FDM, SLA, and SLS are the three most common 3D printing technologies, and they produce parts with very different precision, surface, strength, and cost. FDM extrudes melted plastic through a nozzle, SLA cures liquid resin with a laser, and SLS fuses nylon powder with a laser, so the choice is not "which printer" but "what the part must prove." This guide compares the three processes across accuracy, surface finish, materials, mechanical behavior, and cost, and ends with a selection rule for functional prototypes and low-volume parts.
How Each Process Builds the Part
FDM, fused deposition modeling, feeds thermoplastic filament through a heated nozzle and lays it down layer by layer. The part is built from visible layer lines, and its strength depends on the bond between layers, which is weaker than the solid material in the vertical direction. FDM machines range from desktop units to industrial systems, and the process is the lowest-cost entry into 3D printing.
SLA, stereolithography, cures liquid resin point by point with a laser or light source. The resin solidifies into a dense, smooth part with fine detail, and the surface is the best of the three at small feature sizes. SLA resins vary from brittle general-purpose materials to tough and castable grades, and the process needs support structures that leave small marks to be removed.
SLS, selective laser sintering, fuses nylon powder with a laser, layer by layer, inside a heated chamber. Because the powder supports the part during printing, SLS needs no support structures, and the parts are strong, durable, and consistent, with a matte, slightly porous surface. The trade-offs are the powder handling, the cost, and a coarser surface than SLA.
| Factor | FDM | SLA | SLS |
|---|---|---|---|
| Accuracy | Moderate | High | Moderate to high |
| Surface finish | Visible layer lines | Smooth, fine detail | Matte, slightly porous |
| Typical materials | ABS, PLA, PETG, nylon | Standard and engineering resins | Nylon, nylon composites |
| Mechanical strength | Good in-plane, weaker between layers | Good, resin-dependent | Strong and consistent |
| Support structures | Yes | Yes | No |
| Cost per part | Low | Moderate | Higher |
The table describes tendencies; the specific machine, material grade, and part geometry move every number.
Accuracy and Surface Finish
For fine detail, SLA is the leader. The laser cures resin at high resolution, so small text, sharp edges, and smooth curved surfaces print cleanly, and the part can be sanded and painted to a near-injection-molded look. FDM is limited by the nozzle diameter and layer height, so small features and curved surfaces show stepping, though industrial FDM machines narrow the gap.
SLS sits between the two. The powder process holds good dimensional accuracy, but the surface is matte and slightly porous from the sintered powder, which suits functional parts better than cosmetic ones. A cosmetically critical part is usually SLA or FDM with post-processing; a part that must survive handling is often SLS.
The accuracy callout should match the process: a ±0.1 mm feature that is routine in SLA may need post-machining in FDM, and the drawing should mark the features that actually matter rather than applying one tolerance to everything.
Materials and Mechanical Behavior
The material family is where the three processes separate most.
FDM runs common engineering thermoplastics, including ABS, PETG, nylon, and PC blends, and the layer-bond weakness means strength is directional. For parts loaded in the print plane, FDM nylon and PC can be quite strong; for parts loaded across layers, the part fails at the layer bond before the material limit.
SLA resins span a wide range. General-purpose resins are brittle and suited to form and fit, while engineering and tough resins approach ABS-like performance, and castable resins support investment casting patterns. The resin's datasheet, not the process name, sets the mechanical story.
SLS nylon is the strongest and most consistent of the three for functional parts. The fused powder produces parts with uniform properties in all directions, good impact resistance, and enough durability for hinges, clips, and housings, and glass-filled nylon grades add stiffness for structural use. The material range is narrower than FDM or SLA, but within it, the behavior is the most production-like.
Cost and Lead Time
Cost follows machine time and material. FDM is the cheapest per part and the fastest to a simple prototype, which makes it the default for early iterations. SLA costs more per part but produces a better surface and finer detail, and it is the route when the part must look finished. SLS is the most expensive per part of the three, but the lack of supports and the material consistency often make it the best value for functional parts that would otherwise need multiple FDM iterations.
Lead time is similar across the three for small parts, measured in hours to a day, and the schedule driver is usually post-processing: removing supports from SLA, smoothing FDM layers, or cleaning powder from SLS. A part that needs painting adds a finishing step to any of the three.
Which Process for Which Part
Use the selection rule in order:
- If the part must survive handling, flexing, or repeated assembly, start with SLS nylon; it is the most functional of the three without a machining step.
- If the part is cosmetic, fine-detail, or a master for casting, choose SLA; the surface and detail justify the cost.
- If the part is a cheap geometry check or an early iteration, choose FDM; the cost per iteration is the lowest.
- If the part must hold a tight tolerance that printing cannot guarantee, machine it instead, and use the printed part only for the geometry check.
The rule keeps the processes in their bands. A hinge that must flex is SLS, a display bezel mock is SLA, and a bracket that will be replaced by a machined part is FDM.
Common Misconceptions
- SLA is always more accurate. SLA wins on fine detail and surface, but SLS holds dimensional accuracy well and produces stronger parts. Accuracy depends on the feature and the machine, not the process name.
- FDM parts are weak. FDM nylon and PC are strong in the print plane; the weakness is directional, across the layer bond, and the design should orient the part so loads sit in the strong direction.
- SLS needs support removal. SLS needs no supports at all, because the powder supports the part; the cost is powder handling and a matte surface.
- The cheapest process is the best value. For a functional part, one SLS part that works beats three FDM iterations that fail, so the comparison is cost per working part, not cost per print.
Decision Rule
Print FDM for cheap geometry iterations, SLA for fine detail and cosmetic parts, and SLS for functional parts that must survive handling. Match the material grade to the load, and machine the features that printing cannot hold. The decision rule is cost per working part, not cost per print, which is why a 3D printing partner that runs all three can quote the same drawing across routes.
FAQs
Which 3D printing process has the best resolution?
SLA has the finest detail and smoothest surface because the laser cures resin at high resolution. SLS holds good dimensional accuracy with a matte surface, and FDM shows layer lines that limit small features, so the choice depends on whether the part is cosmetic or functional.
Is SLS stronger than FDM?
Generally yes for functional parts. SLS nylon has consistent, near-isotropic properties and good impact resistance, while FDM strength is directional and weakest across the layer bond. In the print plane, FDM nylon and PC are competitive, but SLS is the safer choice for parts that flex or take repeated load.
Do all three processes need support structures?
No. FDM and SLA need supports for overhangs, and removing them leaves marks and adds time. SLS needs no supports because the powder bed holds the part, which is why SLS suits complex internal geometry and nesting multiple parts in one build.
Which process is cheapest for prototypes?
FDM is the cheapest per part and fastest to a simple prototype, making it the default for early iterations. SLA costs more for better surface and detail, and SLS costs the most per part but often wins on cost per working part for functional designs. The comparison should be cost per working part, not cost per print.
Sources
- 6CProto 3D Printing Services
- 6CProto 3D Printing Materials
- 6CProto Rapid Prototyping Services
- ISO/ASTM 52900:2021 – Additive manufacturing general principles
- ISO 9001:2015 – Quality management systems