Selective Laser Sintering
SLSPBF-LB/PLaser SinteringLSSLS/MJFCO₂ laser, 10.6 µm wavelength, typically 10–70 W. CO₂ lasers are efficient absorbers in thermoplastic polymers (high absorptivity at 10.6 µm). Some modern systems use fibre lasers with adapted scan strategies.
How it works
Semi-crystalline thermoplastic powder (typical D50 55–65 µm for PA12) is spread in thin layers (~80–120 µm) on a heated build platform maintained just below the polymer's melting point (typically 3–10°C below Tm). A CO₂ laser scans the cross-section, raising the powder temperature above Tm and fusing particles. Surrounding unfused powder supports the part — no support structures are required. After the build completes, the entire powder cake cools slowly to prevent warping; this takes hours for large builds. Parts are extracted by breaking the cake and removing loose powder. Powder not fused undergoes thermal degradation and must be blended with fresh powder at a controlled ratio (typically 50:50 for PA12). Near-isotropic properties are a key advantage over FDM.
Parameter envelopes (3 material–machine combinations)
Power
12–35 W (typ. 21 W)
Scan speed
5000–15000 mm/s
Layer thickness
80–120 µm
Hatch spacing
250–350 µm
Max O₂
3000 ppm
Preheat
170 °C
EOS P 396 standard PA12 parameters at 100 µm layer. Build chamber at 170°C — 8–10°C below PA12 Tm (~178°C). Nitrogen inerted to prevent powder oxidation and yellowing. Refresh ratio ≤50% virgin powder for structural parts.
Power
12–30 W (typ. 19 W)
Scan speed
5000–15000 mm/s
Layer thickness
80–120 µm
Hatch spacing
250–350 µm
Max O₂
3000 ppm
Preheat
168 °C
PA11 sinters at ~2–3°C lower chamber temperature than PA12 on the same system. Requires specific OEM qualification — not all PA12 systems have validated PA11 parameters.
Power
20–50 W (typ. 30 W)
Scan speed
3000–8000 mm/s
Layer thickness
80–120 µm
Hatch spacing
200–300 µm
Max O₂
2000 ppm
Preheat
370 °C
EOS P 800 is the only commercially available SLS system qualified for PEEK HP3. Build chamber at 340–380°C — exceptionally high for any SLS system. Requires specialised high-temperature recoater and build environment. Very high operating cost.
Defect modes (4)
Aged Powder Degradation
Cause
Polymer powder exposed to elevated temperatures in the build chamber undergoes partial thermal degradation (chain scission, cross-linking, MFR increase) even without laser exposure. As the proportion of aged powder in the blend increases, molecular weight decreases, crystallinity changes, and elongation at break degrades rapidly.
Indicator
Yellow/brown discolouration of parts or powder. Reduced elongation at break in production batch test coupons. Increased MFR (melt flow rate) of aged powder batch. Rough surface on parts (particle boundary definition increases with aged powder).
Prevention
Enforce strict refresh ratio policy: ≤50% recycled for structural PA12, ≤30% for cosmetic or high-elongation applications. Run tensile coupon qualification at each production batch. Store powder in sealed, desiccated containers. Monitor powder MFR before each use.
Detection
- Tensile testing of batch coupons
- MFR measurement
- visual colour inspection
- particle size analysis
Warping During Cool-Down
Cause
Differential thermal contraction between the hot, freshly sintered part interior and the cooler outer cake during slow cool-down produces internal stresses. Thin-walled or long-span features are most susceptible. Rapid or non-uniform cooling increases risk.
Indicator
Parts visibly warped on extraction from powder cake. Long flat parts (>150 mm) bow in the Z direction. Snap-fit features out of tolerance.
Prevention
Allow full controlled cool-down (do not open chamber until at or below glass transition temperature). Increase wall thickness for flat, large-span features. Avoid thin unsupported spans >150 mm without intermediate supports or gusseting. Optimise part orientation — flat surfaces parallel to XY are more susceptible.
Detection
- CMM or caliper measurement after extraction
- visual inspection
- 3D scanning
Inter-Layer Porosity
Cause
Insufficient energy density at laser/scanner overlap or at the edge of scan tracks leaves unsintered zones between particles or between adjacent scan tracks. Less critical for SLS polymers than LOF in LPBF metals, but increases surface roughness and reduces ductility.
Indicator
Rough surface on down-facing features. Reduced tensile elongation and increased scatter. Visible inter-particle voids in SEM cross-section.
Prevention
Use validated process parameters within OEM specification. Ensure powder is at process temperature before each layer deposition. Check laser power and scan speed calibration regularly.
Detection
- SEM cross-section
- density measurement
- tensile testing
Moisture-Induced Property Degradation (Service)
Cause
Polyamides (PA12, PA11) absorb moisture from the environment, acting as a plasticiser. At equilibrium moisture content (~0.25% for PA12), stiffness and yield strength decrease 10–15% vs. dry-as-built. Not a manufacturing defect, but critical for design.
Indicator
Mechanical property measurements taken from parts stored in ambient conditions are lower than datasheet values (typically measured at dry-as-built condition). Dimensional changes in precision mating parts.
Prevention
Specify design allowables at moisture-conditioned state, not dry-as-built. Condition test specimens per ISO 1110 before testing if service involves humidity. Use moisture-barrier coatings for precision applications.
Detection
- Mass tracking before/after conditioning
- tensile test at conditioned state
- dimensional inspection