How each process actually works
FDM, or Fused Deposition Modeling, is the most common desktop 3D printing process. A spool of thermoplastic filament is fed through a heated nozzle that melts the material and deposits it in thin roads, one layer at a time, building the part from the bottom up. Geometry that overhangs must be supported by breakaway or soluble structures printed beneath it.
Both are additive, but the physics differ in a way that drives every downstream difference. FDM is a melt-and-extrude process where the bond between layers is the weakest direction; SLA is a light-cured photochemical process where the whole layer is fixed at once. That single distinction explains why SLA parts show finer detail and smoother walls, why FDM parts are generally tougher and more impact-resistant, and why post-processing and material selection diverge so sharply between the two. Understanding the mechanism is the first step to choosing correctly.
Surface quality and accuracy
FDM parts carry visible stair-step layer lines on any sloped or curved surface. Even at a fine 0.1 mm layer height, you can feel and see the ridges, and the nozzle leaves a characteristic rounded edge at corners and holes. Typical as-printed accuracy is around ±0.5 mm on small parts, improving to roughly ±0.2 mm with careful calibration, though dimensional drift grows with part size because the material cools and shrinks as it prints.
SLA flips the equation. The laser cures layers as thin as 0.025 to 0.05 mm with a spot measured in tens of microns, so curved surfaces read as smooth and crisp. As-printed accuracy is commonly ±0.1 mm or better, and fine text, sharp edges and small features reproduce faithfully. The trade-off is that the surface is slightly tacky until fully post-cured under UV light. When the brief calls for a presentation model, a lens-like housing, or a master pattern for casting, SLA's finish is usually the deciding factor.
Materials and mechanical properties
FDM draws on a deep bench of thermoplastics. PLA is easy and dimensionally stable but brittle; ABS and ASA add heat and UV resistance for automotive and outdoor use; PETG combines toughness with chemical resistance; and engineering grades like Nylon (PA), PC, and carbon-filled blends bring real structural performance. Because these are the same polymers used in injection molding, an FDM part can approximate the end-use material closely, which matters when you are validating how a product will actually behave.
SLA resins are a different family. Standard resins are rigid and detailed but noticeably more brittle than their FDM cousins, so a thin SLA arm can snap under bending load. The catalog has expanded, though: tough and durable resins mimic ABS-like impact resistance, flexible resins behave like rubber, and high-temperature, dental, and castable (wax-filled) grades serve specialized workflows. What you gain in surface detail you partly give back in long-term stability, because many photopolymers continue to creep and yellow when exposed to sunlight over months.
When FDM is the better choice
Reach for FDM when the part has to survive handling, drops, or mechanical load. A functional enclosure that gets assembled and disassembled, a jig that sees shop-floor abuse, or a bracket carrying weight will all favor the toughness of extruded thermoplastic. FDM also wins for large parts: a build volume measured in hundreds of millimeters is routine on a desktop machine, and the cost of the plastic spool scales gently with the volume of material used.
FDM is also the pragmatic choice when you need several iterations fast and cheap. Filament is inexpensive, machines are ubiquitous, and there is no messy liquid resin or gloves-and-goggles cleanup to slow a team down. For concept models, fit checks, and any prototype where the question is 'does it fit and roughly work' rather than 'does it look finished,' FDM delivers answers at the lowest friction and the lowest cost per hour of machine time.
When SLA is the better choice
Choose SLA when smooth, accurate surfaces are non-negotiable. Consumer product housings that must look production-ready on a shelf, jewelry and dental models where fine detail is the entire point, and master patterns for silicone molding or metal casting all depend on SLA's layer-free finish. The ability to hold tight tolerances on small features also makes SLA the right tool for snap-fit clips, threads, and fluid-tight prototypes that FDM would struggle to resolve cleanly.
SLA is also superior for complex organic geometry and very fine features. Because the part is immersed in a liquid that supports it from all sides, delicate lattices, thin walls, and interlocking mechanisms can be produced without the bulky support structures that FDM needs. Miniature figures, microfluidic devices, and highly detailed architectural or medical models are classic SLA territory. If the part is small, detailed, and must impress, SLA is almost always the answer.
Cost and the break-even point
On a pure per-part basis, FDM is cheaper for most users. Desktop FDM printers are widely available from a few hundred dollars, filament costs only a few tens of cents per gram, and there is no recurring consumable beyond the spool. SLA machines have fallen in price but still carry the ongoing cost of resin, which is several times more expensive per gram, plus the disposable build tray and isopropyl alcohol or cleaning solution used in post-processing.
