Selective Laser Sintering (SLS)
Selective laser sintering fuses nylon powder layer by layer with a laser, so the surrounding loose powder supports the part and no supports are needed.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Additive Manufacturing
- Typical tolerances
- ±0.012 in (±0.3 mm) up to about 4 in (100 mm), then ±0.3% of nominal. Critical bores are normally reamed after the build.
- Surface finish
- Ra 200–500 µin (5–13 µm) as built, a uniform matte with slight grain. Chemical vapor smoothing takes it to roughly Ra 40–120 µin (1–3 µm) and seals surface porosity.
- Typical volumes
- 1–5,000 parts; cost per part is nearly flat with quantity
- Lead time
- 3–7 business days. A full-height build is 20–40 hours of print time plus 8–24 hours of controlled cooling before parts can be removed.
- Materials
- Plastic
What it is
Selective laser sintering (SLS) fuses nylon powder layer by layer with a CO2 laser inside a chamber held just below the polymer's melting point — around 340 °F (170 °C) for PA12. Because the surrounding un-sintered powder cake supports every layer, SLS needs no support structures, which frees the designer to build undercuts, internal channels, captive assemblies, and lattices that no molded or machined part could carry.
The workhorse material is PA12 (nylon 12), with PA11, glass- and mineral-filled nylons, carbon-filled grades, and TPU also in routine production. Parts reach roughly 95–100% of bulk density, and PA12 comes out near 7,000 psi (48 MPa) tensile with 10–20% elongation — functional properties, not merely representative ones.
Typical accuracy is ±0.012 in (±0.3 mm) or ±0.3% of nominal, whichever is greater, at a 0.004 in (100 µm) layer. Surfaces are uniformly matte and slightly grainy. SLS is economical from one part into the low thousands, especially where small parts nest densely in three dimensions.
How it works
- Powder dosing and preheat. Fresh and reclaimed PA12 are blended — commonly 30–50% virgin — and the chamber is brought within a few degrees of the polymer's melting range. PA12 melts at roughly 350–365 °F (178–185 °C) and the bed sits near 340 °F (170 °C). That narrow window is the whole trick: the laser only supplies the last few degrees.
- Recoat. A roller or blade spreads a 0.003–0.006 in (75–150 µm) powder layer, typically 0.004 in (100 µm), across the build area.
- Scan. A 30–100 W CO2 laser traces the cross-section, sintering particles to each other and to the layer below. Contour and fill passes run at different powers to control edge accuracy.
- Index and repeat. The piston drops one layer and the cycle repeats. Because the cake supports everything, parts are nested in all three axes rather than sitting on a platform, and packing density becomes the main lever on cost.
- Controlled cooldown. The finished cake cools in the machine or a take-out frame for 8–24 hours. This is not idle time — cooling too fast produces differential shrinkage, curl, and warp, worst on large flat parts.
- Breakout and cleaning. The cake is broken open, parts extracted, and residual powder removed by brushing and bead blasting. Internal channels are cleared through escape holes with compressed air.
- Finishing (optional). Dyeing (usually black), vibratory tumbling, or chemical vapor smoothing follow. Vapor smoothing seals surface porosity, which matters for cleanability in medical and food-contact applications.
Design guidelines
Wall thickness
0.030 in (0.8 mm) is the practical minimum; use 0.040 in (1.0 mm) for anything load-bearing or larger than a few inches. Thin unsupported walls survive the build but distort during cooldown.
Escape holes
Any enclosed volume traps un-sintered powder. Provide at least two escape holes of 0.20 in (5 mm) diameter on opposite faces so powder can be blown through rather than shaken out. Long internal channels need intermediate ports.
Bulk sections
Avoid solid sections thicker than about 0.8 in (20 mm). Heat accumulates in the cake, the region keeps sintering after the laser has moved on, and the part grows and warps. Core out thick regions or replace them with a ribbed shell or lattice.
Clearances for moving and captive parts
SLS prints assemblies in place. Allow 0.020 in (0.5 mm) clearance on small features and 0.040 in (1.0 mm) on larger ones — the heat-affected zone around each scan grows the surface slightly, and a tight gap sinters solid.
