Why Does an MJF Part's Wall Thickness Tolerance Read Tighter on a Thin Feature Than a Thick One?

The tolerance isn't fixed in absolute terms. It's a percentage of nominal size or a fixed floor, whichever is greater, and on a thin wall the percentage term barely registers while the floor takes over. DigiForge's MJF tolerance is ±0.3 mm or ±0.3% of the dimension, whichever is greater. On a thin wall that floor is most of what you're measuring against, and ±0.3 mm on a 1.5 mm wall is a much bigger relative swing than the same ±0.3 mm on a 10 mm feature.

Why it reads differently depending on feature size

Take a 1.5 mm nominal wall. 0.3% of that is 0.0045 mm, nowhere near the 0.3 mm floor, so the real tolerance band is ±0.3 mm, about 20% of nominal thickness. Now take a 10 mm feature. 0.3% of 10 mm is 0.03 mm, still under the floor, so you're still at ±0.3 mm, but that's only 3% of nominal.

Same absolute tolerance. Wildly different relative tightness. That's the whole effect — the process isn't more capable on thick sections; the same fixed window just looks proportionally bigger on a small dimension.

There's a second, physical reason thin walls drift more in practice. MJF parts cool inside a powder bed after fusing, and thinner sections lose heat and shrink at a different rate than thick ones nearby on the same build. A thin rib next to a thick boss can land slightly undersized relative to where it would sit in isolation, because it's cooling against a different thermal mass. That's a real dimensional effect on top of the percentage-vs-floor math, and it's part of why thin features are the harder dimension to hold tight.

What to do about it

If a thin wall needs to hold close to nominal, don't fight it with a tighter print tolerance call-out. The ±0.3 mm floor doesn't move regardless of what you specify. Design the feature thicker if the tolerance matters more than weight savings, or plan a light machining pass on that one dimension if it's genuinely critical. For most functional nylon parts, brackets, housings, snap-fit features, the as-printed tolerance is fine once you've accepted it's closer to the ±0.3 mm floor than the percentage term on any wall under roughly 100 mm.

The practical takeaway

Treat ±0.3 mm as the number that governs almost every wall thickness you'll design in MJF nylon, not the 0.3% figure sitting next to it. The percentage term only takes over past about 100 mm, where 0.3% finally exceeds 0.3 mm. Design with that floor in mind and a thin feature won't surprise you against a thick one on the same part.