Lesson 8.3ShapeChapter 8 · Additive manufacturing

Selective laser sintering (SLS)

A laser fuses nylon powder in a bed; the loose powder supports the part (powder bed fusion)

Selective laser sintering (SLS) uses a laser to fuse fine plastic powder, usually nylon, one thin layer at a time. The loose powder around the part holds it up, so no supports are needed. It matters because it makes strong, working plastic parts in small batches, such as drone frames, ducts and custom braces.

Photo: newman72, CC BY 2.0, via Wikimedia Commons

SLS build chamber in cross-section: laser, heaters, recoater, pistons and powderCircled numbers match the steps below; steps 3 to 5 repeat for every layer.

How it works

Slice the model. Software cuts the 3D model into thin layers. Many parts can be packed together, even stacked, to fill the whole build volume.
Heat the chamber. Heaters keep the powder bed just below its melting point, about 170 °C for nylon 12. The laser then only has to add a little extra energy.
Spread a layer. The supply piston rises, and a recoater (a roller or blade) sweeps a thin layer of powder, typically 0.1 mm, across the build area.
Fuse the slice. A laser, steered by fast mirrors, traces the part's cross-section. It sinters the grains (fuses them by partly melting them) to each other and to the layer below.
Lower and repeat. The build piston drops by one layer, and steps 3 to 5 repeat. The unfused powder stays in place and props up every overhang.
Cool, dig out, clean. The block of powder, called the cake, cools slowly, often for about as long as the print took. You then dig out the parts, blast off the clinging powder and mix the leftover powder with fresh powder for reuse.

Key facts

Materials: Nylon (PA12, PA11) above all; also glass- or carbon-filled nylon, flexible TPU, polypropylene and high-temperature PEEK
Typical parts: Functional prototypes, housings, snap-fits and living hinges, ducts, brackets, custom braces and grips
Part size: About 15–30 cm per side on benchtop machines; industrial machines up to about 70 cm
Precision: About ±0.3 mm, or about ±0.3% on larger parts
Surface finish: Matte and slightly grainy, like fine sandpaper; often dyed, tumbled or vapor-smoothed
Batch size it suits: 1 to a few thousand parts; one build can hold dozens to hundreds of small parts
Tooling and machine cost: No tooling; benchtop systems cost tens of thousands of dollars, industrial systems hundreds of thousands
Speed: About half a day to a day per build, plus several hours or more of cooling

Strengths and limits

Strengths

No supports, so parts can be nested and stacked to fill the whole volume
Strong, tough nylon parts, nearly as strong between layers as along them
Complex shapes: lattices, inner channels, even hinges printed already assembled
Low cost per part when a build is packed full

Limits

Machines and powder cost far more than FDM
Rough, slightly porous surface that needs finishing and can soak up dirt
Long cooling time before you can unpack the parts
Dusty powder handling, and some fresh powder must be added each build

Where it's used

AerospaceAir ducts and brackets for aircraft and drones
MedicalCustom orthoses, prosthetic sockets, surgical guides
Consumer productsEyewear frames, sports gear, small-batch housings
AutomotivePrototypes, clips, ducts and low-volume production parts
IndustrialRobot grippers, jigs and fixtures, spare parts

How to recognize it

A matte, slightly sandy surface, often off-white, gray or dyed black.
No support marks anywhere, even under steep overhangs.
Faint, soft layer lines on curved surfaces, much less visible than FDM ridges.
Small escape holes on hollow parts, left so the trapped powder could be poured out.

Compare with

FDM8.1far cheaper machines, but it needs supports and gives weaker, less even parts.Laser powder bed fusion8.4the same powder-bed idea with metal; it melts the powder fully, needs supports and costs much more.Injection molding2.1far cheaper per part beyond a few thousand pieces, but it needs a costly steel mold.

Watch it

Dr. D-Flo · The SLS principle explained for beginners

Formlabs Support · Inside the machine: laser, recoater and heating

Vision Miner · The whole workflow, from printing to digging out and blasting

EOS 3D Printing · Industrial SLS used for series production

See real photos

Wikipedia: Selective laser sintering — labeled process schematic and photos of SLS parts
Wikimedia Commons photo search

Remember

SLS fuses plastic powder with a laser, layer by layer, inside a heated bed.
The loose powder supports the part, so no supports are needed and parts can be stacked.
Parts come out strong but grainy, and need hours of cooling and cleaning.