SLS vs SLA vs SLM: How to Choose
An SLS versus SLA versus SLM comparison usually turns into a list of numbers nobody can decide from. Here is a better way: three questions that eliminate two of the three processes, then the numbers that actually separate the survivors — and an honest account of which process is wrong for which part.
Every specification on this page is the published figure from our own process pages — the ones we hold on a calibrated industrial line. They are typical values we work to, not theoretical limits, and tighter values are often possible on specific features. If you are designing to a hard limit, ask our engineers before you commit: the quote form reaches them directly.
What is your part made of, and does it have to be metal?
This eliminates more than any other question, so it goes first.
It has to be metal, or it has to survive heat, load or corrosion that plastic cannot. Then the only real choice is between metal processes, and for most parts that is SLM. The alternative metal route, binder jetting, is a different conversation about machine investment rather than a different material.
It is a prototype, a concept model, or something that needs detail rather than strength. Then look at SLA first. Resin gives the finest surface of the three and the smallest minimum feature size, and it is the fastest route from file to a part you can hold.
It is a functional part, a batch of parts, or something that has to survive handling. Then look at SLS first. Nylon parts come out of the machine already usable, support-free in most cases, and several fit in one build.
It is a cosmetic model, a display piece, or something that will not be handled. Be honest about that, because it is the case where choosing on price is completely reasonable.
What does each process do well, and what is it bad at?
SLA: the best surface, the most fragile result
Resin is cured layer by layer in a vat, so the surface you get needs little work: our published figure is Ra 0.8–1.6 µm after sanding, visibly smoother than SLS. It also holds the smallest minimum feature size of the three at 0.2 mm, and tolerates the thinnest walls at 0.4 mm.
The trade-offs are real. Resin parts are stiff and brittle relative to nylon, they need support for overhangs, and the material is the expensive consumable — which is why resin is usually the largest single line on an SLA quote. Several parts do fit on one platform, so batching does help — but only once the parts can actually be packed flat and printed in the same pass. Tall or tilted parts waste platform height, every part needs its own supports, and each one is washed, cured and de-supported by hand, so the saving is smaller than the nesting saving on SLS and never automatic. Published lead time is 2–4 business days after file approval.
SLS: the best batch economics, and the least surface
Nylon powder is fused where a laser hits it, and the surrounding powder supports the part, which is why SLS needs support far less often than the other two. Parts come out usable. Several fit in one build, so setup cost is shared — this is the process where quantity genuinely changes the unit price.
The surface is the honest cost: matte with a fine granular texture, Ra around 10 µm, and bead blasting is included as standard rather than sold as an extra. Dimensional tolerance is the loosest of the three at ±0.3 mm, the minimum wall is 0.8 mm, and internal channels below 1.0 mm can be cleared but become slow and risky. Available materials are PA12, glass-filled PA12, PA11 and TPU. Published lead time is 3–5 business days.
SLM: metal, and the smallest envelope of the three
Metal powder bed melting gives the mechanical properties you are actually buying when you choose metal: our published relative density is 99.5% or better, and we hold ±0.2 mm as standard, tightening to ±0.1 mm on selected features after machining. Layer height runs 20–50 µm, so it is the finest of the three. Minimum wall is 0.4 mm and internal channels can go to 0.5 mm diameter with a clear powder evacuation path.
The costs are in the envelope, the finishing and the time. Metal needs inert gas, support removal and post-processing, which is why the build volume is the smallest: 250×250×325 mm against SLA's 600×600×400. As-built surface is Ra 6–12 µm, improving to 1.6 micro;m or better after machining. Materials are the metals we hold: Ti-6Al-4V, 316L, AlSi10Mg and CuCrZr. Specifying the grade and its oxygen budget is covered in our metal powder buyer's guide.
How do the numbers actually compare?
