Learn & Understand

Cooling Fan Strategy: Why More Airflow Isn't Always Better

The automatic slowdown a slicer applies to short layers exists specifically to buy cooling time - but cranking the cooling fan to compensate is the right move for some materials and actively harmful for others.

Why the Minimum Layer Time Feature Exists at All

When a layer's calculated print time falls below a configured threshold, the slicer doesn't just hope for the best - it actively slows that specific layer's print speed down, giving the plastic more real time to cool and solidify before the next layer arrives. This is a cooling problem at its core, which means the other major lever for solving it is the part cooling fan sitting right next to the nozzle, blowing air directly onto freshly deposited plastic. Increasing fan speed and increasing layer print time are two different ways of achieving the same goal - more effective cooling before the next layer lands - and slicers often let short layers trigger both a speed reduction and a fan speed increase together.

Why "Just Max Out the Fan" Isn't Universal Advice

For PLA, aggressive cooling is almost always beneficial - PLA prints crisp overhangs and bridges specifically because it can be cooled hard and fast without compromising layer adhesion, since PLA bonds well even when cooled quickly. ABS and ASA behave in the opposite direction: these materials are prone to warping and layer delamination specifically because of uneven cooling stress (the same underlying mechanism covered in the shrinkage guide), and blasting them with a strong part cooling fan can make warping and cracking meaningfully worse, not better, by cooling the outer surface far faster than the interior. This is why ABS/ASA print profiles commonly run cooling fans at low speed or entirely off for most of the print, accepting a slower minimum-layer-time slowdown as the primary tool for small-feature cooling instead of relying on fan airflow.

The Bridging Exception

Regardless of a material's general cooling strategy, most slicer profiles include a specific bridging fan override that boosts cooling substantially just for unsupported horizontal spans, since bridges have nothing beneath them to help hold their shape while cooling and benefit disproportionately from rapid solidification, even on materials like ABS that otherwise avoid strong cooling for the rest of the print.

Applying This to a Small-Feature Print Time Problem

If a model with small, delicate features is printing at a heavily slowed pace due to minimum layer time triggering constantly, check whether your material and profile are already using an appropriate cooling fan strategy before assuming the only fix is slowing down further or increasing the minimum layer time threshold - for cooling-tolerant materials like PLA, confirming the fan is actually running at full effective speed on those small features is often the more direct fix.