SLS process explainer

How Does an SLS 3D Printer Work?

This page explains how an SLS 3D printer works, step by step, for engineers and buyers who need to judge whether a nylon part suits the process. You will see the powder, laser, and thermal steps, then the design rules that decide if SLS is the right route or a poor one.

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Laser fuses powderNo support structuresLayer 0.08–0.15 mmNylon PA12, PA11, TPU

How does an SLS 3D printer work - SLS process design guide

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Key takeaways

Powder is the build material and the supportThe part sits inside unfused powder, so overhangs and internal channels need no dedicated supports.

A laser fuses one cross-section at a timeA CO2 laser traces the sliced contour and melts powder only where the slice says so.

The build chamber stays hotPowder bed and chamber run near the material's melt range to limit warp and curl.

Design rules are looser than FDM or SLAWall thickness from 0.8 mm, holes from 1 mm, no support removal marks on the part.

It is a production route, not just a prototype routeParts print nested in one build, so small runs and 10,000+ runs both stay viable.

Mechanism

What SLS actually does

Selective laser sintering builds a part by fusing thermoplastic powder, layer by layer, with a focused laser. A recoater blade or roller spreads a thin layer of powder across the build chamber, then the laser traces the cross-section of the part and melts the polymer particles together. The platform drops by one layer thickness, and the cycle repeats.

The unfused powder around the part stays in place. It supports overhangs, internal channels, and thin walls without any separate support structure. When the build finishes, the whole powder cake is lifted out and the part is dug out of it. This is why SLS parts have no support marks and why the process handles shapes that FDM cannot print without a raft of extra geometry.

The laser never touches the part. Energy is delivered through the powder surface, so the melt is local and brief. A typical layer is 0.08–0.15 mm thick, and the laser spot is roughly 0.2–0.6 mm wide, depending on machine class. Those two numbers set the surface texture and the smallest feature you can expect.

Nothing here is exotic. The machine is a powder handling system, a heated chamber, a scanning laser, and a control loop that keeps the bed temperature flat. The hard part is holding that thermal uniformity across a full build volume, which is why SLS is run by service shops rather than most design offices.

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Material stateSemi-crystalline thermoplastics that melt and re-solidify without a chemical reaction.
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Heat sourceCO2 laser, typically 30–200 W class depending on build chamber size.
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AtmosphereNitrogen or air, held above the powder's crystallization point to slow cooling.

Materials

Which powders you can run

PA12 is the workhorse. It takes a wide sintering window, holds dimensional stability well, and gives good chemical resistance. PA11 comes from castor oil, so it is bio-based and tougher at low temperature, but it costs more and machines a little softer. Both absorb moisture, so powder is dried before use and sealed after.

Glass-filled and carbon-filled nylons raise stiffness and reduce creep. PA12-GF gives a higher modulus and better heat deflection, at the cost of a rougher surface and more tool wear on downstream machining. TPU grades from roughly 80A to 95A cover flexible parts like gaskets, boots, and living hinges.

You cannot run metal or ceramic on a standard SLS machine. Metal powder needs a different binder system, a different laser power, and usually a debinding and sintering furnace. If a part must be metal, plan for CNC machining or metal injection molding instead, and treat polymer SLS as a prototype-only step.

Powder is not free. In a typical build, only 10–20% of the powder in the bed is fused into parts. Refresh rates vary by material and machine. That ratio drives SLS part cost more than the laser time does.

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PA12General purpose. Good stiffness, stable, easy to dye.
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PA11Higher impact and elongation. Bio-based, higher cost.
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PA12-GF / CFStiffer, better heat deflection, rougher surface.
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TPUFlexible parts, 80A–95A, slower to depowder.

Thermal control

Why the chamber runs hot

Sintering is a thermal process, not just an optical one. The laser raises the powder above its melting point, but the surrounding bed sits just below it. That narrow gap is what keeps the part from curling as it cools. If the bed is too cold, the top layers shrink faster than the layers below and the part warps upward at the edges.

If the bed is too hot, powder outside the scanned area starts to fuse on its own. That is called caking, and it ruins the part detail and the powder for reuse. Machine control keeps the bed within a few degrees across the whole build volume, which is why large SLS chambers are expensive.

Cooling is also controlled. After the build, the powder cake is left in the chamber or in a cooling station to come down slowly. Pulling a part out hot causes the same warp you were trying to avoid. Expect several hours of cooling before depowdering, sometimes overnight for tall builds.

For the buyer, this matters because it sets the minimum lead time. A single SLS build cannot be rushed by turning up the laser. The thermal cycle is fixed by the material, and it dominates the schedule.

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CurlPart edges lift when the bed is too cold or cooling is too fast.
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CakingStray fusion in the powder bed when the bed runs too hot.
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Z-growthParts come out slightly taller than the CAD model; allow for it.

Design

Design rules that match the process

SLS forgives more than most additive processes. Minimum wall thickness runs around 0.8 mm for stable geometry, and 0.6 mm is possible on small parts if you accept some risk. Holes down to about 1 mm print clean in the build direction. Features below that tend to close up or come out undersized.

Unsupported overhangs are fine, but shallow angles still matter. Anything above about 45° from vertical prints without a visible step because the powder holds it. Below that, the bottom face gets a rougher, partially sintered skin. It is cosmetic in most cases, not structural.

