FDM vs SLA vs SLS: An Honest Comparison From a Shop That Only Runs One of Them

A quick disclosure before anything else, because it changes how you should read this article.

We run FDM only. Two enclosed filament printers, no resin, no powder bed. So this comparison is written by a shop with an obvious interest in one of the three answers.

Which is exactly why the honest version is more useful than the flattering one. There are parts FDM shouldn't make, and if yours is one of them you're better served knowing that now than after paying us for a part that disappoints you. By the end of this article you should be able to tell which category your part falls into. And roughly a fifth of the time, the answer will be "not FDM".

The three processes in one paragraph each

FDM (fused deposition modelling) melts a thermoplastic filament and draws each layer with a moving nozzle, building the part up in stacked beads. It's the most common process, uses real engineering thermoplastics, and produces parts with visible layer lines and direction-dependent strength.

SLA (stereolithography) cures liquid photopolymer resin with light, a laser or a masked LCD, one very thin layer at a time. It produces smooth, highly detailed, dimensionally precise parts with no visible layering, in materials that are thermoset photopolymers rather than thermoplastics.

SLS (selective laser sintering) fuses powdered polymer with a laser inside a heated bed of loose powder. The surrounding powder supports the part as it builds, so no support structures are needed at all, and the parts are strong in every direction.

Strength: the difference that matters most

This is where the processes diverge, and it's the least understood distinction.

FDM parts are anisotropic. Strong along the layers, considerably weaker across them, because each layer is a thermal weld to its neighbour rather than continuous material. A well-oriented FDM part performs well; a badly oriented one snaps at a layer line. This is manageable, orientation is a design decision and we make it deliberately, but it's a real constraint.

SLS parts are nearly isotropic. Because the powder fuses fully in all directions, an SLS part behaves much more like a moulded one. For a part that will be loaded from unpredictable directions, this is a significant advantage, and it's SLS's strongest argument.

SLA parts are isotropic but brittle. The resin cures uniformly, so there's no layer-direction weakness. But photopolymers are thermosets, and most of them are noticeably more brittle than engineering thermoplastics. Standard SLA resin will shatter where a PETG part would bend. Tough and durable resin grades narrow the gap but don't close it.

In practice: for a part that has to survive real mechanical abuse over time, a well-oriented FDM part in PA-CF or polycarbonate will usually outlast a standard SLA part. For a part loaded unpredictably in three dimensions, SLS wins.

Surface finish and detail

Here FDM loses, clearly and permanently.

SLA is in a different class. Layer heights down to 25 microns, no visible layering to the naked eye, sharp fine features, and a surface that's essentially ready for paint straight off the machine. If your part needs to look moulded, SLA does it and FDM doesn't.

FDM shows its layers. At our finest setting, 0.08 mm, the lines are subtle but present. At draft settings they're obvious. Our showpiece finish sands and primes the part to hide them, but that's manual work you're paying for, not something the process gives you.

SLS has a uniform matte, slightly grainy surface, no layer lines, but a powdery texture. It looks industrial rather than refined, which suits functional parts fine.

Minimum feature size tells the same story: SLA resolves details under 0.1 mm; SLS manages around 0.5 mm; FDM with a 0.4 mm nozzle needs about 0.8 mm for a reliable wall and 0.5 mm for an embossed feature. Text below about 4 mm cap height stops being legible in FDM.

Geometric freedom

SLS is the clear winner, and this is its other major advantage. Because loose powder supports the part throughout the build, there are no support structures at all. Fully enclosed internal cavities, complex lattices, interlocking assemblies printed pre-assembled, deep undercuts, all trivial. Parts can also be nested densely in three dimensions inside the build volume, stacked above one another, which makes SLS efficient for batches.

FDM needs supports for overhangs steeper than about 50°, and those supports have to be physically removed afterwards, leaving witness marks. Our multi-material AMS improves this considerably, a breakaway support interface snaps off cleanly and leaves a smooth surface, but the fundamental constraint remains.

SLA also needs supports, and they leave small nubs that have to be sanded off visible faces. Resin parts additionally need draining considerations: a fully enclosed hollow cavity will trap uncured resin, so hollow SLA parts need drain holes designed in.

