3D Printing Design for Selective Laser Sintering
This guide covers the design rules that decide whether an SLS part works: wall thickness, hole sizing, clearances and tolerance stack-up. It is written for engineers and buyers who already have a CAD model and need to know what the process can hold. By the end you can tell which features to keep, which to move, and when to send the part to a mill instead.
What SLS does well, and what it does not
SLS builds parts by fusing powder layer by layer with a laser. No support structures, no molds, and no draft angles to design around.
How selective laser sintering shapes your design choices
Selective laser sintering fuses nylon powder with a laser, one thin layer at a time, inside a heated build chamber. Unfused powder surrounds the part and acts as support. That single fact drives most of the design rules below. Because the powder holds the part up, you can stack parts in the build, nest them, and print features that hang in mid-air without a support structure to remove.
For 3D printing design, selective laser sintering is usually chosen when geometry is too complex for a mill, or when part count is too low for injection molding. Internal channels, lattice blocks, and merged assemblies all fall into that bracket. The trade-off is surface finish and dimensional control. An SLS part comes out matte and slightly grainy, and holes tend to run small because the laser melts powder past the nominal edge.
Typical materials are PA12 and glass-filled PA12. Both are tough, slightly flexible, and stable enough for functional testing, jigs, and low-volume end use. If your part needs to hold ±0.005 mm or a mirror finish, SLS is the wrong process. That is a machining job, and we say so at the quote stage rather than after the build.
- 1Best fitComplex organic shapes, internal channels, small batch sizes
- 2Weak fitTight tolerances, optical surfaces, high stiffness parts
- 3Powder supportNo support removal, but trapped powder must escape
- 4Orientation mattersBuild direction sets strength and hole accuracy
Wall thickness, ribs and bosses
Keep walls between 1.0 mm and 3.0 mm for PA12. Below 1.0 mm the laser path can warp the wall or leave it porous, and thin walls rarely survive bead blasting. Above 3.0 mm you are adding cost and build time without much gain in stiffness. A 2.0 mm wall is a safe default for enclosures, brackets and housings.
For stiffness, ribs beat thick walls. Use a rib thickness around 0.6 to 1.0 times the wall, and keep the rib height under three times the wall thickness. Where a rib meets a wall, add a fillet of at least 0.5 mm. Sharp internal corners concentrate stress and are where cracked prototypes usually start.
Bosses for screws or inserts should be at least 2.5 mm thick in the radial direction, with a fillet at the base. If you plan to tap the boss directly, size it for the thread and remember that SLS threads are weaker than machined threads. For anything that will be assembled and disassembled more than a few times, design a pocket for a heat-set insert instead.
Holes, threads and running clearances
Small holes come out undersized. Plan on roughly 0.1 to 0.2 mm of diameter loss on holes below 5 mm, and more on holes under 2 mm. If a hole must accept a pin or a screw, build it at nominal plus the allowance, then ream or drill it after printing. For holes used only for airflow or cable pass-through, the undersize rarely matters.
Minimum hole diameter for a clean round hole is about 0.5 mm, and holes below 1.5 mm are better replaced with a drilled feature after the build. Very small holes may close entirely on a horizontal face. Orient the part so small holes are built on a vertical wall, where the circular cross section is defined by the laser path rather than by the layer stack.
Moving parts need clearance. For a printed hinge or a snap fit, allow 0.3 to 0.5 mm per side. Below 0.3 mm, powder can bridge the gap and fuse the two surfaces into one. If the joint has to move freely on the first try, use 0.5 mm and test the fit before committing to a full run.
