SLS (Selective Laser Sintering) is uniquely forgiving in one dimension — it requires no support structures, because unsintered powder supports every surface during the build. But it has distinct design constraints that catch engineers accustomed to FDM or injection moulding by surprise. At Layer X in Ahmedabad we review hundreds of SLS files per month. These are the rules that prevent reprints, cost overruns, and dimensional failures.
Wall Thickness
Minimum structural wall: 1.0 mm — Below this, the sintered wall may not fuse fully, producing a porous, mechanically weak surface. For walls that see any load (snap fits, bearing surfaces, assembly datum faces), specify 1.5 mm minimum.
Maximum wall: No hard limit — But walls thicker than 8–10 mm accumulate significant internal residual stress and risk thermal warping. For thick sections, hollow the interior with 1.5 mm walls and lattice infill, or add internal rib structure.
Uniform wall thickness: Abrupt section changes (thin wall to thick block) create differential cooling stress and risk sintering defects at the transition. Fillet or taper section changes wherever possible.
Holes and Internal Channels
Minimum hole diameter: 1.5 mm for through-holes. Holes below this diameter may not fully clear during powder depowdering because the unsintered powder column cannot be evacuated.
Internal channels (fully enclosed): minimum 4 mm diameter to allow reliable powder removal with compressed air and vibration. For channels below 4 mm, they must be open-ended (accessible from both ends) and ideally oriented vertically or at 45° in the build to facilitate powder evacuation.
Escape holes: Any hollow cavity in an SLS part must have at least one escape hole of minimum 3 mm diameter for powder removal. Without this, trapped powder adds weight, vibrates during operation, and creates dimensional uncertainty.
Clearance Fits and Moving Parts
SLS is the only polymer 3D printing process that can print assembled mechanisms — nested components, linked chains, hinged joints — in a single build without assembly.
Minimum clearance for free motion: 0.3–0.4 mm per side (0.6–0.8 mm total gap) for parts that need to move after depowdering. With 0.3 mm clearance, parts will usually free up after powder cleaning and break-in; 0.5 mm is more reliable.
Nested rotational joints (like a ball-and-socket): 0.4 mm radial clearance is the tested minimum at Layer X for PA12. Tighter than this and the part sinters together in the build.
Snap Fit Design for SLS PA12
PA12 has excellent fatigue resistance and is the best polymer 3D printing material for snap fits. Design guidelines:
- Cantilever snap arm thickness: 1.5–2.5 mm
- Maximum deflection on engagement: 2–3 mm (PA12 allows 3–5% strain at snap)
- Snap retention angle: 30–45° for permanent; 10–15° for releasable
- Add a 0.5 mm radius at the snap arm root — stress concentration at sharp corners is the primary failure mode
- Orient the snap arm in the X-Y build plane for maximum layer-parallel fatigue resistance
Text, Logos and Fine Detail
Minimum embossed text height: 0.5 mm above surface, 0.8 mm character line width — below this, SLS sintering blurs fine features into the surface. Debossed (recessed) text requires 0.5 mm depth and 1.0 mm line width minimum.
Wire-like features: Minimum 0.8 mm diameter for a feature that extends unsupported from a surface. Below this, the feature sinters but may break during depowdering or post-processing.
Build Orientation and Its Effect
SLS is more isotropic than FDM, but Z-direction (build height) properties are still 5–8% lower than X-Y in tensile strength due to inter-layer sintering. For parts with critical load direction (snap fits, gear teeth, bearing seats), orient the critical feature in the X-Y plane.
Flat horizontal features sink slightly in Z: Parts with large flat faces oriented horizontally in the build can sag 0.1–0.3 mm due to thermal gradient. Add 0.2 mm to Z-dimension on critical flat faces, or specify face machining for flatness below 0.15 mm.
SLS Design Rule Summary Table
| Feature | Minimum | Recommended |
|---|---|---|
| Wall thickness (structural) | 1.0 mm | 1.5 mm |
| Hole diameter (through) | 1.5 mm | 2.0 mm |
| Internal channel diameter | 4.0 mm | 6.0 mm |
| Clearance for free motion | 0.3 mm/side | 0.5 mm/side |
| Snap arm thickness | 1.2 mm | 1.8 mm |
| Embossed text width | 0.8 mm | 1.2 mm |
| Wire/pin diameter | 0.8 mm | 1.5 mm |
| Escape hole for cavities | 3.0 mm | 5.0 mm |
Applying these rules eliminates the most common SLS failure modes before a single gram of powder is sintered. Send your SLS files to Layer X for a free DfAM review with the quote.