Cooling Fan Settings: Part Fan, Auxiliary Fan and Overhang Fan

"Turn the fan up" is the most common quick fix in 3D printing, and it's wrong about as often as it's right. A slicer profile actually controls up to three separate fans with three separate jobs, and maxing all of them is exactly the mistake that turns good ABS into a warped, delaminated mess.

Three fans, three jobs

• Part cooling fan (fan_min_speed/fan_max_speed) blows directly on the freshly-printed layer to freeze it before the next layer lands on top. This is the main lever for bridging, overhangs and fine detail — and the main way to hurt layer adhesion and warping resistance if it runs too hard on a material that needs to stay warm.
• Auxiliary (chamber/side) fan (additional_cooling_fan_speed) exists on enclosed, aux-fan-equipped machines specifically to move air evenly around the part without the aggressive, localized blast of the part fan — useful for ABS/ASA's more even release from the plate without inviting warping.
• Overhang fan (overhang_fan_speed) is a temporary, geometry-triggered boost that kicks in only over steep overhangs and bridges, then drops back to the body's normal fan setting. It's how materials that need low overall cooling (ABS, ASA, PC) can still bridge without needing full-time aggressive cooling.

Per-material fan windows (stock Bambu profile)

Notice the pattern: PLA runs a wide-open fan nearly everywhere because it doesn't need residual heat for layer bonding. ABS inverts that completely — a narrow, low body-fan window protects layer welds and warping resistance, with the overhang fan as a surgical exception rather than a general policy.

What actually needs fixing when a fan-related symptom appears

• Sagging bridges/overhangs on PLA: genuinely a fan problem — check the part fan is reaching full speed. See the bridging guide.
• Sagging bridges on ABS/ASA: check the overhang fan specifically, not the body fan — many downloaded "strength" profiles zero out all cooling including the overhang exception, and that's the actual cause.
• Warping/cracking on ABS/ASA/PC: the body fan window (not the overhang fan) is almost always the culprit if it's been raised above stock. See the warping guide.
• No auxiliary fan on the printer at all: every open-frame machine in our knowledge base lacks one — that's a hardware fact, not a setting to hunt for. Its absence is exactly why open printers lean harder on chamber tents for hot materials.