SLA vs FDM: which print process fits your part?

Choosing between SLA and FDM isn't about which technology is "better" — it's about matching the process to what the part actually needs to do. SLA excels at precision and surface finish on small, intricate parts; FDM trades detail for strength, speed, and material versatility. At FSH Labs we run both on production hardware (Formlabs Form 4 for resin, Bambu P1S and A1 for filament), and the split between them comes down to four things: detail, material properties, volume, and cost. Here's how we think through it.

Key takeaways

• SLA excels at surface finish, sub-millimetre features, and tight tolerances on small parts
• FDM wins on mechanical strength, material variety, cost per part, and large build volumes
• SLA post-processing is more intensive (wash + cure); FDM parts are usable straight off the bed for functional tests
• Materials matter as much as process — PETG for toughness, ASA for outdoors, PLA for prototypes, engineering resins for heat resistance
• When in doubt, describe the end use and we'll pick the process as part of the quote

How does SLA actually work, and what does it get right?

SLA (stereolithography) on our Form 4 uses masked SLA (MSLA) — an LCD photomask backlit by a UV LED array cures an entire layer of liquid photopolymer resin at once. Our Form 4 operates at 25–100 micron layer heights — roughly the thickness of a human hair at the fine end. Because each layer is cured all at once from the digital mask, the process reproduces fine features (threads, logos, living hinges under 0.5 mm) with a consistency that FDM's extruder tip simply can't match.

The practical upshot: if a part has visible text smaller than 2 mm, fine mesh structures, or needs to serve as a master for mold-making, SLA is almost always the right call. Tolerances on our Form 4 run ±0.1 mm on features under 50 mm, tightening further with calibrated print orientation.

Post-processing is non-negotiable for SLA. Every part goes through an IPA wash to remove uncured resin, followed by a UV cure cycle. That adds 30–60 minutes of handling per build and generates solvent waste — factor that into turnaround expectations and cost.

What does FDM do better, and when should you choose it?

FDM (fused deposition modeling) melts thermoplastic filament and deposits it in beads, building up a part layer by layer at 0.1–0.3 mm in practice (finer is possible but rarely worth the time). It sounds cruder than SLA, and for surface aesthetics it is — but thermoplastics give FDM a massive advantage in real-world use.

PETG is our workhorse: it bonds well between layers, resists moisture, and handles light mechanical load without the brittleness of PLA. ASA is what we reach for outdoor or UV-exposed applications — it's chemically similar to ABS but without the warping headaches, and it genuinely survives seasons outside. For parts that live at elevated temperatures (near electronics, engine bays, dishwashers), we move to ASA or specialty high-temp filaments. PLA is still excellent for appearance models and quick functional checks where longevity isn't the goal.

FDM also scales better. A 200 × 200 × 250 mm enclosure bracket is impractical in resin; on the Bambu P1S it's a straightforward overnight print. And because we can plate multiple parts together, small batches of FDM parts share machine time in a way that drives per-part cost down quickly.

What are the real tolerances, and why do they matter for fitment?

Tolerances determine whether your shaft fits in its bore, whether a snap-fit clicks, and whether two mating parts assemble without filing. For SLA we achieve ±0.1 mm reliably on features up to about 50 mm; accuracy degrades slightly on larger spans as resin shrinkage during cure becomes a factor. Orient a part to keep critical dimensions parallel to the build platform and you'll get the best numbers.

FDM on our Bambu hardware holds ±0.2 mm under normal conditions, but that number hides some nuance. Dimensions in the XY plane (the layer plane) are more accurate than the Z dimension, and overhangs beyond 45° introduce sag that affects accuracy. We compensate with print orientation and, where needed, support placement — but complex bridging geometries will always be more accurate in SLA.

A practical rule: if a part has a bore that accepts a standard press-fit or a fine M3 thread, specify SLA. If it's a bracket with 0.5 mm clearance holes and slot features, FDM is fine and considerably cheaper.

How does post-processing differ between the two processes?

SLA requires washing and UV curing after every print — no exceptions. Uncured resin is a skin irritant, so handling requires nitrile gloves, and wash solvent (IPA or Formlabs wash solution) needs proper disposal. After cure, SLA parts are hard, slightly brittle, and typically need support marks sanded. Surface finish straight off the Form 4 is excellent — 25-micron layers are barely visible.

FDM parts come off the bed print-ready for functional tests. Layer lines are visible (typically 0.2 mm in standard settings), so parts that need a smooth appearance get a light sand, filler primer, or acetone vapor treatment depending on material. Support removal on FDM is usually manual and leaves witness marks; for complex internal geometries, soluble support filament eliminates that entirely but adds cost.

Bottom line: SLA has more post-processing overhead but produces better out-of-box aesthetics. FDM is faster to get into service but may need more finishing for customer-facing parts.

Which process costs less, and what drives the difference?

Cost in 3D printing comes from four sources: material, machine time, post-processing labor, and setup amortized over quantity. SLA resin runs significantly higher per kilogram than FDM filament, and the wash/cure labor adds to every build. FDM filament (PETG, ASA) is cheap per gram, and machine time on a Bambu P1S at standard settings is fast — a 100-gram part takes a couple of hours.

For a single small prototype, the price gap between processes is modest. For a batch of ten medium-sized functional parts, FDM will typically be 40–60% less expensive. For parts that genuinely need SLA quality — fine details, mold masters, medical models — the premium is justified.

One common false economy: printing a structural bracket in SLA because it looks better. Resin under sustained mechanical load or repeated impact will crack where PETG flexes. Match the process to the function, not the aesthetic preference.

How do you decide in practice?

We use a simple decision tree on every job:

1. Does the part have features smaller than 1 mm or need a smooth mold-ready surface? → SLA
2. Is the part structural, load-bearing, outdoor, or larger than ~150 mm in any dimension? → FDM
3. Is this a quick functional check where appearance is irrelevant? → FDM in PETG
4. Is quantity above 10? → FDM, unless fine detail is required

When none of those rules is decisive — a medium-complexity part that could go either way — we quote both and let the customer decide with real numbers in front of them.

Not sure which process your part needs? Start an order, describe what the part is for, and we'll pick the process as part of the quote.