Tangled filament and spool knots: why they form and how to fix them

This guide is for anyone who has found a print stopped with the filament pulled tight, a knot on the spool, or an extruder grinding against filament that won't move. It explains how makers say tangles form, how to prevent them, and how Prusa, Bambu Lab, colorFabb, ELEGOO and Overture say to recover. It also explains how to tell a tangle from a clicking extruder or filament that's snapping.

How does a tangle form?

Prusa, colorFabb, Overture and Bambu Lab describe the same cause:

• Prusa: a knot "is caused by letting the filament end go, for example during a filament unloading, causing the filament end to go under another loop."
• colorFabb: never let go of the loose end "or let it hang unsecured, as you run the risk of the filament crossing underneath itself."
• Overture: names two causes. "Losing Track of the End," when the end "gets under another strand in the spool, which can be avoided by proper usage and storage," and "Factory Errors." Overture says it uses "precise winding technology" but that "there is still a certain possibility of tangling due to factory errors."
• Bambu Lab: says that when filament is replaced improperly, "it may become tangled or trapped in the reusable spool."

Can a spool come tangled from the factory?

Prusa and colorFabb both say a knot can't be wound in. Prusa says "the filament in a spool is a single, continuous string wound at once from start to finish. It is physically impossible that, during production, a filament coil can get under another." It adds that a spool without perfectly neat winding is no more prone to tangling.

colorFabb says the filament is kept under constant tension throughout winding, and "there is no loose end to create a knot or tangle during the winding of the spool." It says the only point where an operator could let go is when the end is cut and fastened, and that "if it did happen the knot will be present on the very first winding of the spool and will give issues immediately when loading the material in to the printer."

Overture, as above, says factory errors remain possible. Polymaker says a tangled spool "shouldn't really be possible with how our neat winding works." If one caused a failed print, it asks you to email support with proof of purchase and a photo of the batch number sticker. SUNLU's PLA page says its "neat and precise winding process" uses "tightly and orderly arranged strands that effectively reduce tangling and knotting."

Why the print fails later, not straight away

colorFabb explains why a spool can print for hours before stopping. It says "it is completely possible to print for hours with a tangled spool once it has been allowed to coil freely," because "the tangled bit travels backwards on the spool while the spool is unwinding until tension is built up." Then "at some point the tangled filament will get stuck and pull itself into a knot, leaving your feeder unable to pull the filament through and thus start grinding."

Bambu describes the same thing from the printer's side for the A1 series: "the extruder being unable to pull the filament during the printing process and thus stopping the extrusion." Bambu says the toolhead's filament sensor is usually still triggered in that case, so no run-out reminder appears and the printer keeps moving without extruding, which Bambu calls "air printing." ELEGOO says a tangle during printing means "the filament can no longer be extruded properly, and the model has gaps between layers or overhangs." Polymaker lists "tangled or too tightly wound filament" among the causes of a single layer shift.

Tangle, grinding or snapping?

These three problems can end a print the same way. This table is our quick guide to where to look next.

A tangle can cause grinding too. colorFabb says the knot leaves the feeder "unable to pull the filament through and thus start grinding," and Polymaker lists "resistance at the spool or pathway leading to the extruder" among the causes of extruder skipping. So if the extruder is clicking, check the spool turns freely before you open the hotend.

Some printers can't tell the two apart either. Bambu says that for the P2S extruder motor, "the load characteristics of a clog and a filament tangle are essentially the same — both caused by excessive resistance — so the system cannot distinguish between the two conditions." Bambu's AMS overload error, HMS 0700-0100-0002-0002, says "the filament may be tangled or stuck," and lists tangled or sagging filament alongside spool, tube and buffer causes. Our AMS feeding problems guide covers the other causes.

Snapping is a different problem. Polymaker says filament that snaps too easily "can be from the spool being wound too tight via vacuum sealing over long periods of time," and for a spool that isn't new it suggests "removing the outer 100grams of material."

How do you prevent tangles?

Secure the end every time

These makers all say to secure the end:

• Prusa: "do not let go of the filament tip whenever the filament is not loaded. Once you are ready to store the spool, put the filament tip on the spaces on the spool, for example on any of the honeycombs for Prusament spools."
• colorFabb: "When switching spools on the printer it's very important to always secure the loose end of the filament in the holes on the sides of the spools."
• ELEGOO: after unloading, "Tuck the filament into the holes in the filament spool," and "Always hold the end of the filament or reinsert it in the spool holes to avoid tangles."
• Overture: after use, "Insert the filament end into the small holes on the side of the spool to secure it, preventing it from becoming loose when not in use."
• Bambu Lab: for the AMS, insert the end into one of the holes on the side of the spool "when not in use." Bambu also mentions a filament clip, but adds that "a clip is not really necessary all the time."

Load a new or refill spool carefully

When unpacking a new spool, ELEGOO says to pull the filament out of the holes in the spool and cut off the bent part at the end. Overture says to trim the bent tip "as bent filaments can cause jams inside the PTFE tube or poor print quality."

For refills on its reusable spool, Bambu says to leave the clear protective strap on until the coil is clipped in, push the filament end into the round hole on the spool, and then remove the rest of the strap. Afterwards, check both sides, and "if there is a gap between the filament and the spool, press the edge of the filament coil by hand until the gap is filled. This prevents the filament from getting tangled."

