A 0.2 mm press-fit that works on your CAD screen but binds on the bench is not a “printer problem.” It is an accuracy stack-up problem – process physics, material behavior, orientation, and post-processing all compounding into a part that misses the functional intent.
If you are choosing between stereolithography (SLA) and fused deposition modeling (FDM), accuracy is often the deciding factor – but only if you define accuracy correctly. Engineers usually mean some mix of dimensional tolerance, feature resolution, surface-driven fit, and repeatability across builds. This article breaks down SLA vs FDM accuracy in a way that maps to real requirements: holes that need to accept pins, housings that must close, threads that must engage, and jigs that must locate.
What “accuracy” means on a print
Accuracy is not a single number, and vendor datasheets rarely tell the whole story.
Dimensional accuracy is how close a printed dimension is to nominal. Repeatability is how consistently that dimension lands across multiple parts and builds. Resolution is the smallest feature the process can reliably create. Surface finish affects functional accuracy because a rough surface can effectively reduce a hole diameter or change a sliding fit even if the “measured” dimension is close.
For SLA and FDM, the process can be capable of tight results, but the part must be designed and produced in a way that controls the dominant error sources. The gap between “possible” and “predictable” is where many prototypes fail.
SLA vs FDM accuracy: what drives the differences
SLA cures liquid photopolymer resin with light. FDM extrudes molten thermoplastic through a nozzle. That single difference changes the dominant error mechanisms.
With SLA, you are managing polymerization shrinkage, support interaction, peel forces, and post-cure effects. The part can capture crisp detail and smooth surfaces, but dimensions can shift during curing and post-processing if the geometry is thin, asymmetric, or poorly supported.
With FDM, you are managing bead geometry, thermal contraction, layer adhesion, and anisotropic warpage. The process is mechanically straightforward and materials are often more “engineering-grade” for functional testing, but the extruded road width, layer stair-stepping, and heat history limit fine features and can distort long flat spans.
The practical result is that SLA typically wins on small features, sharp edges, and surface-dependent fits. FDM can be very acceptable on larger features and robust geometries, especially when the design is tolerant to small dimensional drift and the material selection is appropriate.
Typical tolerance expectations (and how to read them)
Most teams want a clean rule of thumb. The honest answer is: it depends on part size, geometry, and what dimensions are critical. Still, you can set expectations.
SLA is commonly selected when you need tight visual and dimensional control on small-to-medium parts: thin walls, fine embossing, small bosses, and complex cosmetic surfaces. In controlled production workflows, SLA can hold tight tolerances on critical features, but you should assume you will need compensation or secondary ops for truly precision fits.
FDM is commonly selected when you need a functional polymer with decent accuracy on larger, less delicate features: brackets, covers, fixtures, and quick test assemblies. You can achieve good dimensional results, but fine holes, thin pins, and sharp internal corners will show the limitations of nozzle diameter and layer height.
Instead of chasing a single number, specify tolerances by function:
If it must locate, seal, snap, or mate with a purchased component, call that out as a critical feature and expect the process plan to revolve around it. If it is cosmetic or clearance-driven, relax the tolerance and let the process do what it does best.
Feature-level reality: holes, pins, threads, and walls
Accuracy problems tend to cluster around a handful of features.
Holes are the classic failure point. Both SLA and FDM often produce holes undersized relative to nominal due to process artifacts and surface effects. SLA holes can come out cleaner and closer to round, but resin cure and support touchpoints can still bias diameter. FDM holes are strongly affected by layer stepping and the tendency for extruded roads to “cut the corner,” especially on small diameters.
If the hole must be accurate, plan on drilling/reaming after printing, or design the hole as a pilot that is intentionally undersized for a finishing operation. This is one of the most reliable ways to turn additive geometry into predictable fit.
Pins and small bosses favor SLA when the features are near the lower limit of what can be printed. FDM can produce pins, but the nozzle diameter and layer adhesion will limit how slender they can be before they become fragile or dimensionally inconsistent.
Threads are a mixed case. SLA can print fine threads with good surface finish, but photopolymers may not survive repeated assembly if high torque is involved. FDM can print robust threads in engineering thermoplastics, but small thread forms can be rough and may require chasing with a tap or using heat-set inserts. If the assembly matters, inserts are usually the most repeatable solution.
Thin walls highlight the trade-offs. SLA can form thin, consistent walls with good surface quality, but they can warp during post-cure if the part is unbalanced. FDM thin walls can be limited by extrusion width and can become porous or inconsistent if you try to go too thin for the nozzle.
Orientation and supports: accuracy is built-specific
In both processes, orientation is an accuracy decision, not just a support decision.
