Why SLS Still Matters
SLS is the most proven production-grade polymer AM technology - with 30+ years of industrial deployment. While MJF has taken market share on throughput and isotropy, SLS remains unmatched in material variety: glass-filled, carbon-filled, flame-retardant, and polypropylene powders that MJF simply doesn't offer. If your application needs a specialty nylon or your parts exceed MJF's build volume, SLS is your process.
How SLS Works - The Deep Dive
SLS uses a CO₂ laser to selectively sinter nylon powder layer by layer - with unsintered powder acting as built-in support for any geometry.
Selective Laser Sintering (SLS) was invented by Carl Deckard at UT Austin in 1986 and commercialized by DTM Corporation (later acquired by 3D Systems). It remains the most widely used powder-bed fusion process for polymer parts. The mechanism: A CO₂ laser (10.6 µm wavelength, 30–70 W) scans each layer's cross-section on a pre-heated nylon powder bed. The laser raises the powder temperature just past its melting point, causing particles to fuse (sinter) together. After each layer, a roller spreads fresh powder across the bed. The powder bed is pre-heated to 5–10 °C below the sintering point, so the laser only needs to deliver a small energy increment - this minimizes thermal gradients and warpage. The self-supporting powder bed is SLS's defining advantage: no support structures are needed, and parts can be nested in 3D throughout the build volume.
CO₂ laser scanning (30–70 W)
The laser rasters the cross-section point by point, with a spot size of 0.3–0.6 mm. Scan speed and power control energy density - too much power over-sinters (making parts brittle and dimensionally inaccurate), too little causes porous, weak parts.
No support structures
Unsintered powder surrounds the part at all times, providing 360° support. This eliminates support marks, enables complex undercuts, internal channels, and allows 3D nesting - you can stack parts vertically through the build volume.
Powder refresh ratio
SLS degrades unused powder through thermal exposure in the heated bed. Typical refresh ratios: 30–50% fresh powder per build (70–50% recycled). Higher refresh ratios improve properties but increase material cost. EOS recommends 50% fresh for production-grade parts.
Cooldown matters
After the build completes, the powder cake must cool slowly (8–24 hours depending on build height) to prevent warpage and crystallization defects. Rushing cooldown is a common cause of dimensional inaccuracy in SLS.
Build orientation still matters
Although SLS parts are more isotropic than FDM, there's still a 10–15% strength reduction in Z (build direction) vs. XY. For load-bearing parts, orient the primary stress axis in the XY plane. SLS isotropy is ~85% - better than FDM (~50–75%) but less than MJF (~95%).
Surface texture
SLS produces a characteristic matte, slightly grainy surface from partially sintered powder particles on the outer skin. Surface finish is 125–300 Ra µin (3.2–7.6 µm) as-built, improving to 50–125 Ra µin after bead blasting.
Pro Tip
SLS parts reach peak mechanical properties when the powder bed temperature, laser power, scan speed, and cooldown rate are all optimized together. A 5 °C bed temperature error can shift part density by 3–5% - always validate mechanical properties on sample coupons from the same build.
SLS Materials Guide
SLS offers the widest material selection of any powder-bed polymer process - more options than MJF, including glass-filled, carbon-filled, and flame-retardant grades.
SLS material selection is one of its key advantages over MJF. While MJF is locked to HP-certified powders (5 options), SLS machines from EOS, 3D Systems, and Formlabs Fuse accept powders from multiple vendors, giving you 15+ validated material options.
