Why Do Push-Fit Tolerances Feel Too Tight or Too Loose Between Identical Nylon Prints?

It's almost always the printed tolerance band stacking against itself in the wrong direction, not a process defect. Powder-bed nylon processes like MJF and SLS hold roughly ±0.3 mm or ±0.3% of the dimension, whichever is bigger, and a push fit designed for machined tolerances gets swallowed by that band long before anything else.

Why the tolerance band is the whole story

A push fit that works reliably in machined aluminum might be sized around a few hundredths of a millimeter of clearance, because machining tolerances are an order of magnitude tighter. Powder-bed nylon printing doesn't hold that. At a nominal fit diameter of 10 mm, the printed tolerance is roughly ±0.3 mm, meaning the actual part could land anywhere across a 0.6 mm window and still be in spec.

If your designed clearance is smaller than that window, some parts fit snug and some rattle. That has nothing to do with print quality. It's the tolerance doing exactly what it's rated to do.

This is worse on a push fit than on most other features because a push fit is inherently a two-part tolerance stack. The pin's diameter and the socket's diameter each carry their own band, and the actual clearance is the sum of both. Two features each at ±0.3 mm can combine to a worst case of ±0.6 mm of clearance variation on the same nominal fit.

What actually causes the scatter within that

Orientation on the build and position within the powder bed both shift dimensional accuracy slightly. That's why two parts from the same file can come out at opposite ends of the tolerance band instead of clustering in the middle. Larger features and features further from the part's own geometric center tend to drift more than small, centrally located ones.

None of that is a defect. It's just thermal history.

What to do about it

Design push fits for powder-bed nylon around the process tolerance, not machined-part intuition. That usually means a deliberately looser fit than instinct suggests, with a chamfer or lead-in on both mating faces so the assembly self-centers instead of depending on a tight diametral tolerance to line up.

If the application genuinely needs a repeatable, tight-clearance fit, a bearing bore or a locating pin, that's a case for machining that one feature, or the whole part, rather than fighting a printed process's tolerance band.

DigiForge runs industrial SLS and MJF nylon at a tolerance of ±0.3 mm or ±0.3%, whichever is greater.

If that number makes you nervous about a push-fit assembly, trust the instinct. Size the fit loose, add a lead-in, and reserve tight fits for machined features.