FDM vs. SLA
Compare filament FDM and cured-resin SLA for a real part: feature detail, surface, material behavior, support workflow, finish and inspection.
Choose FDM or SLA from the delivered part
FDM deposits thermoplastic roads; SLA cures liquid resin into a crosslinked part. Those mechanisms change surface, support workflow, material selection and how a part should be tested.
FDM
FilamentA heated nozzle lays down thermoplastic, one toolpath at a time.
Design check: Plan layer direction and access to remove supports.
SLA
Liquid resinLight cures resin into a solid layer. A bottom-up system is shown here.
Design check: Plan support contacts, washing and the specified cure.
Process principles shown in cutaway. Energy colours are schematic. Supports, hardware and layer thickness are simplified; machine configurations vary.
FDM
Thermoplastic route
Often a practical choice for fixtures, enclosures and large concept parts when a filament grade and visible layer texture fit the requirement.
SLA
Cured-resin route
Often a strong candidate for fine features, smooth cosmetic surfaces, patterns and specialized resin properties when wash/cure is acceptable.
Neither
Know when to compare another route
A loaded nylon mechanism may point to SLS or MJF; a precision metal interface may point to CNC; a stable high-volume plastic part may point to molding.
The visible difference is only one of the trade-offs
Surface appearance matters, but it should not hide the more consequential questions: material grade, cleaning and curing, supported surfaces, environmental exposure and actual load.
| Topic | FDM | SLA |
|---|---|---|
| Part mechanism | Extruded thermoplastic roads form the part; toolpath and interlayer bonding matter. | A light-cured resin forms the part; resin chemistry and post-cure matter. |
| Surface | Layer texture and support contact can be visible, especially on curves and undersides. | A smoother as-built surface can suit detailed visual parts; support contact and finishing still need planning. |
| Materials | Select a specific filament such as PLA, PETG, ABS/ASA, PA, PC or TPU. | Select a specific engineered resin; do not infer behavior from the generic word “resin.” |
| Post-processing | Support removal and cosmetic cleanup may be needed; no resin washing/cure step is inherent. | Washing and post-curing are part of the manufacturing route, with support removal and possibly sanding or coating. |
| Internal voids | Infill and shell strategy can tune mass, but a visible CAD cavity needs an intentional toolpath. | Parts may be solid or hollowed; hollowing adds drainage, cleaning and cure-access requirements. |
| Critical service | Validate anisotropy, heat, creep and adhesion for the named filament/process. | Validate post-cure, environment, brittleness or toughness, and the named resin/process. |
Put the critical feature before the general body
A small connector, seal face, thread, snap arm or optical surface can decide the route even when the rest of the part is simple.
- Fine positive and negative detail: Screen SLA first, then inspect the actual resin and support location. A nominally fine feature still needs cleaning and handling margin.
- Large functional enclosure: Screen FDM with a specific engineering filament, then assess wall toolpaths, warpage, inserts and cosmetic faces.
- Watertight or fluid-facing concept: Specify the pressure, medium, finish and test. A dense-looking surface or high infill setting does not prove a finished seal.
- Threaded assembly: Select the fastener or insert before the process. Confirm pilot, boss, installation and torque using the actual printed material.
Treat finishing as part of the process choice
Sanding, coating, painting, washing and curing can change appearance, dimensions and handling. The drawing should state which faces are cosmetic, sealed, bonded or dimensionally critical after finishing.
Dye changes colour
Dye penetrates accessible polymer surfaces. Base colour, material and cleaning influence the result; a dyed surface is not the same as pigmented bulk material.
Smoothing changes texture
A compatible smoothing process modifies the outer surface. Protect fits and fine details; qualify the final dimensions and function.
Coating adds a layer
Primer and paint add material to exposed surfaces. Specify masking and final fit requirements before coating.
Dyeing and chemical smoothing apply only to compatible materials. They do not replace the specified SLA resin wash and cure steps; coatings and mechanical finishing need their own review.
| Requirement | Decision to make | Risk to avoid |
|---|---|---|
| Show surface | Mark the face and permitted support/contact marks. | Selecting a process for its surface then placing supports on the visible face. |
| Colour match | Define substrate, coating system and approval sample. | Assuming an available filament or resin colour matches the final appearance. |
| Tight fit | State finished dimensions and masked surfaces. | Sanding or coating a mating interface without an allowance or inspection plan. |
| Cured resin performance | Name wash/cure method and environmental condition. | Comparing an uncured or differently cured coupon with a delivered part. |
Buy a comparison sample that answers the design question
If both routes look feasible, make the same geometry in the actual candidate material and finish. Inspect the critical feature after the full workflow—not directly after printing.
Learn from a first article
Use representative geometry, material and finishing to verify the properties that matter before repeating the order.
Control the production recipe
Record material product, build settings, orientation, finishing and inspection criteria. Confirm changes before the next batch.
Schematic illustrations; not to scale. Confirm the design for the chosen material, process and final use.
See the dedicated FDM design guide and SLA design guide for process-specific DFM checks. Use CNC when a critical interface needs machined material behavior or a precision finish.
Frequently asked questions
Is SLA always more accurate than FDM?
No universal accuracy ranking applies to every printer, material, geometry and finish. SLA often suits fine detail and smooth surfaces, while a functional FDM design can meet its own specification when the toolpath, material and orientation are selected deliberately.
Is SLA resin stronger than FDM filament?
Strength depends on the named resin or filament, the print process, orientation, post-processing and service environment. Compare the property that governs the design—such as impact, heat, creep, tensile, tear or stiffness—under relevant conditions.
Can SLA replace injection molded plastic?
SLA can be useful for visual and functional prototypes, patterns and some end-use applications. It does not automatically reproduce the polymer family, durability, molding finish or regulatory basis of an injection-molded part.
Which process is better for a cosmetic prototype?
SLA is often screened first for fine detail and smoother as-built surfaces. Confirm support location, the specific resin, wash/cure workflow and the final coating or color requirement before deciding.
Sources and further reading
- Formlabs — when to choose SLA vs FDMManufacturer comparison; material and workflow claims are process-specific.
- Formlabs — FDM, SLA and SLS comparisonProcess workflow, materials and finishing are formulation- and printer-specific.
- HP — introduction to 3D printing processesOverview of additive-process categories.
- Protolabs — selecting a manufacturing processEstablished process operator guidance; supplier figures are not universal.
Specify the details that make the part work.
Upload your CAD and drawing with the material, quantity, finishing and critical dimensions. Confirm the manufacturing route before production.