SLS Nylon Functional Prototypes: Where the Strength Actually Comes From

• 1
  What Makes SLS Nylon Functional Prototypes Strong
• 2
  PA12 vs PA11: Which Powder Fits the Job
• 3
  How Powder Reuse and Thermal History Change the Result
• 4
  Design Rules That Decide Whether the Part Survives
• 5
  When SLS Nylon Is the Wrong Call
• 6
  Post-Processing Steps That Add Real Strength
• 7
  Nylon Grades for Functional SLS Prototypes
• 8
  SLS Nylon vs Machined Nylon vs Aluminium

Selective laser sintering builds a part by fusing powder particle by particle, layer by layer. There is no binder to burn off and no support structure to cut away, because the surrounding powder holds the part in place. That is why SLS nylon functional prototypes can carry internal channels, living hinges and nested geometry that would be awkward or impossible to mold. The laser does not melt the whole bed. It raises each cross-section just above the melting point of the polymer, long enough for the chains to diffuse across the particle boundaries, then lets the bed cool.

The strength of the finished part comes from how completely those particle boundaries heal. Where fusion is complete, the material behaves close to bulk PA12. Where it is not, the part fails along layer lines and shows low elongation at break. Two parts printed from the same file on the same machine can differ noticeably if the powder was handled differently or the bed cooled at a different rate. That is the part most datasheets leave out.

Nylon is also hygroscopic. A dry PA12 part is stiff but brittle. Conditioning it in air at roughly 50% relative humidity for a few days brings moisture back into the amorphous regions and raises impact resistance. For a functional prototype that will be dropped, snapped onto a clip or cycled in a fixture, conditioning is not cosmetic. It changes how the part behaves in the test you are about to run.

So "strong" is a process outcome, not a number printed on a bag of powder. The variables that matter are the polymer grade, the powder's thermal history, the laser energy density, the chamber temperature, and the cooling curve after the build. Control those and you get repeatable parts. Ignore any one of them and you get a good-looking prototype that cracks at the first assembly.

Polyamide 12 is the default for SLS. It has a wide sintering window, which makes it forgiving to run and gives a dense, low-porosity part. Typical conditioned PA12 shows tensile strength in the mid-40s MPa, elongation at break well above 10%, and good resistance to oils, greases and dilute acids. Housings, brackets, ducts, jigs and snap-fit enclosures are all comfortable in PA12.

Polyamide 11 is bio-based and behaves differently. It is tougher at low temperature, has better impact resistance, and holds up better against hydrocarbons and some fuels. Tensile strength is a little lower than PA12, but elongation is higher. If the prototype is a fuel-line clip, a cold-chain part, or something that will be flexed repeatedly, PA11 is usually the better choice. It costs more and the process window is narrower.

Glass-filled grades change the picture again. Adding 30% glass microspheres or short glass fibers raises stiffness and heat deflection temperature, and cuts warpage. It also drops elongation sharply and makes the surface more abrasive. A glass-filled PA12 bracket is stiff and dimensionally stable, but it will not tolerate a snap-fit being closed and opened many times. Use it where stiffness matters more than toughness.

There is no single best grade. Match the polymer to the load case. A part that sees a slow compressive load wants stiffness. A part that sees impact or repeated deflection wants elongation. A part that sees both needs a geometry change, not a different powder.

In an SLS build, most of the powder is not sintered. It sits in the bed as support and is recovered, sieved and mixed back in. Every cycle at elevated temperature ages that powder. Molecular weight drops, melt flow rises, and the sintering window narrows. A part built from 100% virgin powder and a part built from heavily refreshed powder are not the same part, even at identical laser settings.

The practical consequence is that elongation at break falls before tensile strength does. A reused-powder part can still pass a strength check and then fail a drop test. This is why a documented refresh ratio matters. Shops that track powder age and blend virgin material at a fixed ratio get far more consistent results than shops that simply top up the bed.

