SLS 3D Printing
Also: SLS, selective laser sintering, MJF, Multi Jet Fusion, HP Multi Jet Fusion, powder bed fusion, nylon 3D printing, PA12 3D printing, PA11 3D printing
SLS and HP MJF fuse nylon powder into tough PA12 and PA11 parts with no supports: functional prototypes and production runs into the low thousands.
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Selective laser sintering (SLS) spreads a layer of nylon powder, about 0.08-0.12 mm (0.003-0.005") thick, over a bed held just below the melting point and fuses each cross-section with a CO2 laser. HP Multi Jet Fusion (MJF) does the same job with an inkjet head that prints a fusing agent where the part should be and a detailing agent at its edges, then fuses the whole layer with an infrared lamp. In both, the unfused powder holds the parts up, so no supports are needed and parts can be nested throughout the build volume.
The result is a tough nylon part with strength close to molded nylon 12 but lower elongation, and more uniform properties in every direction than FDM. That makes SLS and MJF the default for functional plastic prototypes and for production runs from tens to a few thousand: housings, ducts, brackets, snap-fit clips, living hinges in PA11, consolidated assemblies and TPU lattice cushions. Because the whole build volume is packed with parts, the cost per part falls sharply when the build is full, and one build can hold hundreds of small parts.
MJF and SLS differ in the details. MJF builds faster, reuses more of its powder and gives slightly finer, more consistent surfaces, and its parts come out gray and are usually dyed black. Laser SLS offers more materials (including PEEK and PEKK on high-temperature machines), larger build volumes and white parts that take dye in colors. Either way the surface is grainy and slightly porous until it is tumbled, dyed, vapor smoothed or painted, large flat sections warp, and thick sections waste powder and time.
At a glance
| Typical tolerances | Standard ±0.3 mm (±0.012") or ±0.3% of the dimension, whichever is greater; ±0.2 mm (±0.008") with care on small parts, where MJF usually holds a little tighter than laser SLS. Large flat sections and thick-to-thin transitions warp, and Z dimensions vary more than X and Y. Bores and fits that matter are reamed or machined. |
|---|---|
| Size limits | HP MJF builds within 380 x 284 x 380 mm (15 x 11.2 x 15"); common laser SLS machines within about 340 x 340 x 600 mm (13 x 13 x 24"), and large-frame machines to about 700 x 380 x 580 mm (28 x 15 x 23"). Minimum wall about 0.7-1 mm (0.030-0.040"); holes under about 1.5 mm (0.060") and narrow channels trap powder; moving parts printed in place need about 0.5 mm (0.020") of clearance. |
| Surface finish | Grainy matte after bead blasting: about 5-10 µm Ra (200-400 µin) on MJF and 8-15 µm Ra (315-600 µin) on laser SLS, with faint layer steps on shallow curves. Vibratory tumbling rounds edges and smooths to roughly 3-6 µm Ra (120-240 µin); chemical vapor smoothing seals the surface and brings it to about 1-3 µm Ra (40-120 µin), giving a molded look and a washable, less porous part. MJF parts are usually dyed black; SLS parts are white and take dye in colors. |
| Lead time | Prototypes in 3-7 business days, including cool-down, depowdering and dyeing; production batches of hundreds to a few thousand in 1-3 weeks. Vapor smoothing and painting add 2-5 days. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
PA12 (nylon 12) is the workhorse: tough, chemically resistant and low in moisture uptake for a nylon. PA11 is more ductile and impact resistant, better for snap fits and living hinges. Glass-filled PA12 adds stiffness and dimensional stability, carbon-filled nylon adds more stiffness, TPU prints flexible lattices, gaskets and grips, and polypropylene prints chemically resistant, weldable parts. High-temperature SLS machines print PEEK and PEKK for aerospace ducting at a few shops. Printed nylons are PA12 and PA11, not the nylon 6/6 of machined stock, which is linked here for comparison. Amorphous plastics such as ABS and polycarbonate do not sinter to dense parts and are not offered for functional work.
What drives the cost
- Packed volume: the bounding box of the part and how densely it nests with other parts in the build
- Part volume: thick, solid sections consume and age more powder
- Build height: parts that share a full build share one heat-up and cool-down cycle
- Material: PA11, TPU, filled grades and PEEK cost more than PA12
- Finishing: dyeing, tumbling, vapor smoothing and painting
- Quantity: setup is small, so the price per part falls mainly by filling builds
- Inspection and documentation: first article inspection, material certs, lot traceability
When to use it
- Functional plastic prototypes that must survive testing
- Production runs of tens to a few thousand parts, and bridge production while molds are built
- Complex geometry: internal channels, lattices, snap fits, living hinges and consolidated assemblies
- Ducts, housings, clips, brackets and enclosures where nylon properties are adequate
- Small parts nested many to a build, such as clips, connectors and covers
When not to
- Volumes above a few thousand a year of a moldable part: injection molding is cheaper per part
- Glossy, clear or highly cosmetic surfaces as-printed
- Tolerances tighter than ±0.2 mm (±0.008") without post-machining
- Sustained load at elevated temperature: nylon creeps and softens far below metal service temperatures
- Long, flat, thin panels, which warp
Design tips
- Keep walls 1-3 mm (0.040-0.120") and as uniform as possible; hollow thick sections and add at least two escape holes of 4-5 mm (0.16-0.20") so the powder can be removed.
- Give moving parts printed in place at least 0.5 mm (0.020") of clearance, more on large parts.
- Use PA11 or TPU for snap fits and living hinges that flex many times; PA12 is stiffer and cracks sooner under repeated bending.
- Add ribs instead of thick walls, and stiffen or split long, flat, thin panels.
- Heat-set inserts and thread-forming screws work well in nylon; tap fine threads rather than printing them.
- Tell the shop which dimensions are critical and which faces are cosmetic, so the part is oriented accordingly in the nest.
- Specify the finish: raw bead-blasted, dyed black, tumbled, vapor smoothed or painted.
Frequently asked questions
- SLS vs MJF: what is the difference?
- Both fuse nylon powder without supports and make similar parts. MJF prints a fusing agent and fuses each layer with an infrared lamp, so it is faster, slightly more accurate and more consistent, and its parts come out gray and are usually dyed black. Laser SLS scans each layer with a CO2 laser, offers more materials and larger machines, and makes white parts that can be dyed in colors. For PA12 parts, price and lead time usually decide.
- Is SLS nylon strong?
- Printed PA12 has a tensile strength around 45-50 MPa (6.5-7.3 ksi), close to molded nylon 12, but its elongation at break is roughly 10-20%, much lower than molded nylon, so it is tough but less forgiving. PA11 stretches further and handles impact and repeated flexing better. Both are strong enough for housings, brackets, clips and ducts.
- Are SLS and MJF parts watertight?
- Not reliably as printed: the surface and the interior are slightly porous, so thin walls weep under pressure. Vapor smoothing, impregnation or a sealing coat makes parts watertight, and anything that must hold pressure should be tested.
- When is injection molding cheaper than SLS or MJF?
- When the tooling cost is spread over enough parts. For small nylon parts, printing often stays cheaper into the hundreds or low thousands; molds start at a few thousand dollars for simple aluminum tools and run much higher for steel and multi-cavity molds, so the crossover moves with part size and complexity. Get both quotes once the design is stable.