Materials

FDM uses genuine engineering thermoplastics, the same polymer families as injection-moulded parts. Our eight: PLA, PETG, ABS, ASA, TPU 95A, PLA-CF, PA-CF and polycarbonate. Service temperatures to 120 °C, genuine UV-stable options, real flexible elastomers. This is FDM's strongest argument: the material properties are well-understood and directly comparable to moulded parts.

SLA uses photopolymer resins, standard, tough, high-temperature, castable, flexible, dental and biocompatible grades. Excellent variety for specialised applications, but they're thermosets with generally lower toughness. Many are also sensitive to prolonged UV, which means unpainted SLA parts are a poor choice for outdoor use.

SLS is dominated by nylon, PA11 and PA12, sometimes glass- or aluminium-filled. Narrower material range, but nylon is excellent: tough, chemically resistant and dimensionally stable.

Cost

Broad shape of it, since the details depend heavily on part geometry:

FDM is cheapest for most parts, especially larger ones. Machines are inexpensive, filament is inexpensive, and material use is efficient. Our pricing runs roughly ₹8-16 per gram of finished part.

SLA costs more per part, driven by resin price and the post-processing labour. Every part needs washing in solvent and UV post-curing before it's finished. For very small, highly detailed parts it can be competitive.

SLS has high setup economics. Machines are expensive and the process is most efficient when the build volume is packed full, so a single SLS part is disproportionately costly while a batch of fifty is very reasonable. It's a volume process.

Post-processing

Worth mentioning because it affects lead time and cost, and because it's often overlooked.

FDM needs supports removed and edges deburred. That's the whole minimum. Optional: sanding, priming, painting, acetone smoothing for ABS and ASA.

SLA requires washing in isopropyl alcohol to remove uncured resin, then UV post-curing to reach final mechanical properties, then support removal and sanding of the nubs. Uncured resin is an irritant and requires handling precautions. This is a more involved workflow.

SLS requires depowdering, removing loose powder from every surface and cavity, usually by bead blasting. Internal channels can be difficult to clear completely.

Which process for which part

Choose FDM when:

• The part is functional and needs real engineering plastic properties
• It will experience heat above 80 °C, or live outdoors
• You need flexibility (TPU) or high toughness (PC)
• Cost matters and the part is medium to large
• You're iterating and want cheap, fast revisions
• Layer lines are acceptable or invisible in service

Choose SLA when:

• Surface finish is the point: appearance models, client presentations
• You need features below 0.5 mm
• Dimensional precision better than ±0.1 mm matters
• The part is small and detailed
• You need a castable pattern for investment casting
• The application is dental, jewellery, or another specialised resin domain

Choose SLS when:

• The part must be strong in all directions
• The geometry has enclosed cavities or complex internal structure
• You need a batch of dozens of complex parts
• Support witness marks are unacceptable but SLA is too brittle
• Nylon's properties suit the application

Where we'll send you elsewhere

Concretely, the parts we'll tell you to take to a resin or powder bureau:

• Jewellery patterns and castable models. FDM can't produce the detail, and we don't stock castable material.
• Dental and orthodontic work. Specialised biocompatible resins and precision requirements we can't meet.
• Anything needing sub-0.1 mm features: micro-fluidics, fine miniatures, small precision optics housings.
• Presentation models where layer lines would undermine the pitch and the budget doesn't allow for sanding and painting.
• Fully enclosed internal cavities that can't be drained or supported: SLS handles these and FDM doesn't.

This isn't false modesty. Roughly one enquiry in five is better served by a process we don't run, and telling someone that costs us one job and earns a customer who trusts what we say about the other four.

Where FDM wins

Equally, don't let the finish argument talk you out of FDM for a part that should be FDM.

For a functional prototype, an enclosure that lives in a cabinet, a jig or fixture, a bracket carrying load, a part that goes outdoors, or a low-volume production run, FDM is usually the right answer on cost, material properties and lead time simultaneously. The layer lines are irrelevant on a part nobody looks at, and the engineering thermoplastics are better suited than photopolymers to anything that has to survive.

The mistake is choosing a process by how the sample part looks on a desk rather than by what your part actually has to do.

Not sure?

Describe the part, what it does, where it lives, what it has to survive, and whether anyone will look at it closely. We'll tell you honestly whether FDM is right, and if it isn't, what to look for in the process that's.

That conversation is free, and the answer is sometimes "not us".