I've been designing print-in-place articulated models for a while, and the same message lands in my inbox every week: "the joints came out fused, the STL must be broken."

It's nearly always one of these four, in order of how often I see it.

1. First layer squish. If your nozzle is a hair too close, that first layer spreads sideways and welds the gap shut before the model has a chance. The joint is 0.25mm. Squish costs you 0.1mm of it. Level, then print a first layer test, then print the toy.

1. Over-extrusion. Flow at 102 to 105 percent feels harmless on a cube. On a ball joint it's the whole clearance budget. Calibrate flow once with a single-wall cube and it fixes half the fused prints people send me.

1. Heat creep on small layers. A 4cm raptor has tiny layers, so the same spot gets re-melted over and over. Print two at once, or add a 10 second minimum layer time. The joints stay crisp.

1. Elephant foot on the bottom joint. The lowest joint fuses while the top ones move freely, which is the giveaway. Set elephant foot compensation to 0.2mm and it stops.

The tolerance itself matters less than people think. 0.25mm works on a tuned Ender. An untuned Bambu will fuse a 0.35mm joint. Tune the machine, then the file works everywhere.

One more thing: if it comes off fused, twist it while it's still warm. Cold PLA snaps, warm PLA breaks the weld and the joint frees up. You get one shot at that.