Holes and detail
Keep holes at 0.060 in (1.5 mm) or larger; smaller ones close up with partially sintered powder. Embossed and engraved detail needs 0.030 in (0.8 mm) of width and depth, and text wants 0.12 in (3 mm) cap height to survive bead blasting.
Living hinges and snap fits
PA12 takes a snap fit well but makes a poor as-built living hinge; the sintered structure carries more void content than molded nylon and fatigues faster.
Threaded features
Below M6, print a clearance hole and use a heat-set or press-in insert rather than a printed thread. If you intend to tap the nylon directly, the tap drill chart gives the pilot sizes.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Wall thickness | 0.040 in (1.0 mm) | 0.030 in (0.8 mm) | Thin walls distort during cooldown |
| Escape hole | 0.30 in (8 mm) | 0.20 in (5 mm) | Powder must be blown out, not shaken |
| Hole diameter | 0.060 in (1.5 mm) | 0.040 in (1.0 mm) | Smaller holes sinter partly closed |
| Clearance, moving | 0.040 in (1.0 mm) | 0.020 in (0.5 mm) | Heat-affected zone grows the surface |
| Embossed detail | 0.040 in (1.0 mm) | 0.030 in (0.8 mm) | Bead blasting erodes finer features |
| Solid section | 0.6 in (15 mm) | 0.8 in (20 mm) | Heat soak causes growth and warp |
Cost drivers
SLS is priced on the volume of build envelope a part consumes, not on setup — there is no tooling and no meaningful per-order fixed cost. Two things follow. First, packing density matters enormously: a bureau nesting parts at 8–12% of chamber volume amortizes the same powder and machine hours across many more parts than one running a single component. Second, unit cost is nearly flat with quantity, the opposite of a molded process.
Powder is the other real cost. Only part of the un-sintered cake can be reused, because powder near sintered regions degrades thermally; refresh ratios of 30–50% virgin are standard, so you effectively pay for powder that never became a part.
Volume breakpoints: SLS is competitive from 1 to roughly 5,000 parts a year. Beyond that — particularly where walls are thin and uniform — injection molding wins on unit cost once tooling amortizes.
- Reduce part volume rather than bounding box: hollow, rib, or lattice bulk sections.
- Batch orders so parts nest into a single build.
- Consolidate assemblies into one printed part; SLS removes fasteners and assembly labor at no geometric cost.
- Skip dyeing and vapor smoothing on hidden components.
- Keep the tallest dimension modest — build height drives machine hours more directly than part count does.
Questions
7 questionsDoes SLS need support structures?
No. The un-sintered powder cake surrounds and supports every layer, so overhangs, undercuts, and internal geometry print without supports. That is why SLS parts can be nested in all three axes and why complex assemblies can be printed as a single piece.
What tolerance can SLS hold?
±0.012 in (±0.3 mm) up to about 4 in (100 mm), then ±0.3% of the dimension. Accuracy depends on where in the build a part sits and how it was oriented, so critical bores are usually reamed afterward.
How big do SLS escape holes need to be?
At least 0.20 in (5 mm) diameter, and you want two on opposite faces of any enclosed cavity so compressed air can drive the powder through. A single hole traps powder no matter how long the part is shaken.
How strong are SLS nylon parts?
PA12 comes out near 7,000 psi (48 MPa) tensile with 10–20% elongation at 95–100% of bulk density. Z-axis strength is lower than XY, but the gap is typically 10–25% rather than the much larger anisotropy seen in FDM.
Why are SLS parts usually gray or dyed black?
Raw sintered PA12 is off-white to light gray with a porous matte surface that picks up handling marks. Dyeing black is inexpensive, penetrates a few tenths of a millimeter, and hides both the porosity and the powder texture.
Can SLS parts be made watertight?
Not reliably as built, because the sintered structure retains interconnected porosity. Chemical vapor smoothing or an epoxy or urethane sealer closes the surface for low-pressure use; for genuine pressure containment, machine or mold the part instead.
How does SLS compare with MJF?
Both fuse PA12 powder without supports. MJF applies fusing agent by inkjet and fuses whole layers with infrared lamps, giving higher throughput, slightly tighter tolerance (±0.008 in / ±0.2 mm), and denser, more isotropic parts. SLS offers the wider material menu, including PA11, TPU, and filled grades.