SLS 3D printing: how it works, and when to choose it

SLS comes up often in requests for technical parts: "I need SLS", "my previous supplier used SLS". It is an excellent process, widely used in industry for functional nylon parts. But it answers specific needs, and many projects do not require it.

This article explains how SLS 3D printing works, what it does better than other processes, its limits, and how to choose. We do not have an SLS machine in our workshop, which prints in FDM. That is precisely why we can talk about it without trying to sell it to you.

How SLS 3D printing works

SLS stands for Selective Laser Sintering. The principle comes down to three steps, repeated layer after layer.

1. A thin layer of plastic powder, around 0.1 mm, is spread across the whole build chamber.
2. A laser traces the cross-section of the part and heats the powder just enough to fuse the grains together. The rest of the layer stays as powder.
3. The build platform lowers by one layer, a new layer of powder is spread, and the cycle starts again.

Throughout the build, the chamber is kept at a temperature close to the material's melting point. The laser only has a small temperature gap to cover, which limits warping.

No supports: the powder does the job

This is the feature that changes everything. The unsintered powder surrounds the part and holds it during printing. There are no supports to design, then remove. As a direct consequence, parts can be stacked and nested throughout the whole build volume, not just laid on the platform.

Cooling takes as long as printing

At the end of the build, the powder cake must cool slowly, often for several hours, before it is opened. Taking parts out too early would distort them. Then comes depowdering, by brushing then bead blasting, to remove residual powder. Part of the unsintered powder is sieved and reused, mixed with fresh powder.

What about MJF?

Multi Jet Fusion (MJF), developed by HP, is a close relative of SLS. Instead of a laser, a print head deposits a fusing agent on the powder, then an infrared lamp fuses the whole layer at once. The result is very close to SLS: nylon parts, no supports, uniform properties, with higher throughput.

SLS materials

SLS works almost exclusively with polymer powders:

• PA12 (nylon 12): the reference material. Tough, slightly flexible in thin walls, good chemical resistance.
• PA11: bio-based, more ductile than PA12, suited to parts that take impacts or repeated bending.
• Glass- or carbon-filled nylon: stiffer and more stable under heat, for mechanical parts under load.
• TPU: for flexible parts, seals, dampers, soles.

Parts come out white or grey, with a slightly grainy texture. They are often dyed black by immersion.

What SLS does better than other processes

• Geometries that are impossible elsewhere. Internal channels, interlocking parts, print-in-place hinges: without supports, almost anything becomes printable.
• Uniform mechanical properties. An FDM part is weaker between its layers than along them. In SLS, fusing the powder produces a part that is far more uniform in every direction. That is decisive for clips, fasteners or parts loaded along several axes.
• Efficient batch production. A chamber filled with dozens, even hundreds of nested parts prints in a single run. For a short run of functional parts, the unit cost becomes competitive.
• No support marks. Every surface has the same finish, with no area scarred by a removed support.

The limits of SLS

• Surface finish. The result is matte and grainy. For a smooth surface or very fine detail, resin does better (see our resin page).
• Colours. Parts are white or grey, possibly dyed. There is no colour choice as in FDM, where each spool brings its own shade.
• Lead times. Printing, cooling, depowdering: an SLS part rarely takes less than a few working days, delivery included.
• Cost for a single simple part. The process pays off when the chamber is well filled. For a single prototype with a simple shape, FDM is often faster and cheaper.
• Design. A closed hollow part keeps its powder inside: escape holes are required. Walls that are too thin (below roughly 0.7 to 1 mm) are fragile.

SLS, FDM or resin: how to choose

The right process depends on what the part has to do, not on the technology's reputation.

Your need	Best-suited process
Functional prototype, one-off part, short lead time	FDM
Specific colour, large simple part	FDM
Fine detail, smooth surface, figurine, detailed model	Resin
Complex geometry without supports, internal channels	SLS
Series of functional nylon parts, loads along every axis	SLS or MJF

In practice, a large share of the technical parts we receive print very well in FDM, with the right material. Nylon or PAHT-CF (carbon-fibre nylon) cover many strength requirements, with a well-chosen print orientation to offset the weakness between layers. Our FDM or resin comparison details the other two process families.

Our position: FDM in the workshop, clear advice on SLS

Our workshop prints in FDM, on a fleet of Bambu Lab machines. We do not have an SLS machine, and we say so clearly.

In practice, when you send us a part, we look at its function before choosing the process:

1. FDM does the job. This is the most common case. We suggest the right material and orientation, with a short lead time. See our FDM page.
2. SLS is genuinely justified. A geometry that cannot be printed without supports, a series of nylon parts under multi-axis loads: we tell you, and we point you to an SLS provider suited to your need.

Have a part in mind? Upload your file on our quote page: we tell you which process fits best before any quote. For a series, our short run page explains how we work.

Frequently asked questions

What does SLS mean in 3D printing?

SLS stands for Selective Laser Sintering. A laser fuses, layer by layer, the grains of a plastic powder (most often PA12 nylon) to form the part. The unsintered powder holds the part during printing, which removes the need for supports.

What is the difference between SLS and FDM?

FDM deposits melted filament layer by layer and needs supports for overhanging areas. SLS sinters a powder and needs no supports. SLS parts are more uniform mechanically. FDM is faster and cheaper for a one-off part, and offers a wide choice of materials and colours.

What accuracy can you expect from SLS?

The usual order of magnitude is ±0.3 mm, with layers of around 0.1 mm. Actual accuracy depends on the part's size, its geometry and the provider. For a tight fit, build clearance into the design.

Do you offer SLS 3D printing?

Not in-house: our workshop prints in FDM. When a part genuinely calls for SLS, we tell you and point you to a suitable provider.

Is SLS more expensive than FDM?

For a single simple part, generally yes. For a series of parts nested in the same chamber, the unit cost of SLS becomes competitive. The right choice depends on volume, geometry and the part's function.