Why your prints snap — and what actually fixes it

The instinct is to turn infill up. It is nearly always the wrong lever.

Walls beat infill

Going from two walls to four does more for strength than going from 20% to 60% infill, and costs a fraction of the time. Think of the walls as the structure and the infill as the packing.

Cooling is a trade-off, not a setting to max

Every degree of cooling you add buys cleaner overhangs and costs you layer adhesion. PLA tolerates 100%. PETG wants 30–50%. ABS wants almost none. Running max fan on PETG is the most common reason a part splits along its layer lines.

Orientation is free strength

• A part is roughly twice as strong across layers as along them
• Print so the load runs across the layers, not trying to peel them apart
• A hook printed flat will hold; the same hook printed upright will snap
• If you cannot re-orient, add walls rather than infill

And sometimes it is the filament

Old PLA that has been through a year of humidity goes brittle and crumbly. If it snaps when you bend a short length by hand, no setting will save it.