Mount the spool so it turns, but not too freely

• Overture: "Secure the spool onto the spool holder to ensure it spins freely but with minimum tension. Completely loose spools can spin more than they should when the filament is pulled due to gravity and its own weight, causing tangling during the print job."
• Bambu Lab (A1 series): after a tangle alarm with an external spool, Bambu says to check whether "the spool is placed directly on the table or in the drying oven," and whether "the PTFE tube length used is too long or the tube is badly bent." With the AMS lite, it also says to check whether "the filament is stuck on the edge of the spool."
• Bambu Lab (AMS): "Verify that the spool is seated correctly in the AMS to allow free spinning," and "Avoid oversized spools that may not fit well in the AMS."
• Polymaker: "Confirm your filament is tight on its spool and that you maintain it in a way that will not allow it to tangle."

Bambu says the original AMS takes up slack itself. When filament is inserted, pre-loading "can also tighten the filament roll on the spool to ensure that the filament on the spool is not loose."

Printers that watch for tangles

Some Bambu printers watch for tangles:

• A1 and A1 mini. A tangle detection module above the toolhead is "enabled by default." Bambu says it doesn't support "TPU, TPE, wet water-soluble filaments, and PVA filaments."
• P2S. Tangle detection is on by default when printing from the AMS, and from an external spool when it feeds through the buffer. Bambu says detection on TPU from an external spool is "less effective," because TPU stretches in a tangle.
• H2D. Bambu says its built-in buffer has a tangle detection function "when printing with an external spool."

Detection doesn't replace checking on the printer. Prusa says the chance of a failed print from a tangle is "minimized by never leaving the printer unattended while it is switched on," which it calls a safety measure for its printers.

How do you untangle a spool?

Prusa's method

Prusa's untangling steps are short:

1. "Never let go of the filament tip during this procedure, or you'd risk further tangling the filament in different spots."
2. "Identify the knotted strands, and gently untangle the knot from that point, without bending the filament."
3. "Rewind the loose strands by gently rotating both the spool and the coil. Never let go of the filament end, until this is secured on a hole on the side of the spool."

colorFabb's method

colorFabb says "the trick is to pull the knot over the side of the spool." Its steps:

1. Find the tangle or crossover on the spool.
2. "Pinch the tangled bit between two fingers, don't let go."
3. Make sure the end isn't fastened in the side of the spool, then "wiggle the spool around to create more slack to remove the tangled bit / cross over over the side of the spool." colorFabb says to push the filament back so the loops forming the knot get bigger, and to make sure the knot is "actually lifted off of the spool," not just slid along it.
4. Check that only one strand comes off the spool. Unwind a couple more meters to make sure there are no more crossovers.
5. Rewind the filament that came off.

Overture and ELEGOO

Overture says to "unwind 8-10 loops of filament to find where it gets tangled and carefully wind them back onto the spool," and to contact Overture if tangling continues or you suspect a manufacturing error. ELEGOO says to unwind several meters, make sure the filament is free of knots, and turn the spool to rewind it. For a tangle during printing, ELEGOO's steps are to stop the print, remove the model, unload the filament from the printer menu, then untangle and reload.

Bambu Lab, by setup

Bambu's tangle guide has a version for each place the spool can sit:

• AMS: find the tangle point, hold it, and "rotate the spool in reverse to release the filament tension." Pull the tangle out from the side, pass the spool through the knot, straighten the filament and put the spool back.
• AMS lite: hold the tangle point and take the spool off the AMS lite. Pull the knot out of the spool, then reinstall the spool "along the filament feeding direction."
• External spool: hold the tangle point, rotate the spool in reverse to release the tension, remove the spool, pull the knot out and put the spool back on the holder.

For a tangle that's too hard to undo, Bambu says to cut the filament at the inlet and tap "Finished, Continue." It says to hold the tangled filament while you cut "to prevent it from springing back as the tension releases." When the printer reports the filament has run out, reinsert it and tap Resume. Bambu's AMS guide says to cut "as close to the tangle as possible" without disturbing the filament that's already feeding, and to make sure the reloaded filament matches the print job.

After recovery, check the print

Bambu says that after an air printing error on the P2S, the model may show under-extrusion from before the printer caught it, so "inspect the print quality before deciding to continue or restart the task." ELEGOO's steps for a tangle during printing begin with stopping the print and removing the model. Decide whether the part is still worth finishing before you resume.

Checklist

• When unloading, keep hold of the filament end and push it into a hole on the spool's side.
• On a new spool, trim the bent tip before loading.
• On a refill, check for gaps between the coil and the spool and press them closed.
• Mount the spool so it turns freely but doesn't spin on by itself.
• If the print stops, check the spool turns before you open the extruder.
• To untangle, hold the crossed strand, make slack, lift it off over the side of the spool, then rewind it.
• If filament snaps instead of knotting, see our brittle filament guide.

How this guide was checked

This is a research-based guide. We have not tested these untangling methods for it. Every cause, step and quote comes from the maker pages listed in the sources, all retrieved on 2026-10-11. SUNLU's statement comes from a product page, not a troubleshooting article. Tangle detection details apply only to the Bambu printers named. Our filament storage guide covers keeping spools dry between prints. Read more in our editorial standards.

Print problems: what to check before you buy ↗