Filament snaps during print

Suddenly I can’t complete a print because the filament snaps off inside the tool head. I can tell immediately when this happens because a scratching noise can be heard as the gear attempts to move the filament either forward or back. I can then see that the spool has stopped turning. This happens with brand new dry and flexible PLA filament, not just my older spools. It also seems to happen when the print program calls for rapid to and fro movement in the tool head. Can the gear be actually severing the filament as it passes through it? I have tried varying the gear pressure from light to heavy, but this has no effect. Please help.

Can you share more details? Which printer model? Which head? Are you using 2.85mm filament? What brand filament?

Some filaments are notorious for breaking … those shiny silky PLA filaments are particularly brittle. Mine usually wont break in the middle of a job … but if I come down the following day after the printer has been sitting unused for many hours … silk pla filament will usually have snapped.

PLA (Poly-lactic acid) is the name of the naked polymer. But this would have no color. To have color … additives are needed. Every vendor does whatever they want. As a result, not all PLA behaves the same. While I’ve never bought the stuff … a good friend of mine (has his own YouTube channel … and something like 30 printers) says Amazon Basics is pretty bad.

A LOT of 3D-printer vendors sell their own branded PLA (usually made for them by someone else). LulzBot doesn’t do that … what they DO is produce pre-tested printing profiles for some of the better filaments on the market so you know what to expect. When in doubt … pick one of the filaments on the list of filament types in Cura LulzBot Edition and see if you have better luck.

Don’t over tighten the tension on the idler arm (this applies tension on the filament … pressing it into the teeth of the hobbed-gear that feeds the filament.) This can result in chewing into the filament more than necessary. A medium amount of tension is what the factory would set … you can gently back off from there if necessary.

“rapid to and fro” sounds like excessive retractions. In the slicer (Cura LulzBot Edition?) you can tune this … in the “Material” category.

I generally don’t set retraction distance to more than 1mm (maybe 1.5mm tops) because direct-drive printers don’t need much.

Set the “Minimum Extrusion Distance Window” to match the Retraction Distance (e.g. 1mm or 1.5mm). You could set it slightly higher if you want (e.g. 2mm) but it usually isn’t necessary.

Set the Maximum Retraction Count WAAAAY DOWN to a reasonable level … like 5. (I find it usually defaults to some outrageous number like 90 … or 99).

This is the maximum number of retractions that can be performed over the same section of filament. Any additional retractions will be ignored UNTIL it extrudes more clear filament than the “Minimum Extrusion Distance Window”. In other words … until it gets to free filament it wont do any more retractions.

Excessive retractions will really chew up the filament … so much that it can result in a jam.

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Sounds logical. I will make the adjustments before next attempt. Thanks.

I use a brand called Filamatrix which I buy from It’s 1.75 colored PLA, my printer is the Sidekick 239 with the SK175 tool head. I have had no problems until recently, now suddenly prints are being interrupted by snapped filament, non-advancing filament, or heating errors. BTW, I do dry my spools at 120 for several hours on a regular basis and have a dry box for storage with many dehydration packets inside. My filament stays pretty flexible. Your retraction issue is the first response I’ve had that actually makes sense.

I normally suspect.

  1. Brittle filament (usually due to the additives).
  2. Excessive retractions in a short distance can grind up the sidewall of the cold filament until it jams or even breaks (you can often see filament stuck in the hobbed-gear on the extruder … some compressed air will blow that clean)

But you’ve added some information that leads to other possibilities.

  • Heat creep
  • Thermistor issues (you mentioned “heating errors”)
  • Nozzle is clogged (partially) creating a bit of a jam.

Heat creep: Extruders have a “cold” side and a “hot” side.

First, the “hot” side: The extruder nozzle is screwed into the bottom of the “heat block”. The heat-block has both a heater-cartridge as well as a thermistor. Both look like a round cylinder embedded in the block. The heater cartridge is, of course, the heater … and thermistor is a variable resistor … and the resistance is temperature-sensitive. The control board on the printer can determine the temperature of the heat-block based on the resistance. These thermistors do occasionally go bad and can be replaced.

