Cooling Fans

Most FDM printers rely on two different fans: the hotend heatsink fan (hardware safety/reliability, usually always on when hot) and the part-cooling fan (print-quality, slicer-controlled). If you can tell which is which and what each one affects, you can fix droopy bridges, stringy details, brittle layers, and heat-creep jams with targeted changes instead of guessing.

TL;DR

Verify the hotend heatsink fan runs continuously whenever the nozzle is hot (to prevent heat creep jams), and tune the slicer-controlled part-cooling fan for overhangs/bridges without overcooling the whole print (which can weaken layer bonding).

Cooling fans: airflow targets and failure modesTopic-specific diagram for the concept, checks, and tradeoffs in this lesson.Hotend fanPart fanHeatsinkHeatbreak
Two fans look similar but cause very different failures. This diagram labels where each fan should blow and what goes wrong when airflow is missing or excessive.

Two fans, two different jobs

Hotend (heatsink) fan: cools the heatsink/cold side so filament stays solid until it reaches the melt zone. This fan is about preventing jams and is typically on any time the hotend is above a set temperature (often around 50 C). Part-cooling fan: cools the just-extruded plastic at the nozzle/part to improve shape (bridges, overhangs, sharp edges). This fan is usually controlled by the slicer and can change speed by layer and by feature.

Quick identification on a real printer

  • Heatsink fan points at the heatsink fins (the upper, “cold” side of the hotend). It should be steady and not vary during the print.
  • Part-cooling fan blows through a duct toward the nozzle tip/print. It often starts off (or low) on the first layer, then ramps up.
  • If changing “Fan Speed” on the printer screen or in the slicer changes a fan during the print, that fan is the part-cooling fan (not the heatsink fan).

What part cooling changes (and what it can break)

More part cooling freezes plastic sooner, which helps bridges span gaps, overhangs hold their angle, corners stay sharp, and small details don’t slump. Too much cooling can keep layers from fusing well, leading to weaker Z strength, brittle parts, and warping or layer splitting—especially with materials that dislike rapid cooling (commonly ABS/ASA, nylon, PC). PLA usually benefits from strong part cooling because it solidifies quickly and is less sensitive to warping.

What the hotend fan prevents: heat creep

Heat creep is when heat travels upward through the heatbreak into the heatsink region, softening filament too early. The filament then swells and drags in the cooler path, causing under-extrusion or a full jam. It often shows up after the printer has been running a while (10–60 minutes), and is made worse by long retractions, slow prints, or high hotend temperatures. A weak, obstructed, or failed heatsink fan is one of the most common root causes.

Slicer starting points (general, adjust per printer)

  • First layer: part cooling off or low to protect bed adhesion; ramp up over the next few layers to avoid sudden warping/curling.
  • PLA: medium to high part cooling for clean overhangs/bridges; use minimum layer time for tiny features so each layer can set before the next arrives.
  • PETG: often moderate or low part cooling; too much can reduce layer bonding and can worsen surface dullness, but some cooling may help control blobs and fine details.
  • ABS/ASA: low or off part cooling unless a specific feature needs it; prioritize stable ambient temperature (often an enclosure) over blasting the part with cold air.

Common fan and duct issues (what to check first)

Blocked intake/exhaust (dust, wire rubbing, shroud contact) easy
  • Fast to inspect with power off
  • Often explains intermittent failures
  • Returns if you don’t clean/route wires
Misaligned, loose, or cracked part-cooling duct easy
  • Direct impact on bridges/overhangs
  • Often causes one-sided cooling
  • Can be hard to notice until you watch the print
Heatsink fan not running when hot (dead fan, wrong port, wiring fault) medium
  • Classic cause of heat creep jams
  • Usually obvious once tested
  • Requires careful electrical troubleshooting
Wrong fan choice for the duct (insufficient static pressure) harder
  • Can improve cooling consistency on complex parts
  • Requires part selection, fit, and airflow checks

Symptom to fan diagnosis (fast triage)

Bridges sag and overhangs look glossy/melted

Likely cause: Insufficient part cooling and/or too much heat/speed

Fix: Increase part cooling for bridges, slow bridge speed, and/or reduce nozzle temp slightly

Parts are brittle or layers split (especially on tall prints)

Likely cause: Overcooling for the material and/or cold drafts reducing layer fusion

Fix: Reduce part cooling, raise nozzle temp slightly, and shield from drafts (enclose if appropriate)

Sudden under-extrusion or a jam after printing fine for a while

Likely cause: Heat creep from weak/failed heatsink fan or blocked airflow path

Fix: Confirm the heatsink fan spins strongly whenever the hotend is hot; remove obstructions and re-test

Corners curl up and get hit by the nozzle on small features

Likely cause: Cooling is too strong/uneven or changes too abruptly; sometimes poor first-layer adhesion

Fix: Use a fan ramp, add minimum layer time, improve bed adhesion, and avoid external drafts