SLS Nylon 3D Printing: PA12, Tolerances & Design Guide

SLS, selective laser sintering, uses a laser to sinter a polymer powder, usually nylon PA12 or PA11, one thin layer at a time. Because the unfused powder surrounds and supports the part as it builds, SLS needs no support structures, which is why it excels at complex, interlocking, and densely nested geometry that FDM and SLA cannot print in one piece. SLS parts are strong and reasonably isotropic compared with FDM, which makes the process a natural fit for functional prototypes and low-volume end-use parts.

The trade-offs are a slightly grainy as-built surface, a higher unit cost than FDM for simple geometry, and a material range centered on nylon. This guide covers how SLS works, the materials and tolerances, the design rules that make the most of the self-supporting powder bed, and a clear set of cases where SLS is the right or wrong choice.

How SLS works

An SLS machine spreads a thin layer of nylon powder, typically 100 to 150 micrometers thick, across a build bed inside a heated chamber. A laser then traces the cross-section of the part and sinters, or lightly fuses, the powder where the part exists, bonding it to the layer below. The build piston lowers by one layer height, a recoater blade or roller spreads a fresh layer of powder, and the next cross-section is sintered. The unfused powder stays in place around the part at all times, holding up every overhang, bridge, and cavity without any added support material.

The chamber is held close to the melting point of the nylon so the laser needs only a small energy pulse to fuse each layer. This near-isothermal build keeps residual stress low and layer bonding strong, which is the physical reason SLS parts are more isotropic and tougher than FDM parts. It is also why the process uses significant energy and why the build must cool slowly before the part can be removed, which sets a floor on SLS lead time regardless of part count.

Self-supporting geometry

Surfaces are self-supporting above about 30 degrees from horizontal, and bridges can span up to about 5mm without sagging. Below 30 degrees, a surface may warp or develop a pillowy, uneven texture, so shallow angles are either steepened, tied to a supporting wall, or accepted with a rougher finish. This self-supporting behavior is what lets SLS nest many parts in one build with no support-removal labor, which is a major cost advantage for small production runs.

Materials

The SLS material range is nylon-centered, and each grade serves a different job. PA12, nylon 12, is the primary material: strong, stable, and the default for functional parts, and it absorbs only about 1 percent moisture, far less than PA6 at about 9 percent, which keeps it dimensionally stable in service. PA11, derived from castor oil, is tougher and more ductile, which suits living hinges, snap-fits, and parts that must flex without cracking. TPU powder prints flexible, rubber-like parts for gaskets, seals, and overmolds.

Filled nylons for higher load and heat

For higher loads and hotter service, glass-filled and carbon-filled nylons add stiffness, strength, and heat resistance over the unfilled grades, at the cost of a rougher as-built surface and faster tool wear if the part is machined later. The filled grades are also more abrasive, which is relevant if a printed blank will be post-machined. The detailed properties for each nylon grade live on the dedicated material pages, including the nylon PA12 page and the engineering plastics overview.

File format and units

Provide an STL with the units stated, and for hollow parts include escape holes so the trapped powder can drain during depowdering. STL carries no units metadata, so a file without explicit units is read against the supplier default, and a millimeter-versus-inch mistake produces a part 25.4 times the intended scale. State the units in the file and the filename, and confirm them in the order, because this single mistake is the most common and most expensive file-format error in custom manufacturing.

Tolerances

Dimensional tolerance is about plus or minus 0.3mm for features under 50mm and about 0.5mm for larger features. Minimum wall thickness is 0.8mm non-structural to 1.2mm load-bearing, minimum hole diameter is about 1.0mm, and escape holes for powder removal are 2 to 3mm. The as-built surface is around Ra 5 to 12 micrometers, rougher than SLA and comparable to MJF, and it improves with tumbling, dyeing, or vapor smoothing. For a mating or bearing fit, plan to machine or ream the printed blank, because the as-built tolerance will not meet a precision fit.

A snap-fit clearance example

For example, a snap-fit housing printed in PA12 with 0.4mm per-side clearance assembles cleanly straight off the build, because the powder held every overhang and the clearance is wide enough for the as-built surface. The same part in FDM would need supports inside the snap pocket and would be weaker across the layers, and in SLA it would be too brittle to survive repeated snapping.

Post-processing

After depowdering, SLS parts are commonly tumbled to smooth the grainy surface, media-blasted for a uniform matte finish, and dyed in a hot bath, most often black, to set the color and seal the surface porosity. Vapor smoothing can gloss the surface for a more finished look and also seals the part against moisture uptake. Dyeing is preferred over paint because it penetrates the surface and adds no measurable thickness, which keeps mating dimensions stable. Critical faces can be machined to size or to a tighter tolerance than the as-built process holds.

A worked SLS design example

Consider a snap-fit battery cover that must clip into a housing, survive repeated removal, and run in a warm device. The requirements point straight to SLS in PA12, and each design choice follows from the process.

Choose the material and process

First, choose the material and process. The cover flexes on a living hinge and snaps repeatedly, so a tough, ductile nylon is required, which rules out the brittle SLA resins and the anisotropic FDM layers. PA12 on SLS is the natural fit, and PA11 is the upgrade if the hinge must flex even more.

