SLS nylon yellowing catches new operators off guard: the first builds come out white, then later ones turn yellow or uneven. In an r/manufacturing thread on starting SLS, a commenter explained that without a nitrogen atmosphere the nylon oxidizes, and that any oxidation turns parts yellow and weakens them. So what’s going on, and what can a shop do?
What causes the yellow color?
Oxidation and powder aging. The commenter noted that antioxidants are added to nylon powder, but oxidation that does happen shows up as yellowing along with lower strength. Research on PA12 in SLS (ScienceDirect, Journal of Manufacturing Processes) describes irreversible oxidation and post-condensation as the dominant aging mechanisms in unsintered powder, changing the polymer’s chemical structure.
Does a nitrogen atmosphere fix it?
It helps a lot. Formlabs recommends printing its Nylon 12 White Powder in nitrogen to keep a consistent white appearance across builds and the most consistent mechanical properties. It says parts printed in air may yellow after multiple builds, and often yellow unevenly as they cool in the chamber.
What if you print in air?
Formlabs says raising the refresh rate to 50% when printing in air may help slow yellowing from build to build. The trade-off is higher powder cost per build, so price it in.
Does yellowing matter if parts get dyed?
Often not for color. Formlabs notes it may not be an issue if the end goal is dyeing. But because oxidation is linked to weaker parts, don’t treat it as purely cosmetic on functional jobs.
What else affects white parts?
- Cross-contamination: Formlabs says white powder is sensitive to it and recommends a dedicated depowdering station and media blaster.
- Over-blasting: it can leave dull patches.
- Aged powder: the r/manufacturing commenter warned that too much aged powder hurts detail and surface finish.
How do you stay ahead of SLS nylon yellowing?
Log the atmosphere, refresh rate and powder age for each build, and note color on inspection. When yellowing shows up, you’ll be able to tell whether it tracks powder age, chamber position or a process change.
Related on the Solvi blog: SLS Orange Peel: Why It Happens and How to Fix It; Should You Dye MJF Parts Black or Vapor Smooth Them?; SLS Powder Cleanup: How to Keep Dust Under Control.
Sources: r/manufacturing SLS thread; Formlabs “Using Nylon 12 White Powder” support page; ScienceDirect PA12 SLS degradation study.