However, the cost story shifts once you account for labor and finishing. An FDM part destined for a showroom may need hours of sanding, filler, and painting to hide layer lines, while an SLA part often needs only a wash and a UV cure to look finished. At low volumes where labor dominates, the 'cheap' FDM print can end up more expensive than SLA once finishing is included. The honest rule is to price the finished part, not the raw print, and to weigh how much hand work each route demands.
Build size, supports and post-processing
FDM support strategy is mechanical. Overhangs beyond about 45 degrees and bridging spans require extruded support material that is later pulled or dissolved away, leaving witness marks that often need cleanup. Large flat areas can warp as the part cools, so a heated bed and proper adhesion are essential. The workflow is forgiving, though: failed prints are easy to diagnose, and failed filament is cheap to throw away and restart. Many teams run FDM printers unattended overnight with few worries.
SLA post-processing is more involved but yields a cleaner result. After printing, the part is lifted out of the resin vat, washed in solvent to remove uncured liquid, and placed under a UV lamp to finish curing -- steps that must happen before the part reaches full strength and stability. Supports are thin and attach to a separate perforated build plate, so they snap off cleanly and leave minimal scarring. The resin itself is a skin and eye irritant, so gloves, ventilation, and careful disposal are part of the real operating cost.
Durability, moisture and end-use suitability
FDM parts made from engineering polymers can serve as end-use components in their own right. Nylon gears, ABS mounts, and PETG fixtures are found on real products and machinery because the material is proven and the part is solid through and through. The main weakness is the interlayer bond: a part loaded perpendicular to the build direction is weaker than one loaded along it, so orientation matters. With sensible design, FDM parts cross the line from prototype to production more often than SLA parts do.
SLA parts are excellent as visual and form-fit prototypes, masters, and low-stress components, but they are generally less suited to long-term structural duty. Many standard resins are brittle and degrade under prolonged UV exposure, turning yellow and losing some strength over a year or two. Specialty tough resins close much of the gap, yet they still lag extruded thermoplastics in pure durability and are slower to print in larger sizes. Treat SLA as the master-and-model champion and FDM as the functional-part workhorse.
Typical applications by industry
In product design and consumer electronics, teams lean on FDM for quick enclosure mock-ups and on SLA for the polished versions shown to investors and at trade shows. The same CAD file often runs both ways: print it rough in FDM to check assembly on Monday, print it fine in SLA to photograph on Friday. This split is the most common pattern in hardware development and explains why many studios keep both machine types on the bench.
Dental and jewelry are almost purely SLA domains, where sub-millimeter detail and smooth surfaces are the product. Manufacturing and automotive use FDM heavily for jigs, fixtures, and end-of-arm tooling that must take abuse, while turning to SLA for ergonomic handles, fluid prototypes, and casting patterns. Medical device makers use SLA for surgical guides and anatomical models. The throughline is simple: detail and appearance point to SLA, function and toughness point to FDM.
A practical decision checklist
Answer five questions before you print. Is the part going to be handled, dropped, or load-bearing? Does it need to look finished straight off the machine? Are fine details, small text, or watertight walls part of the requirement? How large is the part, and how many do you need? And how much hand-finishing time can you afford? Each answer tilts the decision. Toughness, size, and low cost favor FDM; finish, accuracy, and fine geometry favor SLA.
As a rule of thumb, default to FDM for anything functional, large, or iterative, and switch to SLA for anything that must look, seal, or resolve finely. If you are unsure, print the concept in FDM first to validate fit and mechanism at near-zero cost, then produce the presentable version in SLA once the design is locked. Few projects go wrong when they use each process for what it does best rather than forcing one machine to do everything.
How FabProto supports both processes
FabProto runs industrial FDM and SLA systems side by side, so you do not have to buy two machines or guess which is right. Send one CAD model and we will recommend the process that fits your stage and budget -- FDM for functional trials, SLA for presentation-grade parts -- and we can even produce both from the same file so you can compare them directly.
Our team handles the finishing that decides the outcome: support removal, vapor smoothing or painting for FDM, and wash-and-cure plus optional painting for SLA, all to a specified surface standard. We return measured dimensional data and a clear cost comparison per finished part, so the choice between FDM and SLA is based on the real delivered cost rather than the sticker price of the raw print. That is how teams move from prototype to a part they can actually ship.