For SLA, surfaces facing supports risk small witness marks, and large flat areas can show subtle distortion due to peel forces during the layer separation step. If a sealing surface or datum face matters, you typically want it oriented to minimize support contact and to reduce peel-induced stress. Post-cure can also move dimensions slightly, so consistent curing practices are part of repeatability.
For FDM, the Z-axis often has different behavior than X/Y due to layer stacking and thermal gradients. Tall, thin parts can lean or show Z banding. Long flat parts can warp off the build plate if the thermal contraction is not well managed. If a feature must be true, orient it so the most critical dimensions are in the plane that the machine controls best and where warpage is least likely.
Supports themselves affect accuracy. Removing supports can deform small features, and support scars can change functional surfaces. When teams say “the print was accurate but it still didn’t fit,” it is often because the surface that actually touches the mating part is not the surface they dimensioned in CAD.
Material behavior: accuracy over time and under load
Accuracy is not only “as printed.” It is also what happens after the part sits on a bench, warms up, or is assembled.
SLA resins can be dimensionally stable for many applications, but they are photopolymers with specific mechanical and thermal limits. Some resins can creep or become brittle over time, and post-cure is required to reach final properties – which can slightly change dimensions.
FDM thermoplastics often behave more like production plastics. That is a benefit for functional testing, but thermoplastics can have higher thermal expansion and can warp if exposed to heat. If you need an accurate gauge or fixture used near elevated temperatures, material selection and annealing strategy matter as much as the printer.
If your goal is metrology-grade stability, neither SLA nor standard FDM may be the best answer. You may be better served by SLS, HP Multi Jet Fusion, CNC machining, or a hybrid approach where you print for form and machine the datums.
When SLA is the better accuracy choice
SLA is usually the right tool when your “accuracy” requirement is dominated by small features and surface-driven fit. Enclosures with tight seams, cosmetic prototypes that must look injection-molded, microfluidic-like channels (with process-appropriate constraints), and parts that need sharp lettering or tiny bosses are typical examples.
SLA also reduces the ambiguity created by coarse layer lines. If you are trying to validate a snap feature, a thin latch geometry, or a press fit where surface texture would otherwise dominate, SLA’s smoother surfaces can make the test more meaningful.
The caveat is that SLA accuracy is tied to disciplined post-processing. Wash time, post-cure time, support strategy, and handling all influence final dimensions. If you need consistent outcomes across multiple iterations, those steps must be standardized, not left to ad hoc bench practices.
When FDM is accurate enough – and the smarter call
FDM is often the better choice when your part is mechanically functional and your tolerances are realistic for prototype plastics: brackets, guards, housings with clearance, test fixtures, and jigs where you can machine or insert critical interfaces.
If you need material performance closer to end-use thermoplastics, FDM can give you a more relevant mechanical test than standard SLA resins. For assemblies, FDM also supports design patterns like oversized holes plus inserts, captive nuts, and machined datum pads, which can turn “prototype-grade accuracy” into “assembly-grade accuracy.”
FDM also scales well for larger parts where SLA would be slower, more support-intensive, or more expensive. On a large enclosure, the accuracy risk is often warpage and flatness, not micro-resolution. That is a design and process control problem that FDM can handle when planned properly.
How to specify accuracy so you get parts that fit
The fastest way to improve outcomes is to stop asking for “high accuracy” and start communicating functional intent.
Call out the 2-5 features that matter, such as a bearing seat, a locator pin pattern, a gasket land, or a sliding rail. Provide the mating part information if relevant – nominal dimensions, material, and the type of fit you are expecting. If the part will be post-processed (sanded, painted, vapor-smoothed, tapped), treat that as part of the tolerance plan, because finishing changes dimensions.
Also decide early whether you will accept additive-only results or whether secondary operations are allowed. A single drilled hole or faced datum can turn a print into a reliable functional prototype without changing the overall workflow.
If you are using an instant-quote workflow, upload the native geometry (STEP when possible) and add notes on critical features. A good manufacturing partner will respond with manufacturability guidance rather than simply accepting the order and letting variability show up in your assembly test.
At Additive3D Asia, this is typically handled as a process selection and tolerance conversation: identify the datums and interfaces that must be controlled, then choose SLA, FDM, or another in-house process plus post-processing to hit the requirement with repeatable QC.
The decision lens: accuracy is a system, not a printer
If you only compare SLA vs FDM accuracy by layer height or marketing specs, you will miss the variables that actually decide whether your part fits. Accuracy is created by controlling shrink, warp, orientation, supports, and post-processing – and by being explicit about what matters.
A helpful closing thought for engineers moving fast: treat your first print as a tolerance probe. Put your tightest interface features on that build, measure them, and then decide whether to compensate in CAD, add a secondary operation, or switch processes. That small discipline is often the difference between one iteration and three.