- 1Holes under 2 mmExpect 0.1–0.2 mm undersize, drill after build
- 2Holes 2–5 mmAdd 0.1 mm allowance, ream if critical
- 3Holes over 5 mmUsually close to nominal
- 4Mating surfaces0.3–0.5 mm clearance per side
SLS design limits for PA12 parts
Use these as starting values. Confirm critical features with your engineer before release.
| Feature | Recommended value | Note |
|---|---|---|
| Minimum wall thickness | 1.0 mm | 2.0 mm is a safer default |
| Preferred wall thickness | 2.0–3.0 mm | Thicker adds cost, not stiffness |
| Minimum hole diameter | 0.5 mm | Below 1.5 mm, drill after build |
| Hole undersize allowance | 0.1–0.2 mm | Larger for holes under 2 mm |
| Minimum rib thickness | 0.6 × wall | Fillet the rib base at 0.5 mm |
| Moving clearance | 0.3–0.5 mm per side | Prevents powder bridging |
| Typical tolerance | ±0.3 mm or ±0.3% | Whichever is greater |
| Surface finish as built | Ra 6–10 μm | Bead blasting smooths it |
| Minimum text height | 2.0 mm | 1.5 mm for laser marking on machined parts |
| Maximum part size | Depends on build chamber | Large parts can be split and bonded |
What tolerance SLS can actually hold
SLS holds roughly ±0.3 mm, or ±0.3 percent of the dimension, whichever is larger. That is not a defect of the equipment. It comes from thermal shrinkage as the part cools, layer thickness, and the way the laser melts powder slightly past the boundary. Long thin parts can warp more, because they cool unevenly across the build.
If a feature needs better than ±0.3 mm, split the job. Print the body with SLS and machine the critical interfaces afterward. A printed housing with two machined bores is often cheaper and faster than a fully machined housing, and it holds the tolerance where it matters. This hybrid route is worth asking about at the quote stage.
Datum selection matters for inspection. Pick datums that exist after the build, not ones you planned to machine later. If you measure an as-built SLS part against a fully machined model, some of the deviation you see is datum shift, not process error.
Choosing between SLS and CNC machining
SLS wins when geometry is complex, quantity is low, or the part needs internal channels that a cutter cannot reach. There is no tooling cost, so a single part is economical. Changes between iterations cost nothing but build time.
CNC machining wins when you need tight tolerance, a specific metal, a smooth finish, or a part that must survive real load. A 6061-T6 aluminium bracket machined to ±0.005 mm behaves very differently from a nylon part, even if the shapes match. For anything that bolts to a vehicle, a machine, or a patient-contact device, the material usually decides the process.
A practical route for many programs is to print the first article in SLS to check fit and form, then machine the production parts in aluminium or stainless. The printed part proves the geometry quickly and cheaply. The machined part carries the load and the tolerance.
- 1Choose SLSComplex geometry, low volume, internal channels, fast iterations
- 2Choose CNCTight tolerance, metal, load-bearing, smooth finish
- 3Hybrid routePrint for fit, machine for function
Design questions engineers ask about SLS
Can SLS parts be tapped for threads?
Yes, but printed threads are weaker than machined threads and tend to strip after a few cycles. Size the boss at least 2.5 mm thick in the radial direction and fillet the base.
For anything assembled more than a few times, design a pocket for a heat-set insert. That gives you a metal thread in a printed body.
How do I stop small holes from closing up?
Keep holes above 1.5 mm where possible, and add 0.1 to 0.2 mm to the diameter. Orient the part so the hole axis is horizontal, so the round cross section is defined by the laser path.
If a hole must be precise, print it undersize and drill or ream it after the build.
Can SLS parts be painted or dyed?
They can be dyed in the build, which gives a uniform color through the surface. Painting is possible after sanding and priming, but the matte texture shows through thin coats.
Bead blasting before finishing gives a more even surface and better paint adhesion.
What is the largest part you can build?
It depends on the build chamber. Very large parts can be split into sections, printed, and bonded or pinned together. The joint needs a designed mating feature, not just a flat cut.
Send the model and we will tell you whether it fits in one build or needs to be sectioned.
Does build orientation change the part strength?
Yes. Layers bond more weakly in the build direction than within a layer. A part loaded across the layer stack can fail along a layer line.
Tell us the load direction and we will orient the part so the layers run across the stress, not along it.
Can you machine an SLS part after printing?
Yes. We can print the body and machine critical bores, faces or threads to ±0.005 mm. This keeps the complex geometry in the printed section and puts the tolerance only where it is needed.
It is often cheaper than machining the whole part from solid stock.
Send your model for a design review and quote
Upload a STEP file and we will return a quotation with free DFM notes in 12 hours. We will flag wall thickness, hole sizing and any feature better machined than printed.
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