The “cold side” is the end where the still cold filament feeds in. It’s aluminum with a fan which constantly blows air across a heat-sink to dissipate any heat. It is important to keep this side of the extruder cold-enough that the filament cannot melt.

Separating the “cold” and “hot” sides of the extruder is a gap connected only by a threaded metal tube called the “heat brake”. It is important to have this separation as it makes it more difficult for heat to transfer from the hot side to the cold side. But heat DOES creep up (that’s just the nature of thermodynamics) … but since the tube isn’t particularly big it can only creep up slowly (it’s just not a very big surface to transfer a lot of heat). And since there’s a fan constantly blowing cool air across the heat-sink of the “cold” side … any heat that does manage to make it up is quickly dissipated via air convection.

Two things can go wrong.

  1. The heat-sink fan might either be inoperative OR it might be installed backward. You want to make sure it is spinning and blowing air toward the heat-sink. Just occasionally someone will open up a print head to service it (requiring removal of the fan) and re-install it backward.

  2. If the thermistor is either not snugly fit to the heat block OR if it is going bad, it might be under-reporting the temperature … causing the print-head to get MUCH too hot (so now heat can travel through the heat-brake to the “cold” side of the extruder quickly … faster than the fan can handle.)

To test for a jam:

Raise the Z-axis and heat the head up to printing temperature and try to extrude some filament (20-30mm worth should be enough) – just to purge some filament through – does it come out easily? Does the head struggle? Does the hot filament come out flowing straight down … or does it try to bend to one side?

You might have a partial blockage/clogged nozzle. The easiest was to fix that is by doing a “cold pull”.

  1. Heat the head and purge a small amount of filament.
  2. Turn off the nozzle heat and let it cool down (it doesn’t have to be completely cold … but does have to be cold enough that the filament inside is solid – not merely soft).
  3. Release all tension off the hotbed gear (this will make it easier to “pull”)
  4. Start heating the head up again … but watch the temperature and as it approaches the point JUST BELOW where it should soften, start pulling the filament up (I put one hand on the tension wheel to release tension completely … and pull with the other hand).
  5. Just at the point where the filament barely begins to soften (the outside is warm enough to slip but the inside is still solid) the filament should pull up and out of the extruder … and whatever was clogging the extruder should still be embedded in the filament. This cleans the nozzle of debris.

Re-load with fresh clean filament and you’re done.

If you had heat-creep then you may have melted filament on what is supposed to be the “cold” side of the extruder and that needs to be cleaned out. You’ll need to make sure whatever problem caused heat-creep (fan problem, loose thermistor cartridge, or failing thermistor) is corrected. Jamming on the cold-side can be cleared by deliberately inducing heat-creep … basically just prevent the cooling fan from spinning (stop it with your finger) and hold it there for a minute to heat up the “cold” side until you can pull out the jam.

There is special “cleaning filament” (typically this is clear nylon … but sold in small pieces so you don’t have to buy an entire spool just to get some). Nylon is very strong (it is not brittle and will not break) and being “clear” is helpful as you can see what it pulled out.

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i don’t think any of the nozzle issues you describe are the problem. The same filament grinding occured immediately after I installed a brand new nozzle. However, once I turned the retract distance down from 3 to 1 mm, the count from 90 (the default) to 5, and the retract velocity to 10 mm/sec, I successfully printed a model after making these adjustments. We’ll see if all is well as time goes by.

Another problem is drive gear lash in areas with excessive retractions. If your filament drive gear is a bit worn and you can rotate the big gear AT ALL by pushing on the big gear teeth, you need to adjust the 2 gears closer together. If there is no adjustment left, print out a new drive gear. I used nylon, and lubricate the gear teeth with grease.

no problem with gears, tool head is brand new.