FDM and SLA are the two most common 3D printing technologies, and both have a place in a well-equipped shop — we run both in Spencerport for exactly that reason. But they behave very differently, and picking the wrong one for a job can mean paying three to six times as much per gram, getting a part that's too brittle for what it has to do, or waiting an extra half-day for a glass-smooth finish you didn't need. This guide walks through how each process works, what each one is genuinely good and bad at, and a simple way to decide — so by the end you can tell at a glance which one your part wants.
How each technology works
FDM (Fused Deposition Modeling, also called FFF) melts a spool of plastic filament and lays it down in fine beads, layer by layer, through a heated nozzle that moves over a build plate. Each new layer fuses to the one below as it cools. It's the technology most people picture when they think "3D printer" — reliable, affordable, and compatible with a huge range of engineering thermoplastics. The process was developed and trademarked by Stratasys in the late 1980s; "FFF" is the equivalent open term. We run it on Bambu Lab X1 Carbon and Prusa MK4S machines.
SLA (Stereolithography) works completely differently. A vat holds liquid photopolymer resin; a UV light source — a laser on older machines, an LCD/LED panel on modern ones — selectively cures a thin layer of that resin to solid, the build platform lifts a fraction of a millimeter, and the next layer is cured onto it. Because the "voxels" are defined by light rather than a physical nozzle, SLA captures far finer detail and leaves a much smoother surface. The trade-off is in the material: cured photopolymers tend to be more brittle and less UV- and heat-stable than FDM engineering plastics, and they require post-processing (a solvent wash and a UV cure) before the part is finished. We run SLA on a Formlabs Form 3+, and Formlabs publishes the most thorough resin datasheets if you want to go deep on the chemistry.
FDM vs SLA at a glance
| Attribute | FDM | SLA |
|---|---|---|
| Typical layer height | 0.1 – 0.3 mm | 0.025 – 0.1 mm |
| Min feature size | ~0.4 mm | ~0.05 mm |
| Dimensional tolerance | ±0.15 – 0.3 mm | ±0.05 – 0.15 mm |
| Surface finish | Visible layer lines | Smooth out of printer |
| Common build size | 250 – 350 mm | 140 – 200 mm |
| Material strength | Engineering plastics (PETG, ABS, ASA, nylon, PC) | Specialty resins — most are more brittle than ABS |
| Anisotropy (strength by direction) | Weaker between layers (Z); design around it | Nearly isotropic, but brittler overall |
| Outdoor / UV / heat | ASA, PETG, ABS all survive | Most standard resins yellow and weaken in UV |
| Post-processing | Optional sanding, painting | Required: IPA wash + UV cure |
| Material cost (relative) | 1× | 3 – 6× per gram |
| Best for | Functional parts, larger geometry, batch runs | Fine detail, jewelry, dental, smooth presentation models |
When FDM wins
- Functional, load-bearing parts. This is the big one. FDM runs in real engineering thermoplastics — PETG, ABS, ASA, nylon, polycarbonate — that are tough, fatigue-resistant, and behave predictably under stress. Most SLA resins, by contrast, are noticeably more brittle than ABS; "tough" and "durable" resins exist but they're expensive and still don't match a good FDM nylon. If the part has to survive being dropped, clamped, vibrated, or flexed thousands of times, FDM is almost always the answer.
- Larger parts. FDM printers routinely handle 250–350 mm builds, and parts can be split and bonded to go bigger. SLA build volumes are small — typically 145 × 145 × 185 mm on a Form 3+ — and large resin prints are slow, expensive, and prone to warping during the cure.
- Outdoor, UV, or high-temperature use. ASA and PETG shrug off sunlight and weather; ABS and polycarbonate handle heat. Standard SLA resins yellow, get more brittle, and lose strength under UV exposure — fine for an indoor display piece, not for a part bolted to the outside of something.
- Budget and quantity. Filament costs a fraction of resin per gram, and FDM has no per-part wash-and-cure step. For a batch of 50 or 500 end-use parts, FDM is dramatically cheaper and faster. (See our small-batch production page.)
- Big, simple geometry. Brackets, enclosures, jigs, fixtures, housings — anything where you don't need 0.05 mm detail, FDM does faster and cheaper.
When SLA wins
- Fine detail. SLA resolves features down to roughly 0.05 mm — chainmail on a miniature, sculpted fabric folds, fine lettering, jewelry masters, dental models, tiny snap features. FDM physically can't do this at small scale; the nozzle is ~0.4 mm wide.
- Smooth surfaces straight off the printer. SLA parts come out with no visible layer lines — ideal for presentation prototypes, master patterns for casting or molding, and anything that has to look finished without sanding. Getting an FDM part this smooth means a lot of sanding, filling, and priming.
