The short answer: SLA for looks and precision, FDM for function and size

Choose SLA when the part must look and feel like a finished product — fine details, smooth surfaces, crisp edges. Choose FDM when the part must work — take screws, absorb impacts, handle heat, or simply be big without breaking the budget. Both technologies are available at virtually every prototyping service, both deliver a physical part within hours or days, and the price gap is not always dramatic. Yet the parts feel completely different in hand, and the right decision depends not on which technology is "more advanced" but on what this specific prototype must prove: form, feel, function, or fit.

How each technology works

FDM (fused deposition modeling) extrudes a thermoplastic filament through a heated nozzle, laying the part down layer by layer. Common materials include PLA, ABS, PETG, and fiber-reinforced nylon, up through genuine engineering plastics with high heat resistance. SLA (stereolithography) and its resin-based cousins work on a completely different principle: a vat of liquid photopolymer is selectively cured by ultraviolet light, layer by layer, at layer thicknesses several times finer than FDM. That single physical difference explains almost every practical difference between the two. The broader map of processes — including SLS and beyond — is in rapid prototyping techniques compared.

Accuracy, detail, and surface finish

Here SLA's advantage is unambiguous. Layer heights measured in tens of microns reproduce tiny lettering, fine textures, small holes, and sharp edges, and parts come off the printer smooth to the touch with almost no finishing work. An FDM part shows visible layer lines, especially on sloped surfaces, and needs sanding, filler, and paint to look like a product. If the model is headed for a customer meeting, marketing photography, or an investor's desk, resin saves hours of hand finishing. If the model exists to confirm that the lid snaps shut properly, the difference barely matters — which is why the FDM-versus-SLA question gets a different answer on every project, and sometimes a different answer for two parts of the same product.

Mechanical strength and durability

Here the picture inverts. Most standard resins are relatively brittle, sensitive to UV light and moisture, and lose mechanical properties over weeks. A thin SLA wall can crack on its first fall from desk height. FDM parts in nylon or PETG absorb shocks far better, hold threads and screw bosses, and survive extended field testing. Their weakness is anisotropy: the bond between layers is weaker than the material itself, so print orientation often changes the outcome more than material choice does. For functional prototypes that face real users, printed parts are often combined with machined components — see CNC machining for prototypes. Material selection within FDM is its own decision, covered in ABS vs PLA.

Cost, speed, and size limits

FDM printers offer large build volumes and cheap material, so a full-size enclosure or a shoebox-scale chassis will almost always be printed that way. Resin costs more per unit volume, build volumes are smaller, and the process adds washing and post-curing steps that take time and labor. On the other hand, when the job is many small, precise parts on one build plate, SLA can win on cost per part. Realistic budget figures are in 3D printing services for prototypes and what a prototype costs. And when a part needs true metal properties, neither process applies — that is the territory of metal 3D printing.

How we choose in practice

  • Appearance model, marketing mockup, or an optically clear part: SLA.
  • A part that must take loads, screws, repeated clicks, or heat: FDM in an engineering material.
  • Large enclosure or structural frame: FDM, often with internal reinforcement.
  • A batch of small, precise parts for testing: SLA.
  • Quick early fit check: whichever is faster and available — and that is perfectly fine.

On most projects the honest answer ends up being both, at different stages: an early functional print to test the mechanics, and a polished appearance model for presentations. More guides live in our 3D printing hub.

Not sure which technology fits your part — or want an engineering review before you send a file to print? Projects House will go over your design, recommend a process and material, and quote the prototype. Contact us through the form and we will come back with a practical recommendation.