Use the self-supporting bed

Second, use the self-supporting bed. The cover has internal clip features and a recessed pocket that would need supports in FDM or SLA; in SLS the powder holds them, so the part prints in one piece with no support marks. Walls are set to 1.0mm, above the 0.8mm floor, and the snap clearance to 0.4mm per side.

Handle the hollow interior

Third, handle the hollow interior. The cover has a sealed cavity for weight reduction, so two escape holes of 3mm are added so the powder drains during depowdering. Without them the cavity would arrive full of fused powder.

Finish and tolerance the fit

Finally, finish and tolerance the fit. The snap is left as-built because the 0.4mm clearance absorbs the surface texture, while the locating pin that must hold a tight position is drilled to size after printing. The part is dyed black and tumbled, arriving as a functional, repeatable cover that needed no supports and no assembly.

SLS strengths and limitations

SLS is strong where FDM and SLA are weak. It produces functional, largely isotropic nylon parts with no supports, which makes it the best polymer process for snap-fits, living hinges, complex assemblies, and small-batch end-use parts. The ability to nest many parts in one build also lowers the effective cost per part for production runs.

Its limitations are the surface and the cost. The as-built finish is grainy and needs post-processing to look cosmetic, and it cannot match the detail or the transparency of SLA. For simple geometry, FDM is cheaper, and the unit cost of SLS is higher than FDM for a single simple part. SLS is also largely limited to nylon-family materials, so it is not the choice when a part needs a rigid cosmetic resin or a wide material range.

Applications and use cases

SLS wins on functional strength without supports. Snap-fit housings, living hinges, complex assemblies with internal channels, brackets and mounts that carry load, ducts, manifolds, and small-batch end-use parts are all strong applications. The ability to nest many parts in one build with no support removal also makes it economical for short production runs of complex geometry that would be slow or impossible on FDM or SLA.

SLS compared to MJF in detail

SLS and MJF are close cousins, and buyers often choose between them. Both fuse nylon powder across the full build area without supports, both produce strong, largely isotropic parts in the PA12 and PA11 family, and both leave a slightly grainy surface that takes dye well. The differences are in the fusing method, the tolerance, the surface, and the throughput.

Tolerance and surface

SLS holds about plus or minus 0.3mm under 50mm and 0.5mm above, with an as-built surface around Ra 5 to 12 micrometers. MJF holds about plus or minus 0.2 to 0.3mm up to 100mm and 0.3 percent above, with a comparable as-built surface around Ra 4 to 9 micrometers. In practice both land in the same functional range, but MJF tends to be marginally tighter on tolerance and slightly smoother and more consistent in surface, while SLS varies more with machine and powder age (Formlabs Fuse sits at the smoother end, EOS PA2200 at the rougher).

Throughput, color, and the practical choice

MJF fuses a whole layer at once with infrared after jetting the fusing agent, which is typically faster than a moving laser and gives very consistent mechanical properties across a build. MJF parts also come out a characteristic gray that is usually dyed black. SLS offers a wider material spread in some shops, including PA11 and filled grades, and a native surface that responds well to tumbling and dyeing. For most functional nylon parts the two are interchangeable, and the decision usually comes down to which process a given shop runs, what color is needed, and what the lead time allows. The dedicated MJF page carries its own tolerances and design rules.

Powder is recycled between builds, but not indefinitely. Each fusion cycle ages the nylon powder slightly, so shops blend used powder with fresh material to keep the mechanical properties consistent across builds, and powder that has been recycled too many times is retired because it produces weaker, more porous parts. A repeatable SLS outcome depends on tracking powder age and refresh rate as part of the process control, which is one reason batch-to-batch consistency varies with the shop.

When to use SLS, and when not to

Use SLS for functional nylon parts, complex and nested geometry, snap-fits and living hinges, and any part where support removal would be difficult or where many copies are needed. Do not use it for the finest cosmetic detail, where SLA wins, or for the lowest cost on simple geometry, where FDM wins. Expect a slightly rough as-built surface and a higher unit cost than FDM for single parts, and respect the 30-degree self-supporting limit on shallow angles. The full comparison of the three polymer processes is on the FDM vs SLA vs SLS page, and the cost drivers are on the 3D printing quote page.

Design rules for SLS

SLS design rewards geometry that uses the self-supporting powder bed and avoids features that trap powder or warp. Self-supporting angles start at about 30 degrees from horizontal, and the maximum unsupported bridge span is about 5mm. Minimum wall thickness is about 0.8mm for non-structural walls and 1.2mm for load-bearing walls, and minimum hole diameter is about 1.0mm, because smaller holes may close during printing.

Escape holes and hollow parts

Hollow parts save material and weight, but the trapped powder must escape or it stays sealed inside the part. Add escape or drain holes of at least 2 to 3mm, and place them where powder can flow out during depowdering. e.g., a sealed hollow enclosure with no escape holes arrives full of fused-in powder and may weigh almost as much as a solid part, defeating the point of hollowing it. Connect internal cavities to the outside through these holes, or split the part to allow cleaning.

Assemblies, snaps, and clearances

Because SLS needs no supports, co-printed assemblies, living hinges, and chain-link geometries are practical in a single build. Snap-fit clearance is about 0.3 to 0.5mm per side so the parts move freely, and a press-fit uses about 0.1mm of interference. For a moving assembly, print a test coupon first to confirm the clearance on the specific machine and material, because the as-built surface and the dyeing step both affect the real fit.