| Material | Tensile (MPa) | Elongation (%) | HDT (°C) | Key Properties | Best For |
|---|---|---|---|---|---|
| PA12 (Nylon 12) | 48 | 18 | 175 | Workhorse - balanced strength, detail, cost | Enclosures, brackets, functional prototypes |
| PA11 (Nylon 11) | 46 | 30–40 | 180 | Bio-based, more ductile than PA12 | Snap-fits, living hinges, impact-loaded parts |
| PA12-GF (Glass-Filled) | 51 | 3–5 | 175 | 50% higher stiffness, thermally stable | Stiff housings, thermally demanding environments |
| PA12-CF (Carbon-Filled) | 72 | 3–4 | 175 | Highest stiffness-to-weight in polymer AM | Tooling, jigs, fixtures, lightweight structural |
| PA12-AF (Aluminum-Filled) | 48 | 4–6 | 175 | Metallic appearance, improved thermal conduct. | Heat sinks, cosmetic metal-look parts |
| TPU (Flex) | 8–15 | 200–400 | - | Shore 88A–92A, excellent energy return | Gaskets, seals, shoe midsoles, vibration dampers |
| PP (Polypropylene) | 28 | 20–25 | 85 | Chemical resistant, living hinges, low density | Fluid handling, chemical containers, snap-fits |
| PA6 (Nylon 6) | 75 | 10–15 | 190 | Higher strength & HDT than PA12 | High-performance structural parts, automotive |
| PEBA (Flexible) | 14 | 350–500 | - | Shore 40D, very flexible, fatigue resistant | Lattice structures, athletic footwear, wearables |
| FR-PA (Flame Retardant) | 46 | 12 | 175 | UL 94 V-0 rated, self-extinguishing | Electrical housings, aerospace, rail interiors |
Pro Tip
PA12 is the right choice for 80% of SLS applications. Reach for specialty materials only when a specific property demands it: PA12-GF for stiffness, PA12-CF for lightweight tooling, TPU for flexibility, PP for chemical resistance, FR-PA for flammability requirements.
Design Guidelines (DFM)
SLS design rules focus on powder escape, minimum features, and managing warpage - not support structures (which don't exist in SLS).
SLS design freedom is unmatched in polymer AM - no supports means true geometric freedom. The constraints are: minimum feature size (laser spot), powder removal (escape holes), and warpage (thermal management on large flat areas). All values assume PA12 at 0.1 mm layer height.
| Feature | Recommended | Minimum | Notes |
|---|---|---|---|
| Wall thickness | 1.0 mm | 0.7 mm | GF/CF-filled: min 1.2 mm (more brittle) |
| Escape holes (depowdering) | ≥5 mm dia. (×2 per cavity) | 3 mm | Essential - trapped powder = failed parts and added weight |
| Min feature size | 0.8 mm | 0.5 mm | Laser spot (0.3–0.6 mm) is the limit; thin features break easily |
| Living hinges | 0.5 mm thick, ≥3 mm wide | 0.4 mm | PA11 strongly preferred over PA12 for hinge fatigue life |
| Internal channels | ≥3 mm diameter | 2 mm | Below 2 mm, powder removal becomes unreliable |
| Pin/boss diameter | ≥2.0 mm | ≥1.5 mm | Slender pins (aspect ratio >5:1) may break during depowdering |
| Part-to-part gap (nesting) | ≥2.0 mm | ≥1.5 mm | Tighter gaps fuse parts together from residual heat |
| Embossed/engraved text | 0.6 mm wide, 0.6 mm tall | 0.4 mm | Bead blasting fills in very fine engraving - test first |
| Large flat surfaces | Add ribs or curvature | - | Flat spans >100 mm are prone to warpage; stiffen with geometry |
| Hole accuracy | Add +0.1 mm to nominal dia. | - | Holes shrink slightly due to laser over-sintering at boundaries |
Pro Tip
The #1 SLS design mistake: forgetting escape holes. We see this on 20–30% of new SLS designs. Every hollow or semi-enclosed feature needs at least two holes (one for powder flow, one for air) with ≥5 mm diameter. Place them on non-cosmetic faces.
Tolerances & Accuracy
SLS standard tolerance is ±0.010″ (±0.25 mm) - slightly looser than MJF due to the laser's Gaussian energy profile and less uniform thermal processing.
SLS dimensional accuracy is governed by laser spot size, powder bed temperature uniformity, material shrinkage (3.0–3.8% for PA12), and cooldown rate. Parts near the edges of the build volume tend to have slightly worse accuracy than those at the center due to thermal gradients.
| Metric | SLS (EOS P396) | SLS (Formlabs Fuse 1+) | MJF (for comparison) |
|---|---|---|---|
| Standard tolerance | ±0.010″ (±0.25 mm) | ±0.012″ (±0.30 mm) | ±0.008″ (±0.20 mm) |
| Best achievable | ±0.005″ (±0.13 mm) | ±0.008″ (±0.20 mm) | ±0.004″ (±0.10 mm) |
| Surface finish (as-built) | 125–300 Ra µin | 150–350 Ra µin | 100–250 Ra µin |
| Surface finish (bead blasted) | 50–125 Ra µin | 60–150 Ra µin | 50–100 Ra µin |
| Isotropy (Z vs XY) | ~85% | ~80% | ~95% |
| Layer height | 0.06–0.12 mm | 0.11 mm (fixed) | 0.08 mm (fixed) |
| Shrinkage (PA12) | 3.0–3.8% | 3.2–4.0% | 2.5–3.0% |
| Max build volume | 340 × 340 × 600 mm | 165 × 165 × 300 mm | 380 × 284 × 380 mm |
Position matters
Parts built near the center of the powder bed have better dimensional accuracy than those at the edges. For critical tolerance parts, request center-of-bed placement from your service bureau - some vendors charge a premium for this.
