SLS material testing: why powder handling and finishing change your test results

Inframotion 3D has published a guide on qualifying SLS materials. Its main point: test the part as it is delivered, because powder condition, cooling, reuse and finishing all shift measured values. Datasheet figures such as Nylon 12 tensile strength are only usable with that process context.

Context SLS (selective laser sintering) fuses preheated polymer powder layer by layer with a laser, while the surrounding unfused powder supports the part during the build.

What should an SLS test cover?

Tensile tests give the baseline: how much load a feature carries, how stiff it is and how far it stretches before failing. Impact and fatigue behaviour matter for long-term use. Flexible grades such as TPU 88A are judged on tear resistance, abrasion, elastic recovery and damping, since they serve as seals and protective features. Heat resistance, warpage risk, repeatability and moisture effects are added where the application needs them, as are chemical exposure tests against the real media.

Which process steps change the numbers?

Powder particle size, shape and flowability decide how evenly the bed packs and how consistently layers fuse. Poorly controlled cooling raises warpage, built-in stress and batch variation. Aged powder shifts fusion behaviour, so the refresh or reuse strategy belongs in the test plan. Depowdering needs the same discipline to avoid contamination. Blasting, dyeing or smoothing then alter surface texture, sealing and some measured values. With third-party powders, the printer and powder combination should be judged as one system.

How should Nylon 12 data and failure modes be handled?

PA12 is the reference SLS material for its balance of strength, durability and dimensional stability. Its tensile value only means something next to stiffness, elongation, impact or fatigue results and the part geometry, and only when grade, settings, cooling, powder strategy and finish match. Early checks should look for warping, shrinkage, distortion and oversintering at thick-to-thin transitions, trapped powder in hollow parts, and rough or porous surfaces that limit sealing. Thin walls and fine details can fail even when coupon data looks fine.

How is a practical test matrix built?

Start from service conditions: load, temperature, chemical contact, permitted deflection, surface needs and lifetime. Combine coupon data with checks on the printed part, such as dimensions, batch consistency and functional tests tied to its geometry. Run these on parts in the delivered state, using the intended printer, material, powder strategy and finishing.

What it means

For small and mid-sized manufacturers, the practical message is that a datasheet value is not a qualification result. Budget for testing printed parts in the delivered finish, and record the powder refresh strategy and post-processing as fixed process inputs. Changing either can invalidate earlier data, which affects cost per part and lead time for certification. Without someone owning these records, results from different batches stay hard to compare. The guide is general and gives no numbers, so it works as a checklist, not as acceptance criteria.

Frequently asked questions

Why is Nylon 12 tensile strength not enough to select an SLS material?

It must be read with modulus, elongation, fatigue or impact behaviour and part geometry. Values also vary with grade, process settings, cooling, powder refresh and finishing, so only like-for-like data compares.

Which SLS materials does the guide discuss?

It compares PA12, Nylon 11, PA12 GlassFilled and TPU 88A, with TPU judged on tear resistance, abrasion, recovery and damping.

What failures should an SLS validation plan catch early?

Warping, shrinkage, distortion and oversintering, trapped powder in hollow parts, rough or porous surfaces that limit sealing, and fragile thin walls or details.