth, and thin walls below 0.8 mm become unreliable. If the part needs to be black, dyeing is standard; other colours are usually painted.
FDM: size and cost first
Fused deposition is the cheapest way to make something large. It is the right process for jigs, fixtures, form checks and parts where the envelope matters more than the finish. Layer lines are visible, the minimum wall is around 1.2 mm, and the tolerance is quoted as a percentage as well as a number, because the error grows with the part. For a 300 mm bracket, ±1.5 mm is realistic — fine for a fixture, not fine for an assembly.
| SLA | SLS | FDM | |
|---|---|---|---|
| Surface | Smooth, paintable | Matte, slightly grainy | Visible layer lines |
| Toughness | Low to moderate | High | Moderate |
| Min wall | 0.6 mm | 0.8 mm | 1.2 mm |
| Typical tolerance | ±0.1 mm | ±0.15 mm | ±0.3 mm / ±0.5% |
| Best for | Cosmetics, detail | Snap fits, brackets | Large fixtures, form checks |
Four questions, in order
- Does the part have to flex, snap or survive a drop? If yes, start at SLS.
- Does the visible surface have to look moulded? If yes, start at SLA.
- Is the part larger than about 300 mm in any direction? If yes, FDM is usually the only economical route.
- Will the part be handled daily for months? If yes, skip standard resin and price SLS or a machined part.
When the answers conflict — a large cosmetic shell, for example — the usual solution is to split the part: print the shell in SLA in sections, or print a cheap FDM form check first and finish only the winning geometry. Ask for both prices on the same file; the comparison usually makes the decision obvious.