SLS vs SLM 3D Printing: 5 Critical Differences You Must Know

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  SLS vs SLM 3D printing at a glance
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  Material state: polymer powder versus metal powder
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  Laser physics: sintering versus full melting
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  Support structures: the self-supporting myth
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  Dimensional accuracy and surface roughness
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  Cost economics and production scale

The first split in SLS vs SLM 3D printing is the material itself. SLS handles polymers: PA12, PA11, TPU and filled grades such as glass-filled or carbon-filled nylon. These parts are tough, chemically resistant and fine for housings, ducts, brackets and snap fits. They are not metal, and they do not become metal after any post-process.

SLM handles metal powder. On our side that usually means Ti-6Al-4V, 316L, 17-4PH or AlSi10Mg. The laser melts each layer into a weld pool, so the finished part is a real metal component with a metal's stiffness, thermal conductivity and fatigue behavior.

The expensive mistake is not sending a metal file to a polymer machine. It is using an SLS nylon prototype as a stand-in for a metal part during functional testing. A fit check is fine. A load test at temperature is not. Nylon softens, creeps and fails in a way the metal design never would.

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  SLS is a polymer processTough, light, chemically stable, non-conductive.
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  SLM is a metal processReal metal properties, real metal density.
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  Do not mix them in test plansFit checks yes, load and heat tests no.

In SLS the laser power is deliberately held below the melting point. Polymer particles soften and the surface energy pulls them together, forming necks between grains. The result is a connected structure with microscopic voids. Porosity in an SLS part typically runs 3% to 7% by volume.

That porosity is not cosmetic. It changes how the part behaves under pressure and how it handles moisture. A sealed pneumatic manifold printed in SLS can leak through the walls. If the design needs a pressure boundary, plan on a coating or choose a different process.

In SLM the powder is fully melted. The laser tracks overlap and fuse into dense metal, with porosity normally under 1% and lower again after hot isostatic pressing. The trade is residual stress. Steep thermal gradients build stress into the part, and thin sections can curl or crack during the build.

This is why metal parts get stress relief before they leave the machine, and why support and orientation matter far more in SLM than in SLS.

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  Sintering leaves 3–7% porosityLeaks and moisture uptake are real risks.
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  Melting gives near-full densityResidual stress becomes the main concern.
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  Stress relief is standard for SLMPlan it into the schedule, not after.

SLS parts sit in a powder cake. The unsintered powder supports overhangs, so the part needs no anchors and you can nest parts tightly in the build volume. Support removal is a brush, a bead blast and a lot of patience with internal channels.

SLM is different. Metal powder conducts heat away from the melt pool, so supports do two jobs: they hold the part to the plate and they drain heat out of the downfacing surfaces. Remove them too early and the part warps. Design them too thin and the part tears off the plate mid-build.

The practical rule for metal is to keep overhangs above roughly 45 degrees from the build plate, or add support. Internal channels below about Ø1.5 mm are difficult to clear and should be avoided unless you have a proven cleaning route.

With SLS you can break that rule. Channels down to about Ø1 mm can be printed and cleared with compressed air, which is why polymer powder bed fusion is popular for manifolds and ducting.

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  SLS overhangs need no anchorsPowder cake supports the part.
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  SLM supports anchor and coolThey are process hardware, not scaffolding.
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  Keep metal overhangs above 45°Otherwise expect curl and delamination.

As-built SLS parts usually hold about ±0.3% of the nominal dimension, with a floor around 0.3 mm. Metal parts are tighter, often ±0.1 mm on small features, but they are more sensitive to orientation and to the thermal history of the build.

Surface finish is where both processes lose to machining. As-built SLS lands around Ra 8–12 μm. SLM is similar on upfacing surfaces and noticeably rougher on downfacing surfaces, because that is where the support scars and partially fused powder sit.

If a surface has to seal, slide or mate, abrasive finishing, bead blasting or machining is normal. We machine metal powder bed parts on our 5-axis centers when a bore, a face or a thread has to meet ±0.005 mm.

The deciding question is simple. Does the feature carry a tolerance, a seal or a sliding contact? If yes, budget for secondary machining, in either process.

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  Polymer: ±0.3% with a 0.3 mm floorFine for housings and fit checks.
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  Metal: about ±0.1 mm small featuresOrientation still moves the result.
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  Machined critical featuresBores, faces and threads to ±0.005 mm.

SLS is cheaper per part at low volume for small parts. No support anchors, high nesting density and inexpensive polymer powder keep the cost curve flat. A hundred nylon brackets can be one build.

SLM costs more for three reasons: metal powder is expensive, the build is slower, and parts need support removal, stress relief and often hot isostatic pressing. Inconel and titanium push that further.

Volume changes the answer. Above a few thousand parts a year, die casting or machining usually wins on unit cost, and we quote all three routes so the comparison is honest. Between one and a few hundred parts with metal properties, SLM is often the only sensible path.

For prototypes that need to look and feel like a production part, vacuum casting from a machined master is a common middle option. It is a mold, not a powder bed, but it beats both processes on cost at 20 to 50 units.

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  SLS wins small polymer batchesCheap powder, tight nesting, no anchors.
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  SLM carries post-processing costSupport removal, stress relief, HIP.
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  Above a few thousand partsDie casting or CNC usually wins.