Stringing on 3D prints: what causes it and how to stop it

Stringing happens when molten filament keeps oozing out of the nozzle while the printhead is travelling between features. The plastic is hot enough to stretch but cool enough to leave a wispy trail behind. You end up with hairs across the gaps in a Benchy, fuzz between the tines of a comb, or a fine web inside a part that should be clean.

The good news is that stringing is almost always a settings problem, not a broken printer. You can usually clear it up in an afternoon with a temperature tower and a retraction test.

Why stringing actually happens

There are really only four things going on inside the nozzle when you string.

The plastic is too hot, so it stays runny long after the extruder stops pushing. Pressure inside the melt zone keeps forcing filament out. The hot end drags through that ooze and pulls it into a thread. Or the filament has absorbed water, and the steam is pushing extra material out of the tip.

Most prints that string badly are hitting two or three of those at once. That is why a single setting change rarely solves it on its own.

What to try first

Before you start tuning retraction, do the cheap checks. They take five minutes and often fix the problem on their own.

1. Look at the filament. If it has been on the shelf for more than a few weeks in a humid garage, dry it. PETG and Nylon are the worst offenders. A food dehydrator at 55-65 degrees C for four to six hours usually sorts it. 2. Wipe the nozzle. A burnt-on blob of plastic on the outside of the nozzle will smear itself across the print on every travel move. A quick clean with a brass brush while the nozzle is hot does the job. 3. Check the nozzle is tight. A loose nozzle leaks from the threads and dumps plastic where it should not be. Heat the hot end to printing temperature first, then nip it up with a spanner. Never tighten a cold nozzle. 4. Re-slice the model with the slicer defaults for the exact filament you are running. If you copied a profile from PLA across to PETG, the retraction and temperature will both be wrong.

If the print still strings after that, it is time to tune.

Drop the temperature

The single most effective change is usually printing five to ten degrees cooler. Hotter plastic is runnier plastic, and runnier plastic oozes more.

Most slicers ship a temperature tower test print. It prints the same shape at different temperatures up the Z axis, so you can see exactly where stringing starts and where layer adhesion falls off. Run one for each new spool. The number on the side of the box is a starting point, not a setting.

As a rough guide on a healthy printer with dry filament:

• PLA tends to print clean between 190 and 210 degrees C.
• PETG sits between 230 and 245 degrees C on most machines.
• ABS likes 230 to 250 degrees C, but you have other problems if it is stringing.

If the tower shows your filament printing cleanly at 200 degrees C but stringing badly at 215, do not run it at 215 just because the spool says so.

Get retraction right

Retraction is the extruder briefly pulling filament back up the throat when the printhead is about to travel. It drops the pressure in the nozzle so plastic stops being squeezed out. Too little and you string. Too much and you get gaps in the next feature because the nozzle starts dry.

The right numbers depend on the type of extruder you have, not the brand of printer.

For a direct drive extruder (the motor is on the printhead, like a Bambu X1 or a Voron), start at 0.6 to 1.0 mm of retraction at around 35 to 45 mm per second. The filament path is short, so you do not need much pull.

For a Bowden setup (the motor is on the frame and pushes filament through a long PTFE tube, like most Ender 3 variants), start at 4 to 6 mm of retraction at 35 to 45 mm per second. The tube flexes, so you need more travel to actually move the filament tip.

The quickest way to dial it in is a retraction tower or the test in OrcaSlicer's calibration menu. It prints two pillars with a gap, varying retraction up the Z axis. Pick the lowest setting that gives a clean gap.

A word of warning. Endlessly cranking up retraction distance is a trap. Past about 2 mm on direct drive or 8 mm on Bowden, you are mostly just grinding the filament against the extruder gear and making the problem worse. If you are not getting a clean result by then, something else is wrong.

Speed up travel, slow down nothing

If the printhead spends longer travelling, more plastic oozes out during the travel. Bumping travel speed up to 150-250 mm per second on a CoreXY, or 120-150 on a bedslinger, often cleans up the last bit of stringing on its own.

While you are in there, enable the slicer's "avoid crossing perimeters" or "avoid crossing walls" option. It tells the printhead to route around the part rather than over it during travel, so any tiny string it does leave is hidden inside.

When it is the hardware, not the slicer

A few hardware faults look like a settings problem but are not.

A worn PTFE tube inside the hot end (on a Creality-style setup with a Bowden-into-heatbreak design) leaks plastic into the gap and causes endless stringing. Pull it out, cut a fresh end square, and reseat it cold then re-tighten hot.

A cracked or partially blocked nozzle drips. If the stringing is bad at any temperature and no amount of retraction helps, swap the nozzle. They are a couple of quid and last a few hundred hours at best on abrasive filament.

A failing heatbreak that has lost its thermal break (the thin neck between the cold and hot sections) lets the melt zone creep upwards. Plastic that should be solid is soft, and it keeps feeding under gravity. This is more common on cheaper all-metal swaps that were not assembled with a torque check.

When to mail it in

If you have run a temperature tower and a retraction test, dried the filament, and changed the nozzle, and the print is still stringing across every travel move, the hot end itself is probably the issue. That usually means a worn PTFE liner, a leaking nozzle thread, or a heatbreak that has lost its grip.

Those are jobs we are happy to do for you in the workshop. Send the printer in and we will strip the hot end, replace anything that is worn, and send it back with a clean test print so you can see it is sorted. Get in touch through our contact page and we will talk you through what to send and how to pack it.