Why Does Dye Color Wear Off SLS Nylon Parts Over Time but Not MJF Parts the Same Way?

Dye in SLS nylon sits mostly in the surface pores of the material. MJF's porosity profile and surface finish hold pigment more evenly through a thin surface layer, so SLS color wears thin at high-contact points faster than MJF color does. Neither process is dyed all the way through.

What's actually happening at the surface

SLS sintering leaves a rougher, more open surface structure than MJF's fused-powder finish. That open structure is exactly what lets a post-process dye bath penetrate. It also means the dyed layer sits closer to the surface, in a less uniform pattern. Some pores take up more dye than others.

The exposed high points on that rough texture are the first material removed by handling, rubbing, or ordinary abrasion against whatever the part contacts in use. Once those high points wear down, the lighter substrate underneath shows through. The part looks faded at the worn spots well before the bulk color has gone anywhere.

MJF's finer, more uniform surface takes dye more evenly and doesn't have the same high-and-low texture for abrasion to selectively strip. The dye layer wears more uniformly. A casual look sees it holding color longer, and that's the whole basis for the reputation MJF has for not fading the way SLS does. It isn't immune. It's just more even about wearing.

What this means for a functional part

Neither process is through-dyed. If a part is going to see real wear, handled constantly, rubbing against a mating part, or exposed to repeated abrasion, don't expect dyed color on either process to hold up as a cosmetic promise over the part's service life. Treat color as cosmetic. Expect it to fade quickest wherever the part actually contacts something, dyed SLS more visibly than dyed MJF.

If color matching matters for a visible part and needs to stay consistent over time, that's a reason to specify MJF over SLS for that particular part, separate from any dimensional or mechanical reason to prefer one process.

This also affects how you spec an assembly with multiple dyed parts. If one part is SLS and a mating part is MJF, in the same color, expect them to drift apart visually well before either one is structurally worn out. Matching process as well as color keeps the whole assembly fading at roughly the same rate, even if it still fades.

The practical takeaway

It's not complicated.

For anything handled or rubbed against another surface regularly, budget for the dye to fade at contact points regardless of process, and lean MJF when color consistency over the part's life matters more than it would for a shelf part.

DigiForge's 3D printing line runs both MJF and SLS nylon among its material options, with a lead time of 5-7 business days either way. The dye choice doesn't change the schedule, only how the part looks a year from now.