Shrinkage compensation
PA12 shrinks 3.0–3.8% linearly during cooldown. On a 100 mm part, that's 3.0–3.8 mm before compensation. Service bureaus apply scaling factors in the build software - but residual errors of ±0.1–0.2 mm are common.
Z-height and warpage
Tall builds (>400 mm Z-height) experience more thermal variation top-to-bottom, leading to ~20–30% worse tolerances at the top of the build vs. the bottom. Split tall assemblies if tolerances are critical.
Pro Tip
For SLS parts requiring tolerances tighter than ±0.005″, plan for secondary CNC machining on those features. Callout "as-printed" vs. "post-machined" dimensions on your drawing to avoid confusion and unnecessary cost.
Cost Analysis
SLS economics are driven by build-volume utilization and powder refresh ratio. Packing density is the #1 cost lever - not machine rate.
SLS cost structure is similar to MJF: machine time + material + post-processing. The key difference is that SLS build times are longer (laser scanning vs. MJF's full-bed fusing), but SLS machines accept powders from multiple vendors, creating pricing competition that can offset the speed disadvantage.
| Cost Component | SLS (EOS P396) | SLS (Formlabs Fuse) | MJF (for comparison) |
|---|---|---|---|
| Machine rate | $25–60/hr | $10–25/hr | $20–50/hr |
| Material (PA12) | $50–100/kg | $80–130/kg | $50–90/kg |
| Refresh ratio | 30–50% fresh | 30–50% fresh | 20–30% fresh |
| Post-processing | $5–15/part | $5–12/part | $5–12/part |
| Est. cost (1 part) | $30–80 | $20–50 | $25–65 |
| Est. cost (50 parts) | $15–40/ea | $12–30/ea | $12–30/ea |
| Est. cost (500 parts) | $10–25/ea | $8–18/ea | $8–20/ea |
Desktop SLS changes the economics
Formlabs Fuse 1+ ($18K) and Sinterit Lisa ($10K) put SLS in-house at a fraction of EOS pricing. For engineering teams printing 5–20 parts/week, desktop SLS payback is 6–12 months vs. service bureau costs.
Powder waste is the hidden cost
At 50% refresh ratio, you throw away ~50% of the recycled powder each build. On a full EOS P396 build (~6 kg powder used), you're consuming 9–10 kg total (6 kg used + 3–4 kg waste). Budget 30–50% on top of material cost for waste.
SLS vs injection molding crossover
For PA12 parts, SLS is cheaper than injection molding below ~3,000–8,000 units (depending on geometry and mold complexity). Above that, IM's $0.50–$5/part dominates. SLS is perfect for bridge production while mold tooling is being built.
Pro Tip
If you're producing 50+ SLS parts per month, evaluate bringing a desktop SLS machine in-house. A Formlabs Fuse 1+ at $18K pays for itself in 6–12 months vs. service bureau pricing - and you get 24-hour turnaround instead of 5–7 days.
SLS vs MJF - Head to Head
The most common question in powder-bed polymer AM. Both produce engineering-grade nylon parts - the differences are in throughput, isotropy, materials, and cost.
SLS and MJF produce similar nylon parts from similar powders - they compete directly for functional polymer applications. Here's the engineering comparison to help you choose:
| Factor | Choose SLS When… | Choose MJF When… |
|---|---|---|
| Material variety | You need GF, CF, PP, PA6, FR, or PEBA - SLS has 15+ options | PA12, PA11, PA12-GB, or TPU covers your needs (5 options) |
| Build volume | Parts exceed 380 × 284 × 380 mm (EOS goes to 340 × 340 × 600 mm) | Parts fit within 380 × 284 × 380 mm |
| In-house printing | Desktop SLS ($10K–$18K) is more accessible than MJF ($250K+) | You're using a service bureau (MJF is widely available) |
| Throughput | Low-volume prototypes (1–20 parts) where speed isn't critical | Production runs (50+) where 1.5–2× faster builds matter |
| Isotropy | ~85% isotropy is acceptable for your loads | You need ~95% isotropy for consistent multi-axis loading |
| Part color | You need white or light-colored parts (natural PA12 is white) | Gray/black default is acceptable |
| Tolerance | ±0.010″ standard is adequate | You need ±0.008″ or tighter consistently |
| Cost at volume | Low volumes (<50) or in-house desktop workflow | Production volumes (100+) where MJF is 20–40% cheaper |
Pro Tip
For prototyping (1–20 parts), SLS and MJF are interchangeable - pick whichever your service bureau has available faster. For production (100+), MJF typically wins on throughput and per-part cost. For specialty materials (glass-filled, carbon-filled, PP), SLS is your only option.
