Some days ago I was printing a model that required an automatic spool change since I didn’t have enough left on spool 1. This happened overnight and I woke up with a message saying “Failed to extrude AMS A Slot 1 filament; the extruder may be clogged or the filament may be too thin, causing the extruder to slip.”.
At first I feared it pulled part of the end tape into the extruder but I was lucky since I had the newer, white tape. It was still attached to the spool. Checking the machine itself I could indeed confirm that once I pressed resume there was no or barely any filament being extruded. I cancelled the print and removed the nozzle at which point a short piece of filament just fell out.
I thought that was causing the blockage and continued on printing other models without any issues. However, today I noticed severe under extrusion when printing solid infill at the higher speed the HF nozzles offer. When I set the printing speed to 50% this problem was solved immediately.
This lead me to believe that something must’ve have gone wrong after all when it switched the filament some days prior. Somehow the extruder isn’t able to handle those high speeds anymore. I then went on going through the following troubleshooting steps:
printed same model from unaffected left HF nozzle… same result!
this excluded the nozzles from being the issue
inspected the extruder gear for any debris or tape, nothing found
did a cold pull using PETG, came out clean
did automatic dynamic flow calibration: failed! (only right nozzle failed, though left nozzle finished with a 0.010 value, which seems rather low for PLA)
manual dynamic flow calibration was successful with a value of 0.032
test printed a small 50 x 50 x 10 mm object, but starts under extruding every time at layer 2 when printing the solid infill. Used the default 0.20 profile.
At this point I’ve reached the end of what I could possibly troubleshoot. For some reason I now get severe under extrusion on both nozzles when printing long continuous solid infill sections. I have not yet tested the regular nozzles but I assume these will be fine since they print slower.
Thanks in advance for helping out a fellow 3D printing enthousiast.
Have you checked if your nozzles are properly seated against the heating element?
This seems like the nozzle cools down too much on higher speeds. Assuming your profile settings are correct, the nozzle becomes too cold during high material flow.
Yeah, I’ve checked that as well. Once the clamps are closed it’s not moving at all. Only the left nozzle wobbles ever so slightly to the left and right and up, but I believe it’s normal for that nozzle. Perhaps worth doing another experiment where I bump up the temperature by 5 or 10°C? Also note that I’m only using Bambu filament and their profiles.
I still feel like whatever is triggering this has to do with when it switched filament to a new spool and that lead to the extruder error message. Apart from removing the extruder gear cover, is there still another spot where debris could build up? Like I said, I already cleaned the nozzle using the cold pull maintenance step.
Update: printing the same test cube at 240°C (up from 220) completely eliminates the under extrusion a high speeds! But that then leads to the next question: what happened that suddenly forces me to print a higher temperatures? I’ve printed many hundred hours using the HF nozzles at the stock 220°C setting.
The left nozzle should not be able to move left and right. You can push it up with some force, and you hear a click.
You might want to check if the screws of your heating element are not loose. You should have 4 on the left and 3 on the right.
If you are sure that there is good contact between the nozzle and the heater block, I have no clue what could cause this. Especially if this was not the case before.
Well I should clarify, it only moves like less than 0.5 mm. I already checked multiple times if it is really seated well and it appears it is. The clamps close without any force.
Screws are fine as well. So indeed, it’s a mystery why the printing temperature is now suddenly too low.
Thanks for that suggestion though! At least now I have a way to continue using the HF nozzles. Though I’m not sure if they are always the best option. For quality prints you don’t want to print super fast anyway (I know they mainly achieve those speeds when doing infill).
After more testing, upping the temperature didn’t really solve it as I had to go as high as 250°C, at which point the filament obviously became too soft.
That’s when I decided to run the max flowrate test in Bambu Studio. As you can see, it failed. This happened at 40 mm height which corresponds to a flow rate of 25 mm³/s… which also corresponds to the max flowrate when using the standard nozzle for printing PLA Matte. Perhaps this is pure coincidence, but it’s interesting nonetheless. It’s as if the slicer is incorrectly using the standard nozzle preset, even though I have the HF nozzles set correctly in the left and right nozzle box.
From experience, Bambu Studio can sometimes overwrite existing profiles with seemingly random values, and this can also cause sudden issues. It is therefore a good idea to double‑check your profiles, especially the MVS and Flow Ratio settings. There was even a firmware version that overwrote PA values, so verifying those as well can be helpful.
If you have access to cleaning filament, that is also a useful process to run. A cold pull and/or using cleaning filament can help remove any contamination inside the nozzle. With HF nozzles, even very small partial clogs can cause inconsistent extrusion which, at high print speeds, may result in both under‑extrusion and blobs.
That’s also something I was thinking of yesterday. I’ll try starting a print from a different pc using a clean install of Bambu. Looking at the profiles itself they do look fine (25 max FR for standard and 40 for HF). PA/K value is manually set to 0.032 after calibration.
As mentioned before I did do a cold pull using PETG and it came out fine.
It might simply be filament slip caused by a dirty extruder gear. If the drive gear has built‑up plastic dust it can lose grip and the filament will start to slip when the back pressure increases, which then shows up as under‑extrusion at higher flow rates.
It could be worth opening the extruder, carefully cleaning the drive gears, and then rerunning your test print or max flow test to see if the issue improves.
Also, check the idler wheels in the filament path—they can accumulate a thick layer of residue that causes slip as well. In my experience, I had a thick film on mine that caused slipping, plus black buildup around the extruder gears.
It all looked fairly clean, but indeed some fine particles and some string (filament/tape?) around the idler wheels. I had previously already removed some but then I didn’t remove the entire mechanism.
Your extruder gears look quite clean, so that’s good news! However, I’d still recommend giving the idler wheels some extra attention. Even if they look relatively clean, they can accumulate a thin, almost invisible film that reduces grip at higher flow rates.
Also, check along the right side of the mechanism (visible in your first photo)—I had a black greasy residue there that felt like melted filament or grease, and it was affecting performance. Worth making sure everything in that area is completely clean.
That’s exactly the thing I removed from the idler gears! So we’re hopefully on the right track.
And yep, the photo was taken after I removed that black residue, thanks for confirming you had the same issue. Fingers crossed now for when I’m doing the max flow rate test
Seems like I repeat myself in various posts and sometimes that’s not it at all but I experienced almost the same thing when my extruder gear started failing and the wheel was not fully secured to the gear. I had underextrusion at first only on higher speed paths then almost at any speed when it failed completely.
If you have not reinstalled everything can you check by holding the black gear that you cannot rotate the metal wheel?
Just tried and they’re both very secured to each other, but thanks for the suggestion! The only thing I do notice is that the gear is not very secured when installing it in the toolhead, it wobbles a bit. I assume that’s correct?
