Process guide
FDM vs SLA 3D printing: which is right for your project?
FDM and SLA are the two 3D printing processes most projects come down to. They make parts in completely different ways, and the right pick depends on whether you're chasing strength and cost or surface finish and fine detail.
The short answer
Choose FDM for functional parts, jigs, brackets, enclosures, and anything that needs to bear load or survive heat — especially when you care about cost per part. Choose SLA when the part is small, highly detailed, or has to look injection-moulded straight off the printer. Our 3D printing service runs on production FDM because it wins on price, material choice, and durability for almost every real-world use case.
How each process works
FDM (Fused Deposition Modeling) melts a thermoplastic filament — PLA, PETG, ABS, TPU — and lays it down layer by layer. The part is solid engineering plastic once it cools.
SLA (Stereolithography) cures liquid photopolymer resin with a UV laser or LCD, one thin layer at a time. The part comes out of a bath of resin and needs washing and post-curing.
Strength and durability
FDM parts made from real engineering plastics (PETG, ABS, nylon) are tough, impact-resistant, and hold up to sun, heat, and repeated use. SLA resin parts are typically stiffer but more brittle — most standard resins crack under impact and degrade in sunlight. For anything mechanical, load-bearing, or outdoors, FDM is the safer bet.
Surface finish and detail
This is where SLA wins. SLA resolves features down to a fraction of a millimetre and comes off the printer with a smooth, near-glossy finish — ideal for miniatures, dental models, jewellery masters, and tiny mechanical assemblies. FDM shows visible layer lines that need sanding, filler, or acetone smoothing (for ABS) to hide.
Material variety
FDM has a huge material library: PLA for prototypes, PETG for weatherproof functional parts, ABS for heat resistance, TPU for flexible parts, plus carbon-fibre and glass-fibre reinforced blends. SLA is limited to resins — standard, tough, flexible, high-temp, castable — but you can't get true engineering thermoplastics.
Cost and lead time
FDM is cheaper per cubic centimetre and scales well for larger parts and batches. SLA parts carry the cost of resin plus mandatory post-processing (wash, cure, support removal), which adds labour and time. For a functional bracket or enclosure, FDM often lands at a fraction of an SLA quote.
Quick comparison
| Property | FDM | SLA |
|---|---|---|
| Material | Real thermoplastics (PLA, PETG, ABS, TPU) | UV-cured resins |
| Surface finish | Visible layer lines | Smooth, near-injection-moulded |
| Detail | Good to ~0.4 mm features | Excellent, sub-0.1 mm |
| Strength | Tough, impact-resistant | Stiff but brittle |
| Heat resistance | Up to ~100 °C with ABS | Standard resin softens ~50 °C |
| Cost per part | Lower | Higher (resin + post-processing) |
| Best for | Functional parts, enclosures, jigs | Miniatures, dental, jewellery |
Dimensional accuracy & tolerances
Accuracy is usually the first concern when a part has to mate with something else — holes for bearings, slots for rails, threads, snap fits, or embossed text. FDM and SLA deliver very different tolerance budgets, so it is worth designing for the process rather than hoping the printer will compensate.
FDM tolerances on a well-tuned production machine are roughly ±0.2 mm, with a practical minimum feature size around 0.4 mm. That budget is driven by nozzle diameter, layer height, belt tension, bed levelling, and material shrinkage. Holes printed vertically tend to come out slightly undersized, and dimensions that cross layers are more stable than dimensions measured across the build plate. For press fits, aim for 0.2–0.3 mm of interference on small diameters; for clearance fits, add 0.2–0.4 mm. Always allow for shrinkage — about 0.5–1% for PLA and PETG, 1–2% for ABS.
SLA tolerances are tighter, typically ±0.05 mm with features resolving below 0.1 mm. That makes SLA the better choice for fine mechanical assemblies, jewellery masters, dental models, and tiny snap fits. The trade-off is resin shrinkage during cure (often around 2%), support nubs on mating faces, and the risk of distortion in thin walls during post-curing. For sliding fits use 0.05–0.1 mm clearance; for light press fits use 0.1–0.2 mm interference. Orient support-heavy surfaces away from mating areas and print a small test gauge before committing to a full batch.
For a deeper walkthrough — including tolerance tables, shrinkage allowances, and print settings for millimetre-sized parts — see our 3D printing tolerances guide.
Which should you order?
If the part has to work — hold a load, survive heat, live outside, or clip onto something — order it in FDM. If it has to look perfect at a small scale and never sees real force, SLA is worth the premium. For the vast majority of engineering, product, and hobby projects, FDM is the right call.