Internal channels are one of the strongest reasons to choose SLS. A channel of 2 mm or more prints reliably with powder that can be cleared afterward. Below 1 mm, the powder packs in and you may not get it out. If the channel carries fluid or air, design for a larger diameter and a straight path.

Tolerance is where SLS is weakest. Expect roughly ±0.3% of the part dimension, with a floor around ±0.3 mm on small features. If a bore needs to be tight, print it undersized and ream or bore it on a CNC afterward. That hybrid route is common for functional prototypes.

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Wall thickness0.8 mm minimum; 1.2 mm if the part takes a load.
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Holes1 mm minimum as printed; expect 0.1–0.2 mm undersize.
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ClearanceAllow 0.3 mm between parts that must move after printing.
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TextEngraved characters below 1.5 mm are hard to read.

Process order

Step by step: from CAD to finished part

Follow this order. Skipping the drying or cooling step is the most common cause of a failed build.

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1. Model and check for wall thicknessExport a watertight STL or STEP from CAD. Check every wall against the 0.8 mm minimum. Thin ribs are the number one geometry problem we see in incoming files. Fix them in CAD, not in the slicer.
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2. Slice and nest the buildSlice at 0.08–0.15 mm. Thinner layers give a smoother Z surface but roughly double the build time. Nest parts with 2–3 mm spacing so the recoater has room and heat stays even across the bed.
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3. Dry the powderNylon absorbs moisture. Dry PA12 at about 70–80 °C for several hours before loading. Wet powder foams at the laser spot and leaves porosity in the part.
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4. Heat the chamber and spread the first layersBring the bed to its set point and let it stabilize. The first few layers set the flatness of the whole part. A cold start shows up as a curled base you cannot fix later.
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5. Run the laser scanThe laser traces each cross-section, and the platform drops one layer. Watch for a rise in chamber temperature drift across the build. Uniform bed temperature matters more than laser power.
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6. Cool the cake slowlyLeave the powder cake in the chamber or a cooling station for several hours. Pulling it hot warps the part. For tall builds, cooling overnight is normal and should be planned into the schedule.
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7. Depowder, blast, and finishBreak out the part, brush off loose powder, then bead blast to a uniform matte finish. Dyeing, tumbling, or CNC finishing of critical bores comes after. Recovered powder is sieved and blended before reuse.

Process fit

SLS compared with FDM and SLA

Use this to pick a route before you commit a design to one process.

Criterion	SLS	FDM	SLA
Support structures	None needed	Required for overhangs	Required, removed by hand
Typical layer	0.08–0.15 mm	0.1–0.3 mm	0.05–0.1 mm
Small holes	1 mm and up	2 mm and up	0.5 mm and up
Surface finish	Matte, slightly grainy	Visible layer lines	Smooth, glossy
Strength in Z	Near isotropic	Weak between layers	Brittle, low impact
Best for	Functional nylon parts	Cheap form checks	Fine detail, small models
Tight bores	Machine after printing	Rarely holds	Rarely holds

When SLS is the right call

Choose SLS for complex nylon parts with internal channels and no support marks. Choose CNC when tolerance, metal, or surface finish decides the part. For hybrid parts, print in SLS and machine the critical features.

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FAQs

Common questions

Do SLS parts need support removal?

No. The unfused powder around the part holds overhangs and internal features during the build. There is nothing to cut away and no support witness marks on the surface.

The trade-off is depowdering. Internal channels and blind pockets need compressed air or vibration to clear, and small channels may trap powder. Design channels at 2 mm or more if they must be clear.

What tolerance can I expect from an SLS part?

Plan for roughly ±0.3% of the dimension, with a practical floor near ±0.3 mm on small features. Warp and Z-growth both push parts off nominal.

If a bore or a mating face needs to be tight, print it with stock and finish it by CNC. We hold ±0.005 mm on machined features, so the hybrid route gives you the geometry of SLS and the tolerance of machining.

Can SLS print metal parts?

Not on a polymer SLS machine. Metal powder needs a binder, a different laser, and a debinding and sintering furnace. Those are separate processes.

For metal prototypes and production parts, CNC machining is usually faster and gives better tolerance. We machine aluminium, stainless, steel, titanium, and copper alloys from one part upward.

How long does an SLS build take?

The laser time depends on part height, not part count. A build volume full of small parts costs about the same laser time as one tall part. Nesting is what makes small runs economical.

Add several hours for cooling and depowdering. The thermal cycle cannot be shortened, so plan the schedule around it rather than around the print itself.

Is SLS powder reusable?

Yes, with limits. Unfused powder is sieved and blended with fresh powder at a refresh rate set by the material and the machine. Reusing powder indefinitely shifts the part colour and mechanical properties.

That refresh ratio is one reason SLS pricing is not purely a function of laser time. It also means quoting an SLS part without knowing the material grade is guesswork.

When should I choose CNC instead of SLS?

Pick CNC when the part needs tight tolerance, a metal material, or a smooth machined surface. Pick SLS when the geometry is complex, the part count is low to medium, and nylon is an acceptable material.

Many projects use both. Print the complex housing in SLS, then machine the bores and mating faces to final size. We quote the CNC side from a STEP file and give DFM feedback within 12 hours.