How to Tune Part-Cooling Fan Settings for Overhangs and Bridges

A clean overhang or a flat bridge almost always comes down to one thing the slicer hides in plain sight: the part-cooling fan. Air solidifies each new layer before it can sag, but the right amount of air depends entirely on the filament in the spool. Crank it for the wrong material and you get warping or split layers; starve it on the right material and your overhangs droop into a mess. This guide walks through the fan numbers that actually work, material by material.

Quick Answer

Run the part-cooling fan at 100 percent for PLA and at 30 to 50 percent for PETG (push PETG to 70 to 100 percent only over overhangs and bridges). Keep ABS and ASA near zero, enabling only a 10 to 20 percent burst from about layer five onward. In every case, leave the fan off for the first 3 to 5 layers so the print bonds to the bed.

Why The Fan Matters For Overhangs And Bridges

An overhang is plastic extruded with little or nothing beneath it. A bridge is a span stretched between two points with open air below. Both rely on the filament cooling and stiffening in the fraction of a second before gravity pulls it down. The part-cooling fan is what supplies that fast set. Without it, a steep overhang curls upward at the edges and a bridge sags into drooping strands.

The catch is that every plastic has a different glass transition temperature, the point where it goes from firm to soft. PLA softens around 60 degrees, so aggressive air helps it. ABS and ASA stay rubbery up past 100 degrees and crack apart if you cool them too fast. That single fact explains every setting below.

Fan Speeds By Material

PLA

PLA is the most forgiving filament and the most fan-hungry. Set the part-cooling fan to 100 percent for general printing and keep it at 100 percent across bridges and overhangs. The low softening point means there is almost no downside to maximum air once the first few layers are down. If you see warped corners on a large flat PLA part, that is usually bed adhesion, not too much fan.

PETG

PETG is stickier and softens around 80 degrees, so it splits and weakens along the Z axis if you blast it with cold air. Start at 30 to 50 percent for walls and infill. When the toolpath reaches an overhang or a bridge, that is where extra air earns its keep: ramp to 70 to 100 percent for those specific moves, then drop back. Watch the layers; if you see them separating or the part going brittle, pull the fan back down.

ABS And ASA

These engineering plastics hate part cooling. Too much air causes the layers to contract at different rates, which shows up as cracks and delamination, and on big parts it lifts the corners off the bed entirely. Keep the fan at 0 percent through the body of the print. For bridges or steep overhangs, a brief 10 to 20 percent from around layer five is a reasonable compromise, ideally inside an enclosure so the chamber stays warm. If a bridge sags slightly, accept it rather than ruining the whole part with a draught.

Ramp The Fan, Do Not Slam It

The single most useful habit across every material is to keep the fan off for the first 3 to 5 layers, then bring it up gradually rather than jumping straight to full speed. The opening layers need heat to fuse to the bed; a cold blast there guarantees a print that pops loose halfway through. Most slicers expose this as a fan-speed ramp or a "disable fan for first N layers" field. A staged increase over the next few layers gives you adhesion at the base and crisp detail higher up.

For PLA and PETG, also enable a minimum-layer-time setting so thin sections do not get re-printed before the previous pass has cooled. On tall, skinny features this prevents the molten blob effect even when the fan is already maxed. You can compare entry-level and enclosed machines across the 3D printers in the components range at Evetech, since an enclosure is what makes low-fan ABS work in the first place. For tools, spare nozzles and cleaning kit, the accessories best sellers list at Evetech covers the consumables that keep extrusion consistent.

Frequently Asked Questions

Should the part-cooling fan ever be at 100 percent for ABS?

No. ABS and ASA crack and warp under heavy cooling. Keep them near zero, with at most a short 10 to 20 percent burst over bridges from about layer five, ideally inside an enclosure.

Why do my PLA overhangs still droop at full fan?

If the fan is already at 100 percent, the next levers are slowing the print speed on overhang moves and lowering the nozzle temperature a few degrees. A weak or partly blocked fan duct also starves the print of airflow, so check the duct points at the nozzle tip.

Do I need an enclosure to print PETG well?

No. PETG prints fine in open air with moderate fan. Enclosures matter mainly for ABS and ASA, where keeping the chamber warm prevents the cracking that part cooling would otherwise cause.

How many layers should the fan stay off at the start?

Three to five layers is the usual range. This lets the base bond to the bed before any cooling air hits it, which is the most common cause of prints lifting mid-job.

Can I set different fan speeds for overhangs only?

Yes. Most modern slicers have an overhang or bridge fan-speed override. This is exactly how you keep PETG at 40 percent for walls while jumping to 90 percent only on the spans that need it.