Not sure which material to pick? Read the full materials guide or compare specific filaments like PLA vs PETG and PLA vs ABS.
Frequently asked questions
What's the difference between FDM and SLA 3D printing?
FDM (Fused Deposition Modeling) melts thermoplastic filament and lays it down layer by layer, producing durable parts in real engineering plastics like PLA, PETG, ABS, and TPU. SLA (Stereolithography) cures liquid photopolymer resin with UV light, producing smoother, more detailed parts but in more brittle materials.
Is FDM or SLA more accurate?
SLA is more accurate for small features and fine detail — typical tolerances are around ±0.05 mm, versus ±0.2 mm on a well-tuned production FDM printer. For most mechanical parts, brackets, and enclosures, FDM's accuracy is more than sufficient and cheaper.
Which is stronger, FDM or SLA parts?
FDM wins on strength and impact resistance when printed in engineering plastics like PETG, ABS, or nylon. Standard SLA resins are stiffer but brittle — they tend to crack under impact and degrade in UV. For load-bearing or outdoor parts, choose FDM.
Is SLA more expensive than FDM?
Yes. SLA costs more per cubic centimetre because resin is pricier than filament and every part requires wash, cure, and support-removal labour. FDM scales more cheaply for larger parts, thicker walls, and small batches.
When should I choose SLA over FDM?
Choose SLA when the part is small, highly detailed, and appearance-critical — miniatures, dental models, jewellery masters, or fine mechanical assemblies. For anything functional, structural, heat-exposed, or cost-sensitive, choose FDM.
How do FDM and SLA compare for dimensional accuracy?
SLA is the more dimensionally accurate process, typically holding ±0.05 mm on small, well-supported features. Production FDM is closer to ±0.2 mm, and accuracy depends on nozzle size, layer height, print orientation, and material shrinkage. For precise snap fits, threaded holes, or mating parts, SLA gives you more headroom; for brackets, enclosures, and structural parts, FDM accuracy is usually sufficient.
What post-processing effects should I expect from FDM and SLA?
FDM parts show visible layer lines and may need sanding, filler primer, painting, or acetone smoothing (ABS only) for a finished look. SLA parts come off the printer smooth and detailed, but they require washing and UV curing, and support points can leave small nubs. SLA resin also yellows and becomes more brittle with prolonged UV exposure, so functional outdoor parts need coating or should be printed in FDM.
How should I design tolerances for FDM prints?
Design holes about 0.2–0.4 mm larger than the mating feature and pins or shafts about 0.2 mm smaller. For press fits, aim for roughly 0.2–0.3 mm of interference on small diameters. Keep critical dimensions in the XY plane rather than across layers, and size vertical features in multiples of your layer height. Allow for material shrinkage — roughly 0.5–1% for PLA and PETG, and 1–2% for ABS.
How should I design tolerances for SLA prints?
SLA tolerances can be tighter: use 0.05–0.1 mm clearance for sliding fits and about 0.1–0.2 mm interference for light press fits. Account for resin shrinkage during cure — often around 2% — and the fact that post-curing can slightly distort thin walls. Orient support-heavy surfaces away from visible or mating faces, and add test-fit gauges before committing to a full batch.
What are typical tolerance ranges for FDM and SLA?
On a well-tuned production FDM printer, expect dimensional tolerances around ±0.2 mm, with a minimum feature size near 0.4 mm. SLA is tighter, typically holding ±0.05 mm and resolving features below 0.1 mm. These are practical ranges for real-world prints; tighter claims require specialised resin, calibration, and post-processing control.
What affects dimensional accuracy in FDM and SLA?
For FDM, accuracy is driven by nozzle diameter, layer height, print orientation, belt tension, bed levelling, and material shrinkage. Thin walls printed across layers or tall vertical holes often come out undersized. For SLA, accuracy depends on resin shrinkage during cure, exposure settings, support placement, peel forces between layers, and whether the part is washed and cured evenly.
How do I choose print settings for millimetre-sized parts?
For tiny FDM parts, use a 0.25 mm nozzle, 0.08–0.12 mm layer heights, slow outer walls, and thin-wall detection so small features are not skipped. Print multiple copies at once to let each layer cool. For millimetre-sized SLA parts, choose a high-resolution resin, orient to minimise support scarring on mating faces, and use light touchpoints. Always print a small test gauge in the same orientation before a full batch.