Small and Medium Volume Production: How SLS Actually Works

• 1
  What happens inside an SLS build chamber
• 2
  Tolerances, surfaces, and where SLS loses accuracy
• 3
  Why small and medium volume fits SLS so well
• 4
  When to choose SLS and when to choose CNC
• 5
  Finishing steps that change the part
• 6
  SLS compared with CNC for small and medium volume

Selective laser sintering starts with a thin layer of polymer powder, usually 0.08–0.12 mm deep, spread across a heated build platform. A CO2 laser scans the cross-section of the part and melts the powder along that path. The melted polymer fuses, then cools. The roller or blade lays down the next layer and the cycle repeats until the build finishes.

The powder bed does two jobs at once. It feeds new material into the laser path, and it supports every overhang because loose powder surrounds the part. That is why SLS parts need no breakaway supports and no orientation compromise for simple geometry. A ball joint inside a housing can be built in one piece and moved after cleaning.

Parts cool slowly inside the build cylinder, which keeps internal stress lower than in a laser melting process with a solid substrate. Nylon 12 shrinks roughly 3 percent as it cools, so the machine applies a scale factor in software before the build starts. Get that factor wrong and the whole nest drifts out of tolerance together.

The build chamber stays at an elevated temperature, typically 160–180 °C for PA12. Uniform heat matters more than laser power for repeatability. A cold corner in the chamber produces a weak, porous part that looks fine coming out of the powder cake.

After the build, the cake cools, the operator breaks it out, and loose powder goes back to a sieve and a mixer. Refresh rates of 30–50 percent virgin powder keep mechanical properties stable across runs.

A well-tuned SLS machine holds about ±0.3 mm on a part up to 100 mm, and roughly ±0.3 percent of the longest dimension beyond that. That number depends on geometry, not just machine spec. Long thin walls warp. Large flat panels bow. A 200 mm unsupported rib can move 1 mm or more after cooling.

Layer thickness sets the surface finish. At 0.10 mm layers, as-built SLS surfaces sit around Ra 10–15 μm, with visible stair stepping on shallow angles and curved tops. Bead blasting brings that down to roughly Ra 4–6 μm. Vapor smoothing can reach Ra 1–2 μm but it rounds sharp edges and changes dimensions slightly.

Holes are a common trap. A nominal Ø5 mm hole built horizontally comes out undersized by 0.1–0.2 mm because of thermal contraction and unfused powder at the edge. Designers who need a sliding fit should expect a reaming pass or specify the hole oversized.

Minimum feature size is another boundary. Walls below 0.8 mm thick may build but break during depowdering. Small pins and snap fits survive better when oriented so the layer lines run across the bending axis rather than along it.

Moisture affects nylon parts after the build. PA12 absorbs water from air, which changes dimensions by a few hundredths of a millimeter and softens the material. Parts meant for dimensional inspection should be dried and measured quickly, or conditioned to a stated humidity before measuring.

Small and medium volume sits in an awkward zone for injection molding. A steel mold costs tens of thousands of dollars and takes weeks before the first good part. At 50 to 2,000 pieces, the amortized tooling cost per part is often higher than the part itself. SLS skips tooling entirely, so the cost curve starts at the first part and stays nearly flat.

Build volume drives the economics more than part count. A nest fills with many different parts from different orders, so one build can serve several customers. That is why the per-part price drops when the geometry packs well: tall thin parts use the same powder volume as short dense ones.

Material choice narrows the field. SLS covers PA12, PA11, glass-filled PA12, TPU, and some flame-retardant grades. It does not cover metals, high-temperature engineering resins above roughly 180 °C service, or optical clarity. Those needs go to other processes.

The cost of SLS is dominated by machine time, powder, and labor for depowdering and finishing. A part that needs extensive manual smoothing or fitting erases the tooling savings fast. Design for the process and the economics hold.

For quantities past a few thousand, the picture flips. Molded parts cost cents each once tooling is amortized, with better surface finish and tighter repeatability. SLS remains useful for bridge production while a mold is being cut, and for design changes that would otherwise scrap a mold.

Pick SLS when the geometry is organic, internally channeled, or impossible to reach with a cutter. Lattice structures, ducting with swept bends, and housings with captured internal features are natural fits. Nylon also tolerates impact and chemical exposure better than most machined plastics.

Pick CNC when the part carries a tolerance tighter than ±0.05 mm, needs a metal substrate, or must survive high temperature. Machined aluminum 6061-T6 or 17-4PH stainless holds ±0.005 mm at GreatLight and finishes at Ra 0.8–1.6 μm without a secondary smoothing step.

A hybrid route often works best. Use SLS for the complex shell and CNC for the mating faces, bearing bores, and threaded inserts. The two processes complement each other because each handles the features it does well.

Watch the interface between the two. A machined insert pressed into an SLS boss needs a wall thick enough to resist hoop stress, typically at least 2.5 mm around the bore. Threads formed directly in SLS nylon strip under load; use a metal insert or a machined thread.

Volume is the last filter, not the first. One hundred complex nylon parts favor SLS. One hundred aluminum brackets with three critical bores favor CNC. The decision usually comes down to tolerance, material, and how much post-processing the part can absorb.

Dyeing gives nylon parts a uniform color, most often black. The dye penetrates only a few tenths of a millimeter, so a later sanding pass exposes the lighter core. Dye after any abrasive step, not before.

Bead blasting removes caked powder and evens out the surface. It also rounds sharp edges slightly. A part that needs a crisp 90° corner for assembly should keep that corner masked or accept a small chamfer.

Threaded inserts, helicoils, and heat-set brass fittings are common on SLS parts that will be assembled and disassembled. Each insert needs a boss with enough wall thickness and a pilot hole sized to the manufacturer's data, not to a generic chart.

Sealing matters for parts that hold fluid or air. As-built nylon is slightly porous, so a pressure-tight SLS housing usually needs a sealant, a vapor-smoothed skin, or a machined mating surface.

Inspection closes the loop. SLS parts are usually checked with calipers and a CMM on the critical features only, because the as-built surface makes full dimensional mapping slow. Agree on the inspection features before the build, not after.