KEMI Docs · 3D printing practice
SLA vs FDM — Choosing by Precision, Strength, Finishing and Cost
Last updated: 2026-09-16
The same STL printed in SLA gives a smooth precision model; in FDM, a tough functional part. Neither is better — they serve different jobs. This article compares the two on six criteria and gives verdicts for the uses we quote most often: enclosures, gears, jigs and show prototypes.
1
Six criteria
| Criterion | SLA (UV resin) | FDM (filament) |
|---|---|---|
| Dimensional accuracy | ±0.05-0.1 mm | ±0.2-0.3 mm (XY), 0.2 mm layers |
| Surface | layers barely visible, paint-ready | layer lines, needs sanding/filler |
| Strength/toughness | standard resin is brittle, tough resin ≈ ABS | PLA stiff; PETG, ABS, nylon tough |
| Heat | standard 60°C, high-temp resin 120°C+ | PLA 55°C, PETG 75°C, ABS 95°C, nylon 120°C |
| Max size | typically ~200×120×200 mm | 250-300 mm, large machines 400 mm+ |
| Cost feel | good for small precision parts, rises steeply with volume | cheapest per volume, best for large parts |
2
Verdict by use
| Use | Recommended | Why |
|---|---|---|
| Electronics enclosure (prototype) | FDM (PETG) | survives repeated screws and snap-fits, any size |
| Enclosure (display/appearance) | SLA + paint | looks injection-moulded with no layer lines |
| Gears, ratchets, snap hooks | FDM (nylon/PETG) or SLA tough resin | standard resin snaps under impact |
| Assembly and test jigs | FDM (PLA/PETG) | dimensionally stable, cheap, fast, reprint when worn |
| Small precision parts (covers, buttons) | SLA | 0.5 mm walls, small text, curved surfaces |
| Clear windows, light guides | SLA clear resin + polishing | FDM clear is translucent at best |
| Hot parts (motor brackets) | FDM ABS/nylon | standard SLA resin deforms at 60°C |
3
What each process asks of the design
- Minimum wall: SLA 0.5 mm (1.0 recommended); FDM with a 0.4 nozzle 0.8 mm (1.2 recommended, two perimeters).
- Holes come out ~0.1 mm small in SLA and 0.2-0.3 mm small in FDM. Add clearance for fits or plan to drill afterwards.
- Support marks: SLA leaves dots (one pass of sandpaper), FDM leaves patches. Orient so visible faces do not face down.
- Hollow shapes: SLA needs drain holes (3 mm+); FDM handles interiors with infill automatically.
- Screws: heat-set inserts in both. FDM tolerates a few M3 tapped cycles; standard SLA resin cracks when tapped.
4
Finishing and lead time
SLA requires IPA washing and post-curing, so even a same-day print adds a day. One pass of 400-grit before paint is enough. FDM parts are usable immediately, but appearance work runs 240 → 400 grit → filler → primer. Lead-time feel for small single parts: FDM 1-2 days, SLA 2-3 days, add 2 days for paint.
KEMI's decision order
① functional → FDM, visible appearance → SLA ② above 80°C → FDM ABS/nylon ③ over 200 mm → FDM ④ walls under 1 mm or small text → SLA. Four questions settle 90% of cases.
⚠ When neither is right
Above about 50 units, or when injection-grade strength and surface are required, look at silicone moulding (vacuum casting) or CNC. 3D printing is the tool for 1-30 pieces.
FAQ
Resin is said to be harmful — are finished parts?
Uncured resin irritates skin, but washed and post-cured parts are stable. Food contact or prolonged skin contact needs specifically certified resins.
Do FDM parts leak water?
They can, through micro-gaps between layers. Use 3+ perimeters and 100% infill, or seal with epoxy. If sealing is critical, SLA is the better choice.
Can I get both quotes from one STL?
Yes. Upload to the quote console for an instant FDM estimate; SLA is calculated separately from volume and size and returned by reply.
See also
This article summarises the working rules KEMI uses in real design and fabrication work. For production, part datasheets and fab specifications take precedence.