Applications & Use Cases
SLS is the original production-grade polymer AM process - proven across automotive, aerospace, consumer, and medical industries since the early 2000s.
Functional prototyping (the classic use case)
SLS PA12 is the standard for engineering prototypes that need to survive real-world testing: snap-fits, clip assemblies, living hinges, and threaded bosses. Parts are tough enough for EVT/DVT testing and representative of production nylon.
Aerospace ducting & brackets
SLS PA12 and FR-PA parts are used in aircraft ducting, cable routing, and non-structural brackets. FR-PA (UL 94 V-0) meets FAA flammability requirements for cabin interiors. Weight savings of 30–50% vs. traditional CNC aluminum brackets.
Automotive interior & under-hood
Clips, grommets, cable guides, fluid connectors, and HVAC ducting. PA12-GF for thermally stable housings under the hood. SLS has been used by BMW, Porsche, and Mercedes for low-volume production and after-sales spare parts.
Consumer products & footwear
TPU and PEBA lattice structures for athletic midsoles (Adidas 4DFWD). Complex geometries impossible with injection molding - customizable, lightweight, and energy-returning. SLS enables mass customization at 100–10,000 unit volumes.
Medical devices & orthotics
Patient-specific orthotics, prosthetic sockets, and surgical planning models. PA11 (bio-based, ductile) for body-contact applications. Each device is unique - AM eliminates per-patient tooling cost.
Tooling, jigs & fixtures
PA12-CF jigs and fixtures replace machined aluminum at 10–20% of the cost with 1–2 day turnaround. Glass-filled PA12 provides excellent dimensional stability for CMM fixtures and assembly aids.
Pro Tip
SLS's broadest competitive moat vs. MJF is material diversity. If your application needs glass-filled nylon, carbon-filled nylon, polypropylene, or flame-retardant nylon, SLS is your only powder-bed polymer option.
Common Mistakes
These SLS-specific mistakes increase cost, delay delivery, or produce parts that fail in the field.
Forgetting escape holes on enclosed features
Trapped powder cannot be removed after the build. Parts will be heavier than expected, and trapped powder can absorb moisture and swell over time, cracking the part from the inside. Add ≥5 mm holes to every enclosed cavity.
Using cheap recycled powder for critical parts
Over-recycled SLS powder (>5 cycles without adequate refresh) becomes "orange peel" - parts look fine but have 20–40% reduced elongation and impact resistance. For production parts, insist on documented powder refresh ratios from your vendor.
Designing large flat surfaces without stiffening
Flat surfaces larger than 100 × 100 mm warp during cooldown. Add ribs (1 mm wide × 3 mm tall), curvature, or honeycomb patterns to the underside. A 3 mm crown on a 200 mm flat panel prevents visible warpage.
Assuming SLS PA12 = injection molded PA12
SLS PA12 has different crystallinity, porosity (3–5% typical), and UV resistance than injection-molded PA12. Elongation at break is 15–20% vs. 100–300% for IM PA12. Don't substitute SLS parts for IM parts in safety-critical applications without testing.
Rushing the cooldown cycle
SLS parts must cool in the powder cake for 8–24 hours depending on build height. Removing parts early causes crystallization defects: warped geometry, reduced impact strength, and inconsistent dimensions. Never rush cooldown.
Not specifying surface finish requirements
As-built SLS is grainy (125–300 Ra µin). Bead blasting improves to 50–125 Ra µin. If your application needs smoother surfaces, specify "bead blasted" or "chemical smoothed (AMT)" on your drawing - and budget 10–20% extra.
Pro Tip
Create an SLS spec sheet for your service bureau with four must-have items: (1) powder refresh ratio (≥30% fresh), (2) cooldown protocol (slow, ≥12 hours), (3) surface finish requirement (as-built vs. bead blasted), (4) mechanical property certs (tensile coupons from the same build). This prevents 90% of quality issues.
Frequently Asked Questions
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