SLS vs FDM in one paragraph: Both build parts layer by layer, but they aren’t interchangeable. SLS (selective laser sintering) fuses nylon powder — no support structures, near-isotropic strength, and complex geometry printed in a single build, which makes it the choice for functional and small-batch parts. FDM (fused deposition) extrudes thermoplastic filament — far cheaper per machine, anisotropic along the Z-axis, and hard to beat for fast, large, or one-off prototypes.
We run both on the floor in our Kyiv shop — a rack of FDM machines and a Formlabs SLS line — so this isn’t a spec-sheet comparison. It’s what we see when the same part comes through on each process. Here’s the short version, then the engineering behind it.
| Criterion | SLS (nylon powder) | FDM (filament) |
|---|---|---|
| Process | Laser sinters polymer powder, layer by layer | Extrudes molten thermoplastic, layer by layer |
| Supports | None — loose powder holds the part up | Required for overhangs; leave scars |
| Strength & isotropy | Near-isotropic; ~48–50 MPa (PA12) | Strong in XY, weaker in Z (layer bond ~50–70% of XY) |
| Geometric freedom | Channels, lattices, captive/moving parts in one build | Limited by supports and removal access |
| Materials | PA12, PA11, TPU 90A, PA-CF / GF nylon | PLA, PETG, ABS/ASA, nylon, PC, CF-filled |
| Surface finish | Uniform matte, slightly grainy; no support marks | Visible layer lines + support scars |
| Tolerance | ~±0.3% (min ±0.3 mm), repeatable across a batch | Machine-dependent, usually a touch looser |
| Cost — 1–2 parts | High (you pay for a whole build + powder refresh) | Lowest |
| Cost — batch | Lower per part (parts nest in 3D) | Higher per part (one at a time) |
| Lead time | Hours per build, but dozens–hundreds of parts out | Fast for one small part; scale by adding machines |
| Best for | Functional/end-use, complex geometry, small–mid batch | Prototypes, large/simple parts, jigs, cheap iteration |
Which is stronger, SLS or FDM?
SLS, for most functional parts — and the reason is direction, not just a headline number. A sintered nylon part is close to isotropic: it carries roughly the same load whether you pull it along X, Y, or Z. PA12 lands around 48–50 MPa tensile with ~11% elongation, and it behaves predictably under load.
FDM is anisotropic. In the print plane a well-tuned PETG or nylon part is genuinely strong, but the weak axis is layer adhesion — the bond between deposited layers. Depending on settings and material, Z-strength often sits at 50–70% of the in-plane figure. For a bracket loaded in one known direction you can orient around it; for a part loaded every which way, that layer plane is where it cracks. That one fact decides a lot of functional jobs.
When is FDM the right choice?
- One or two prototypes — cheapest, fastest path to a part in your hand.
- Large, simple parts — a big FDM bed prints a half-metre bracket cheaply; SLS build volumes are smaller and powder cost adds up.
- Cost-sensitive iteration — PLA/PETG is cheap, so reprint twice a day while you tweak the design.
- Jigs, fixtures, visual models — where layer lines simply don’t matter.
When does SLS win?
- No supports, full geometry. Loose powder holds everything up, so internal channels, lattices, and captive or pre-assembled moving parts come out of one build with nothing to cut off.
- Functional and end-use parts. Isotropic nylon, living hinges (PA11), seals and grips (TPU 90A), or stiff glass/carbon-filled nylon (PA-CF) for under-the-hood loads.
- Small-to-mid batches. Parts nest through the whole build volume in three dimensions, so a single run yields dozens to hundreds.
What about cost?
People get this wrong by quoting one number. There’s a crossover.
At one or two parts, FDM is cheaper — a little filament and an hour or two of machine time. SLS makes you pay for a whole build and a powder refresh whether you fill it or not, so a single part is expensive.
As quantity rises, it flips. SLS nests parts through the entire powder bed; FDM still lays one part at a time on a plate. By a few dozen functional parts, SLS is usually cheaper each — and the gap widens from there. Modern industrial SLS has pushed this hard: a current-generation machine can run on the order of several thousand parts per week, roughly 3× the throughput of legacy SLS at about half the cost per part. That’s what moved SLS from “prototype only” into genuine low-volume production. Before you pick a process, work out your crossover quantity.
Surface finish and post-processing
FDM comes off the bed with visible layer lines and scars where supports met overhangs — fine for a fixture, less so for a customer-facing part without sanding or vapour smoothing. SLS has a uniform, slightly grainy matte finish straight out of the powder, no support marks anywhere, and it takes bead-blasting, dyeing (black is common) and tumbling well. Neither is “better” — they fail differently, and which failure matters depends on whether anyone ever sees the part.
Tolerance and repeatability
Both are dimensionally honest enough for engineering work, with SLS generally tighter and — more importantly — more repeatable across a batch. Typical SLS accuracy is around ±0.3% (min ±0.3 mm); FDM is machine-dependent and usually a little looser. For press-fits and assemblies that have to go together without fettling, that batch-to-batch repeatability is a big part of why functional work tends to land on SLS.
How to choose — the short rule
Choose FDM when the part is large, simple, one-off, cost-driven, or just a shape you need tomorrow. Choose SLS when the part is functional, geometrically complex, support-hostile, or you need more than a handful that all have to perform like the design. Still unsure? The two deciding questions are almost always: how many? and does it carry load in more than one direction?
We print both in-house in Kyiv, so if you’d rather not guess, send us the model — we’ll tell you straight which process fits and what it costs on each. More on our SLS 3D printing service, or the Formlabs Fuse 1+ if you’re weighing bringing SLS in-house.