Why Does an MJF Part's Press-Fit Boss Loosen After a Few Insertions When a Machined One Holds?

Because MJF nylon is a sintered powder material with lower stiffness and more creep than machined metal or Delrin, so the interference that generates clamping force in a press fit relaxes over repeated cycles instead of holding its shape.

What a press fit actually relies on

A press-fit boss works by elastic interference. The boss is sized slightly larger than the bore, and inserting it forces both surfaces to deform elastically, generating a clamping force from that stored stress. The fit holds as long as the material keeps pushing back against the deformation. Metal and hard engineering plastics like machined Delrin have high enough stiffness and low enough creep that the interference stress stays close to its original value insertion after insertion.

MJF nylon (PA12) doesn't behave the same way. It has a noticeably lower elastic modulus than Delrin or metal, and nylon in general is prone to creep. Under sustained stress the material slowly deforms plastically rather than staying purely elastic. Every time you seat the boss, you're loading that interference stress right up near the material's yield behavior, and every insertion cycle lets a little of that stress relax permanently instead of springing back.

Why it gets worse with repetition, not just time

A single insertion into an MJF boss might feel fine. The problem shows up over several insertion cycles because each cycle adds to the cumulative creep. The bore's effective diameter increases slightly. The boss's outer diameter compresses slightly. The interference that generated your clamping force shrinks a little more each time. After enough cycles the fit that started snug is loose enough to rattle.

Layer orientation and part porosity make this worse in some geometries than others. A boss printed with its load direction across weaker interlayer bonding creeps faster than the same boss oriented so the load runs with the strongest print direction. Two nominally identical parts can behave differently depending on how they sat in the build.

What to do about it instead

If a joint needs to survive repeated insertion cycles, not a one-time assembly, a press fit in as-printed MJF nylon by itself is the wrong feature. A few options actually work.

Use PEM-style threaded inserts or heat-set metal inserts instead of relying on plastic-to-plastic interference. That moves the repeated-cycle wear onto a metal thread, not a creeping nylon bore.

If the joint really is a one-time assembly, a press fit sized generously for the first insertion is fine, since the creep problem is specifically a repeated-cycle problem and you're not asking it to survive one.

If the part needs both structural stiffness and dimensional stability under repeated fastening, a machined Delrin part is the better fit. Delrin's stiffness and much lower creep hold an interference fit through far more cycles than MJF nylon, and DigiForge machines Delrin as a standard material.

Match the fastening strategy to how the material actually behaves under sustained and repeated stress, not just its as-printed dimensions.