Swipe the chart sideways to read it →
| Specification | SLA | SLS | SLM |
|---|---|---|---|
| Layer height | 50–100 µm | 60–120 µm | 20–50 µm |
| Dimensional tolerance | ±0.1 mm | ±0.3 mm | ±0.2 mm |
| Minimum feature | 0.2 mm | 0.4 mm | — |
| Minimum wall | 0.4 mm | 0.8 mm | 0.4 mm |
| Minimum internal channel | — | 1.0 mm | 0.5 mm dia |
| Surface | Ra 0.8–1.6 µm | Ra ≈10 µm | Ra 6–12 µm |
| Build envelope | 600×600×400 | 340×340×600 | 250×250×325 |
| Relative density | — | — | ≥99.5 % |
| Lead time | 2–4 days | 3–5 days | — |
All figures are the published values on our SLA, SLS and SLM process pages (millimetres unless stated). Tolerance is stated as the standard figure; each page also states the percentage-of-nominal alternative, whichever is greater. Nothing here is a quotation.
Two rows are worth reading twice. Minimum feature versus minimum wall is the pair that most often invalidates a design — in SLS a 0.4 mm wall is not achievable — the figure is 0.8 mm — while a 0.2 mm slot fails on every process, and the two numbers come from different geometry limits anyway. Layer height versus tolerance is the second: layer height controls the stair-step effect on curved surfaces, tolerance controls how far a dimension drifts. They are not the same thing and one does not buy the other.
Which process is wrong for which part?
The more useful half of this comparison is knowing when to rule a process out.
| If the part is… | Wrong process | Why |
|---|---|---|
| A loaded, fatigue-critical metal component | SLA, SLS | Neither is a metal process; the material is the whole point |
| A cosmetic display model, no handling | SLM | You are paying metal performance for a surface nobody will stress |
| A snap-fit or living hinge in plastic | SLS | PA11 holds it — toughness and elongation are why it is chosen for repeated snap cycles and living hinges. PA12 is the wrong grade for it: it is stiff and it will take a set. TPU if the flex is severe or continuous |
| A part with0.3 mm slots and 0.4 mm walls | SLS | Below its minimum feature and wall, so it becomes a support and tolerance problem |
| A batch of twenty identical parts | All three | SLS nests them and shares one setup. SLA nests them too, but each still needs supports, washing and curing — the saving is real but smaller, and it depends on how the parts can be laid out |
| A part larger than the envelope | none, automatically | All three can be sectioned and joined; that is a conversation, not a refusal |
The last row matters. All three processes grow beyond their published envelope by printing in sections and then bonding or welding, with re-machining on metal parts. We would rather tell you that up front than quote it as a surprise — see how it works for what that involves.
How do the three fail differently?
Choosing well means knowing the failure mode you are accepting, because each process has a characteristic one.
- SLA fails at the thin features and the unsupported overhangs, and in brittle fracture rather than gradual deformation. Support removal can mark a surface that was chosen for its finish.
- SLS fails at the layer interfaces and at powder removal from internal channels. Parts are tough, so they rarely break — they flex, then crack where a feature is too thin to behave.
- SLM fails at the heat-affected layer, at the residual stress that comes with big metal parts, and at porosity if the powder is not what it was specified to be. The powder side of that is covered in how to specify and buy metal 3D printing powder, including what to ask for on a certificate of analysis.
None of these is a reason to avoid the process. They are reasons to specify for it — wall thickness, orientation and grain direction in metal, feature size and support strategy in resin, channel size in nylon.
When it looks like a tie, what decides it?
- Material, first. If it must be metal, SLS and SLA are out. This decides most cases.
- Surface, second. If appearance or fine detail is the point, SLA wins and the cost of resin is worth paying.
- Quantity, third. If you need more than a handful, SLS nesting usually wins on unit price — and finishing is what decides the surface question instead.
- Size, fourth. If the part does not fit the envelope, that is a conversation about sectioning, not an automatic no.
- Cost, last. Comparing SLS versus SLA versus SLM on price per part is only meaningful once the other four agree. If two processes both meet the requirement, the cheaper one is the right answer — and we will tell you so rather than quoting the more expensive option.
On cost, the honest framing is that the process is rarely the dominant line. Material, machine time, support, finishing and yield are all in there, and they move with geometry in ways that matter more than the process name. We set that out in our guide to what a 3D printed part actually costs.
Where this leaves you
If you take one thing from this: choose the process from the function and the quantity, not from the material name. The material is a consequence of the decision, and so is the price.
Send the drawing and tell us how many you need. We will propose a process, tell you which specification on this page it is being held to, and say so if a different process would serve you better. Our process pages carry the full specifications, and the material pages carry the grades.