Cooling is the second thermal variable. A build pulled hot from the chamber cools unevenly, and the outer skin of a thick section shrinks while the core is still warm. That leaves residual stress, which shows up later as warpage, curled edges, or cracking along a layer line during assembly. Controlled cooling inside the machine, with the part left in the powder cake until the chamber drops steadily, removes most of it.

For a functional prototype, ask for the powder refresh policy and the cooling practice. If neither is documented, the parts may look correct and still behave differently from one build to the next. Repeatability is what makes a prototype useful as a design decision, not just a visual check.

SLS has no support structures, but it does have a minimum feature size below which a section simply does not form. Walls under about 0.8 mm are risky. Between 1.0 mm and 1.2 mm is workable for non-structural skins, and 2.0 mm and up is where the part starts behaving like the material datasheet suggests. Thin walls also cool faster than the surrounding powder, so they warp and lift.

Bosses and ribs are the usual failure points. A vertical boss with a thick solid root creates a sharp thermal gradient between the boss and the wall. It cools, shrinks, and pulls a dish into the panel. Core the boss, thin the rib to roughly 60% of the wall thickness, and add a fillet where it meets the surface. Those three changes do more for strength than switching powders.

Orientation decides anisotropy. The bond between layers is weaker than the bond within a layer, so a part loaded across the build direction fails earlier. For a bracket, place the build so the main tensile load runs in the plane of the layers, not across them. If the design cannot avoid a cross-layer load, thicken the section or add a machined insert at that point.

Holes and threads need clearance too. Small holes under about 1.5 mm may close or come out oval, so design them slightly oversize and drill to size if the tolerance matters. Printed threads in nylon are usable for a few assembly cycles, but a metal insert or a tapped post is the better answer where the joint will be opened repeatedly. Add a chamfer to the hole entry, and keep countersinks shallow.

If the prototype has to hold a tolerance tighter than the process can repeat, SLS will disappoint. Sintering leaves a slightly porous surface and a dimensional spread that varies with orientation, position in the bed and powder age. A sliding fit, a bearing seat or a sealing face usually needs machining afterward. Budget for that step or choose a different process from the start.

High stiffness in a thin section is another mismatch. Nylon is tough, but it is not stiff. A long unsupported arm in PA12 will deflect under load no matter how the build is oriented. Adding glass filler helps a little and costs elongation. If the part has to stay rigid, aluminium 6061-T6 or a steel grade machined on a 5-axis center will hold shape far better and take a fine finish.

Heat is a hard boundary. Unfilled nylon softens well below the temperatures an engine bay or a soldering fixture will see. A prototype that will sit near a heat source needs a different material, and often a different process. Testing an SLS part in that environment tells you nothing useful about the final design.

Surface finish is the last one. As-built SLS has a grainy, matte texture. Bead blasting smooths it, and dyeing gives a uniform color, but it will not look like an injection-molded cover. If the prototype is going in front of a customer for a visual review, plan for post-processing or machine the visible faces.

Post-processing is where a lot of prototype strength is either protected or thrown away. Bead blasting removes the loosely bonded surface particles that would otherwise flake off during handling. It also cleans the part without cutting into it. Keep the pressure moderate; aggressive blasting on a thin wall can round edges and thin the section.

Dyeing is cosmetic and slightly dimensional. The dye bath is warm, and nylon absorbs moisture during it, so parts come out a little larger and softer than they went in. That is usually fine, but it means a dyed part should be measured after conditioning, not before. If a critical dimension is near the limit, dye before the final check.

Vapor smoothing and tumble polishing are sometimes used to improve surface finish. Both can reduce edge sharpness and slightly change wall thickness on thin features. They are worth it on a visible cover, less so on a bracket where the texture does not matter and the geometry does.

For prototypes that will be tested to failure, the most useful post-processing is simply conditioning plus a documented build record. Knowing the powder batch, the refresh ratio, the orientation and the cooling curve lets you compare two builds honestly. Without that record, a passing test result is hard to reproduce and a failing one is hard to explain.