- Tight tolerances on small parts. Below about 20–30 mm, SLA's dimensional repeatability beats most FDM setups — the layers are thinner, there's no nozzle-width compensation to dial in, and shrinkage is more uniform.
- Watertight, seamless geometry. SLA layers fuse into essentially solid material with no extrusion seams, so SLA walls seal better — useful for fluid-handling parts, manifolds, and pieces that need to hold a vacuum or pressure.
- Intricate shapes that don't carry load. Display models, art pieces, architectural detail, scale figures — where the geometry is complex and beautiful and nothing's trying to break it.
Materials: what's actually available in each
FDM materials we stock: PLA (cheap, great finish, low-stress indoor parts and display pieces); PETG (the all-round workhorse — tough, dimensionally stable, mild chemical and UV resistance); ABS and ASA (heat-resistant, paintable; ASA adds excellent UV stability for outdoor parts); nylon (high impact resistance and fatigue life — living hinges, gears, abuse-resistant parts); polycarbonate (maximum strength and heat resistance); TPU (flexible, rubber-like — gaskets, bumpers, grips). That's six engineering plastics covering most mechanical, thermal, and aesthetic requirements.
SLA resins come in families rather than a single material: standard (best detail, most brittle), tough/durable (more impact resistance, still not a true engineering plastic), high-temp (for heat-resistant masters and tooling), flexible/elastic, castable (burns out cleanly for lost-wax casting — jewelry), and dental/biocompatible. Each trades off something — you don't get fine detail, toughness, heat resistance, and low cost all at once.
Post-processing: the hidden time cost
An FDM part is essentially done when it comes off the bed — pop it loose, snip any supports, and it's usable. Optional finishing (sanding, vapor-smoothing for ABS, painting) is just that: optional. An SLA part is not done off the printer: it has to be washed in isopropyl alcohol to remove uncured resin, then UV-cured to reach full strength, then have its supports removed (and the support marks cleaned up, since SLA supports leave nibs rather than the fuzzy patches FDM does). That workflow adds the better part of a day to a small part that would have printed in an hour on FDM. We handle all of it — but it's why "I need 100 of these by tomorrow" almost always means FDM.
Cost and turnaround
For parts over about 30 mm, FDM is usually both faster and cheaper — lower material cost per gram, no wash-and-cure step, larger build plates so more parts run at once. SLA's cost advantage only appears on small, highly detailed parts where an FDM equivalent would need extensive hand-finishing to look acceptable — at which point the labor swings it back toward SLA. For a production batch of end-use parts, FDM wins on cost almost every time. There's a fuller cost discussion (including where CNC and injection molding fit) in our post on 3D printing vs CNC vs injection molding, and our tolerances guide covers the dimensional-accuracy differences in depth.
A simple decision tree
- Does the part need to be strong, outdoor-rated, heat-resistant, or larger than ~100 mm? FDM.
- Does the part need micro-detail, a glass-smooth finish out of the printer, or jewelry-/dental-level precision? SLA.
- Is it a small visual prototype where appearance matters more than strength? SLA.
- Is it a production batch of functional end-use parts? FDM in almost every case.
- Is it a master pattern for casting or molding? SLA (castable or high-temp resin).
- Still genuinely torn? It's probably a part where either would work — pick FDM for cost and durability unless the finish is the whole point.
FAQ
Can SLA parts be used for functional, load-bearing applications? Sometimes, with tough/durable resins — but they're still generally more brittle and less heat-stable than FDM engineering plastics, and they cost more. For a part that has to take real mechanical abuse, FDM in nylon or polycarbonate is usually the better and cheaper call.
Why are SLA parts so much more expensive? Resin costs roughly 3–6× as much as filament per gram, build volumes are smaller (fewer parts per run), and every part needs a wash-and-cure step that adds labor. The cost is real, which is why we only recommend SLA when the detail or finish genuinely needs it.
Will an SLA part hold up outdoors? Most standard resins won't — they yellow and get brittle under UV. A clear-coat helps somewhat, but for anything that lives outside, ASA (FDM) is the right answer. There are weather-resistant resins, but they're a niche option.
Can you print the same part in both and let me compare? For a small part, sometimes that's worth doing — and we'll suggest it if your project is genuinely on the fence. Mention it when you quote.
Which one is more accurate? SLA on small parts (under ~20–30 mm); FDM is competitive and often better on larger parts because resin can warp during the cure. See the tolerances guide for the numbers.
Not sure? We'll recommend
Tell us what the part does — how it's loaded, where it lives, how big it is, how it needs to look — and we'll recommend the right process when we quote it, free. If you're prototyping, our rapid prototyping service runs both technologies, so a single project can start in FDM for fit-and-function and finish in SLA for the photo-ready units. Send your file through the quote form and we'll take it from there.