Selective Laser Sintering uses a high-power CO₂ or fiber laser to fuse polymer powder particles into solid geometry. Unlike FDM and SLA, SLS builds parts entirely within a powder bed — unsintered powder surrounding each layer provides support, eliminating the need for support structures. This characteristic makes SLS particularly suitable for complex organic geometries, interlocking assemblies, and thin-wall structures that would require extensive support removal with other methods.
Process Mechanics
The build chamber is pre-heated to just below the powder's melting point — for PA12, this is approximately 168–172 °C. A counter-rotating roller or blade deposits a thin layer of powder (typically 0.10–0.15 mm) across the build surface. The laser then scans the cross-section of each part at that height, raising the local temperature above the sintering point and fusing particles. The platform descends, a new powder layer is deposited, and the process repeats. Thermal management of the entire powder bed is critical: uneven temperature distribution causes part warping and dimensional deviation.
After the build completes — which can take many hours for a fully packed chamber — the entire cake of sintered parts and loose powder must cool in a controlled manner before unpacking. Rapid cooling introduces thermal stresses. Cooling phases of 12–24 hours are standard on industrial SLS systems before the powder cake can be opened.
Nylon Powders: PA12 and PA11
PA12 (Polyamide 12)
PA12 is the dominant material in commercial SLS. It sinters reliably within a consistent process window, produces parts with tensile strength of 44–50 MPa and elongation at break of 15–20%, and exhibits good chemical resistance to oils, fuels, and weak acids. The material's white appearance post-printing allows dyeing to a range of colors. PA12 is produced from laurolactam, a petrochemical derivative; it is not bio-based but is recyclable.
EOS PA 2200 and 3D Systems DuraForm PA are the most widely referenced PA12 grades, with publicly available material data sheets showing tensile modulus around 1700 MPa and Charpy notched impact strength of approximately 3.5 kJ/m². These values are consistent across X, Y, and Z build orientations — the isotropic behavior that distinguishes SLS from FDM.
PA11 (Polyamide 11)
PA11 is derived from castor oil, making it a bio-based material. It exhibits greater elongation at break (30–50%) than PA12, lower modulus, and better impact resistance at low temperatures. PA11 powder is used in oil and gas industry components, flexible ducts, and applications requiring resistance to dynamic fatigue loading. It is less widely available than PA12 and typically more expensive due to feedstock sourcing.
Powder Refresh Rate and Cake Ratio
Unsintered powder from an SLS build can be reused, but repeated thermal cycling degrades it: particle size distribution shifts, flowability decreases, and sintered part properties degrade with higher proportions of aged powder. Manufacturers publish maximum refresh ratios — the proportion of fresh virgin powder that must be blended with recycled powder. For PA12 on most industrial systems, a 50% refresh rate (equal parts fresh and recycled) is typical for maintaining certified material properties. Some high-performance applications require 70–100% virgin powder.
The economics of SLS are heavily influenced by the cake ratio — the percentage of build volume occupied by actual parts versus unsintered supporting powder. A densely packed chamber amortizes machine time and material cost across more parts; a sparse build is expensive per part. Production bureaus using SLS typically nest multiple customer jobs into single builds to maintain economic cake ratios above 10–15%.
Dimensional Accuracy and Post-Processing
SLS parts exhibit shrinkage during cooling as the sintered polymer contracts. Industrial systems apply shrinkage compensation factors — typically 3–4% for PA12 — to the build file. Residual shrinkage variation across a large build results in tolerances of ±0.3 mm for standard industrial SLS, improving to ±0.1 mm on calibrated systems with temperature-stabilized chambers. Surface roughness on unfinished SLS parts is in the Ra 10–15 μm range, resulting from partially fused surface particles. Shot blasting or tumble polishing reduces roughness to Ra 3–6 μm. Dyeing, coating, and surface sealing are common finishing operations for consumer-facing components.
Industrial Applications in the Polish Manufacturing Sector
SLS is used in Polish automotive manufacturing for low-volume brackets, sensor housings, and air duct prototypes that require end-use mechanical properties for fit-and-function validation. The process avoids the lead time and tooling cost of injection molding for pre-series components. Medical device manufacturers in Poland use SLS for surgical guide geometries that require sterilization compatibility — PA12 withstands EtO sterilization and, with appropriate surface sealing, autoclave cycles.
The aerospace maintenance sector uses SLS for cabin interior replacement parts where original tooling is obsolete and part volumes are too low to justify new injection molds. The absence of support structures is particularly valuable here: complex clip geometries and ducting with internal channels can be produced as single pieces. EU Aviation Safety Agency (EASA) requirements for additive parts used in aircraft apply; certification documentation requirements are substantially more demanding than for non-safety-critical applications.
| Property | PA12 SLS | PA11 SLS | FDM PA12 |
|---|---|---|---|
| Tensile Strength | 44–50 MPa | 48–52 MPa | 36–48 MPa |
| Elongation at Break | 15–20% | 30–50% | 5–30% |
| Isotropy | High | High | Low |
| Support Structures | Not required | Not required | Required |
| Surface Finish (as-built) | Ra ~12 μm | Ra ~12 μm | Ra ~10 μm |
| Typical Tolerance | ±0.1–0.3 mm | ±0.1–0.3 mm | ±0.2–0.5 mm |
References: EOS PA 2200 Material Data Sheet · ASTM F3092 Standard Specification for Additive Manufacturing Powder Bed Fusion