Troubleshooting: Porosity in SLS Nylon Parts

What SLS Porosity Looks Like in Nylon Parts

SLS porosity shows up as small internal voids, surface pinholes, or irregular pockets inside a nylon part. In a functional component, these voids reduce load-bearing strength, create leakage paths, and introduce inconsistent density. In a decorative or cosmetic part, they may only show as a rough texture after surface finishing. The problem is that SLS porosity is not one defect. It is a cluster of related failure modes that behave differently depending on where they appear and how they form.

A typical industrial part might look fine on the outside. A machined flange, a housing with internal channels, or a bracket with thin walls can pass visual inspection and still contain voids in the core. That is why cross-sections matter. A part that fails in service often fails in a way the surface never showed.

Common Symptoms of SLS Porosity in Nylon

Before you change a machine setting, map the symptom to the location. The same defect in a different place has different root causes.

Symptom	Likely cause	What to do
Internal voids in thick sections or corners	Low laser energy, poor powder packing, or poor flowability	Inspect powder humidity and flowability. Increase laser energy or scan speed adjustment. Check for powder clumping.
Surface pinholes or rough texture	Powder contamination, moisture, or over-powdering	Clean powder bed, replace or dry powder if needed. Reduce powder layer thickness or improve raking.
Porosity concentrated near part edges or overhangs	Laser energy too low for unsupported geometry, or poor powder density at edges	Adjust scan strategy for overhangs. Increase local energy. Check powder bed compaction at part perimeter.
Voids in thin walls or small features	Insufficient energy to fully sinter thin geometry	Increase energy for small features. Check laser focus and power calibration.
Porosity only in certain builds or lots	Powder lot variation or machine drift	Compare lot certification data. Run machine calibration and laser power verification.
Porosity after heat treatment or post-processing	Thermal stress or residual stress concentration	Review heat treatment curve. Check for thermal cycling stress in the part design.

Internal voids are the most common form of SLS porosity in nylon. They form when powder particles do not fully sinter into a continuous structure. The result is a part with pockets of unsintered powder trapped inside. Surface pinholes are more common when powder is contaminated, too moist, or too loosely packed in the laser path.

Likely Causes of Porosity in SLS Nylon Prints

Porosity in SLS nylon is almost always a combination of powder state, machine condition, and part geometry. No single cause is responsible in every case. The goal is to isolate which variable changed.

Powder flowability is the starting point. Nylon powder is hygroscopic. If the powder sits exposed to humid air, moisture bonds particles into clumps. Clumps do not flow evenly. They do not pack consistently. The laser then hits a patch of powder that is either too loose or too dense. Both conditions produce porosity.

Laser energy is the second major cause. Too little energy leaves gaps between particles. Too much energy can melt the surface and create warping, but it does not usually cause internal voids. The problem in most SLS porosity cases is insufficient energy for the specific geometry. A thick section needs different energy than a thin wall. A corner needs more energy than a flat plane.

Machine condition matters. The laser power calibration drifts over time. The laser focus shifts. The powder bed compactor or rake system wears. These changes are subtle, but they change how the powder receives energy. A machine that produced dense parts last month may produce porous parts this month if the laser power has drifted.

Part design is the fourth cause. Overhangs, sharp corners, and thin walls are high-risk zones. Nylon powder sintering is a surface melting and bonding process. Geometry that blocks powder flow or creates stress concentration will produce porosity even on a well-tuned machine.

How to Diagnose SLS Porosity

Diagnosis starts with the part, not the machine. Take the part apart if you can. A machined cross-section reveals internal voids that visual inspection misses. If the part is expensive or too complex to cut, use X-ray inspection or CT scanning. Even a small inspection window can show the void pattern.

Once you see the void pattern, ask three questions.

1. Where are the voids located?
2. Are they random or patterned?
3. Do they correlate with a specific feature or build area?

Random voids across a large area usually point to powder state or laser energy. Patterned voids, such as voids only in overhangs or only in a specific corner, point to scan strategy or geometry. Voids in one build area and not another may indicate a local machine issue, such as a powder feed problem or a lens contamination.

Do not skip the powder check. Take a sample of the powder from the machine. Measure its flow rate, moisture content, and particle size distribution. Compare it to the lot certification data. If the powder state changed, the machine settings may be fine but the material is not.

Fixes for SLS Porosity in Nylon Parts

The fix depends on the cause. If the powder is contaminated or too moist, drying or replacing the powder is the first step. If the powder is fine, adjust the laser energy. Increase energy for areas with known porosity. Reduce energy if the surface is too melted. If the problem is geometry, change the scan strategy. Add supports for overhangs that are too aggressive. Break thin walls into smaller features. Increase the number of layers in thick sections to improve energy distribution.

For parts that already have porosity, reprocessing is sometimes possible. A part with surface pinholes can sometimes be re-sintered in a secondary pass. A part with deep internal voids is usually not worth reprocessing. The voids have already compromised the structure. In that case, the part is scrapped and the root cause is documented.

Post-processing matters too. Heat treatment can relieve residual stress, but it can also make existing porosity worse if the thermal curve is too aggressive. A part with small voids may survive a moderate heat treatment. A part with large voids may crack or deform. Always review the heat treatment curve against the part design and the defect pattern.

Prevention Tips for SLS Quality Control

Prevention is cheaper than rework. It also gives you a defensible record for buyers and quality teams. Keep the following checks in your routine.

1. Verify powder lot data before every build. Record lot number, moisture content, and flow rate.
2. Calibrate laser power and focus on a schedule. Record the results.
3. Inspect powder bed compaction after each build. Look for uneven packing.
4. Use a standard test part for each new lot and each machine change.
5. Document every parameter change. If a part fails, the log tells you what changed.
6. Keep material certification records and machine calibration logs together.
7. Review part design for high-risk geometry before slicing.
8. Run a post-build inspection on critical parts. Cut a cross-section if the part is structural.

The test part is the most underused tool in polymer 3D printing issues. A small coupon with a known set of features gives you a baseline. If the coupon passes, the machine is likely fine. If it fails, you know the machine or powder is the problem before you run a full production job.

When to Escalate SLS Porosity Issues

Some porosity is within the acceptable range for a given application. A cosmetic housing may tolerate small surface pinholes after sanding. A pressure-rated part cannot. The difference is the function. If a part fails in service because of SLS porosity, the issue is no longer a surface finish problem. It is a structural defect.

Escalate when the defect appears in a critical feature, when the same defect repeats across multiple builds, or when the part fails in a way that cannot be explained by the defect pattern. In those cases, the root cause is not a single parameter. It is a system issue. Powder handling, machine maintenance, and process control all need review.

Keep the documentation. A defect report that includes the part number, build date, powder lot, machine ID, parameter set, and defect location is worth more than a verbal explanation. It lets you compare builds, identify trends, and make informed decisions.