Selective Laser Sintering builds plastic parts layer-by-layer by scanning a CO₂ laser across a bed of fine polymer powder, fusing particles together at the laser's focal point. After each layer, a roller or blade spreads a fresh ~100 µm layer of powder, the build platform drops, and the laser scans the next cross-section. The unfused powder around the part stays in place — and that's the magic.

Why no support structures? Unlike FDM (where overhangs sag) or SLA (where unsupported features fall off the build platform), SLS parts float in a sea of loose powder that acts as its own support. Overhangs, internal channels, captive moving parts, and interlocking assemblies all print in one shot. A working pair of pliers with a live hinge? Print it assembled.

The dominant material is Nylon 12 (PA12), prized for its toughness, chemical resistance, and ability to be reused (typically 50% virgin + 50% recycled powder per build). Other materials include PA11 (more ductile, bio-based), glass-filled nylons (stiffer, more dimensionally stable), and TPU (flexible).

Critical process parameters:

• Bed temperature: held just below the polymer's melting point (~170 °C for PA12) — the laser only needs to add a few degrees to trigger fusion
• Laser power and scan speed: determine energy density (J/mm³); too low and parts are porous, too high and they warp or burn
• Cooldown: a full build cake can take 12+ hours to cool — yank parts out hot and they warp permanently

Real-world example: Boeing prints over 60,000 SLS parts per year for the 787 Dreamliner and other aircraft — ducts, brackets, and cabin components in flame-retardant PA. The geometric freedom lets engineers consolidate what used to be 10-bracket assemblies into single parts, cutting weight by 30–50% and eliminating fasteners.

Rule of thumb — minimum wall thickness: 0.7 mm for small features, 1.0 mm for general walls. Minimum clearance for moving assemblies: 0.4 mm — tighter than that and the parts fuse together during the build.

Limitations: surface finish is grainy (Ra 5–15 µm) — like fine sandpaper. Parts are slightly porous unless infiltrated or vapor-smoothed. Trapped powder in internal cavities must have escape holes (≥4 mm diameter) or you'll never get it out. And SLS is anisotropic: Z-axis tensile strength is typically 70–85% of XY strength.