3D printing design SLS: rules that hold up on the shop floor

Selective laser sintering builds parts inside a heated powder bed. A laser traces each cross-section and fuses the polymer, then a roller spreads the next layer. Because unfused powder stays packed around the part, overhangs and internal channels do not need dedicated support.

The build chamber sits just below the melting point of the polymer, so the part cools slowly and internal stress stays low compared with a process that melts and resolidifies quickly. That is why SLS parts rarely warp the way a large flat FDM print does.

The trade-off shows up in two places. Powder has to be removed from every cavity you create, and the outer surface stays slightly grainy because it is formed against loose powder, not a smooth tool. Design decisions should start there.

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  No support removalOverhangs down to about 45° from vertical print without extra geometry.
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  Powder escape pathsEvery enclosed void needs at least two openings so trapped powder can leave.
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  Layer directionBuild orientation changes hole roundness and surface texture on vertical faces.

A wall thinner than 0.8 mm prints but flexes and can curl at the edges. For PA12, 1.0 mm is a practical floor for a load-bearing wall, and 1.5 to 2.0 mm is comfortable for parts that will be handled. Anything above 4 mm of solid section starts to trap heat and increases the risk of a hard, brittle core.

Bosses around a screw need more care. A boss wall under 1.2 mm may crack when a thread-forming screw is driven in. Keep the boss outer diameter at roughly twice the screw diameter, and add a fillet at the base instead of a sharp corner.

Ribs can be thinner than walls because they are supported on one edge. A 0.8 to 1.0 mm rib with a 0.5 mm tip radius usually survives handling. Sharp rib tips create a thin sliver of material that the laser cannot fuse cleanly.

Small holes sinter shut. In a vertical orientation, a Ø1.0 mm hole often comes out undersized or blocked, while a Ø2.0 mm hole stays open with reasonable roundness. Give holes 0.1 to 0.2 mm of extra diameter if the fit is loose, and expect to ream or drill any hole that has to mate with a precision pin.

Holes built horizontally come out oval because the top of the circle is formed against powder and sags slightly. On a Ø5 mm horizontal hole, expect 0.1 to 0.2 mm of ovality. Orienting the part so critical holes run vertical is usually worth more than any tolerance callout.

General SLS tolerance in PA12 sits around ±0.3 mm, or ±0.3% on long dimensions, whichever is larger. That is a working figure for a whole part, not a promise on a single feature. Do not dimension an SLS part like a machined one.

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  Mating surfacesLeave 0.3–0.5 mm clearance between two SLS parts that slide.
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  Press fitsSkip them. Print oversize and machine the bore.
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  Datum choicePick a flat face, not a curved powder-facing surface.

Enclosed hollows are the most common reason a part gets rejected at the powder station. A sealed void keeps powder inside, adds dead weight, and can release dust later. Put two openings on opposite ends of any cavity so compressed air and vibration can clear it. A single opening rarely works.

Internal channels for fluid or wiring need a minimum Ø2 mm bore and smooth transitions. Sharp 90° corners in a channel act as powder traps. Where a channel turns, use a radius at least equal to the channel diameter.

As-sintered surfaces sit around Ra 10 to 15 μm and feel like fine sandpaper. Bead blasting brings that down and evens out the color. For tighter finishes, we machine the critical faces on a CNC after printing, which also holds ±0.005 mm on those features. Dyeing, sealing, and tumbling are available when the part needs a specific look.

SLS wins when the geometry is organic, has internal channels, or would need five setups on a mill. It also wins when you need ten copies of a bracket with no tooling cost. Complex geometry comes out of the build at the same cost per part as a simple block of similar volume.

CNC wins when a face has to be flat and precise, when the material must be aluminium, steel, or PEEK, or when the part is a simple prismatic shape. Machining that part often costs less than printing it, and the tolerance is an order of magnitude tighter.

A hybrid route is common for functional prototypes: print the body in PA12 for fit and ergonomics, then cut the load-bearing insert or the sealing face on a 3-axis or 5-axis mill. That keeps the expensive tolerance only where it is needed.

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  Pick SLSLattice, internal channels, undercuts, low-volume duplicates.
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  Pick CNCFlat sealing faces, tight bores, metal parts, simple shapes.
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  Pick bothPrinted body plus a machined insert or sealing face.