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Why does filament sometimes get clogged in the hot-end?

cs2000cs2000 Posts: 56🌟 Super Member 🌟

   Hey guys,

Not an issue currently as ive fixed it, but I'm curious to understand what happened here. Sorry for the long read, but im trying to get all the facts down.

After several weeks/months of troubleshooting my A5, needing to re-lube the lead screws, a failed stepper motor, upgrading to TMC2130 and installing an IR bed level sensor, I have been printing flawlessly for around 10 days now (yay). My machine has been on literally almost non stop during this time creating various things I have had saved up during the downtime I had. I have been switching colours after almost every print as was required by my models. All filament was PLA, all from the same manufacturer who ive always had good, consistent print results with.

Typically, the last thing I printed was in black, I did two 4-5hr prints. These completed OK, but the second of the long prints had weird extrusion on it, almost blobby rather than smooth when it was doing the outline around the parts for example.

at the end of the print, i retracted that material and tried to insert some ABS, but it got stuck. Weird I thought! I heated up the nozzle to 250 (think this is as high as I can go with the PTFE throat) and still stuck. Odd...

I undid the Bowden tube on top of the extruder carriage and tried to manually push the filament in, but no dice, it was completely stuck up.

I then removed the nozzle using the recommended method of holding the heater block with an adjustable "monkey wrench" (as we call them in the UK) and a socket set to remove the nozzle and tried pushing the filament in again. Nothing, totally stuck!

So, my new pack of nozzles came with something they called "cleaning picks". The package clearly says they're acupuncture needles :D :D Anyway, I shoved one up the throat and it went through with a bit of resistance, but I still couldn't get the damn filament through.

Lastly, I actually unscrewed the Bowden tube connector on the extruder carriage and shone a torch up from where the nozzle (now removed) would normally be and it was clearly blocked.

I ended up showing a 2mm drill bit (smooth end first) all the way up, through the throat and back out the top of the extruder assembly, out with it came a huge chunk of black PLA!!!

My question is, where did this come from!?!

As mentioned, I had been printing flawlessly for 7-10 days almost non-stop. I haven't adjusted any settings in firmware or in my slicer from start to finish through all the models I have printed and this is "known good" filament I'm printing with.

There were no other colours in this chunk of plastic, purely black, but I cannot figure out why it would have jammed when it had been printing so long in all kinds of colours just fine.

It was an absolute pain to clear it, I'm just trying to be sure that its not something that I did, so I can avoid doing it again.

The only thing I can think of is pure and simple heat soak, but I'm sure after an hour or two, the machine would already be heat soaked, it wouldn't take 10 days to reach this state.

Yours, confusingly, Cs2000 :D

Thanked by 1Samuel Pinches


  • Der_MuckDer_Muck Posts: 265🌟 Super Member 🌟
    Thats strange, but first, for the normal PTFE tube, 250 is to much, it allready degrades and soften if you have the original white tube in it. Better to use the blue capricorn it would be able to reach 250 but thats the max. also for that tube.
    How long do you print with the PTFE throat, maybe it was just ready to be changed and you missed that point.
    Which retraction settings are you using? Maybe thers a to high setting where you get some melted filament stick in the throat? 
    I have a all metal throat, with that system its 100% harder to find the exact retraction setting. So when I set it to 3mm the PLA filament totaly stuck and you have only the oprion for a cold pull. 2,4mm are totaly ok and no problem.  So that can also be a problem with a PTFE tube. In a wider dimention ofcause.   PLA filaments are never always the same, some seems to have a much lower glas temp. 
  • cs2000cs2000 Posts: 56🌟 Super Member 🌟
    Appreciate the reply :smile:

    my normal PLA printing temp is 200-215, I only raised it to 250 to try and melt anything that was stuck in the hot end, I never print at that temp. When doing ABS, I tend to sit at 240, but never more. In the case of the blockage, it was PLA at 210.

    i have never actually changed the  PTFE throat, perhaps done a total of maybe 150 hrs of printing with it. How long do they tend to last?

    you May have hit the nail on the head with retraction from your explanation, I use 7mm as my retraction distance, is this too far?
  • Der_MuckDer_Muck Posts: 265🌟 Super Member 🌟
    Hm 7mm with the PTFE tube seems to be on the higher end but its passible.
    Lastly I had stringing with PLA as I had to reset my filament settings because I loose them in S3D (another story)
    I printed at 200 with strings and exrudion problems what is strange because it means to hot and to cold at the same time :D 
    So I changed to 207 and got no stringing and no extrusion problems. At the same time I reduced the retraction because of the extrusion problems but it worked.
    Maybe you can try that to see if it helps. As I said, with the all metal throat everything is much more critical but should work the same way as it is with the ptfe tube. The best is always a minimum of retraction distance.
    I think with 7mm, you may pull the melted area out of the nozzle, to far in the cold zone. 

    Also passible is, when you print ABS and you change from ABS to PLA, heat the hotend up to 245 minimum and extrude enough PLA to clean the whole hot area from ABS. It can be that there is still only a little ABS in the hotend area and you print till the ABS spot get free and clogge the nozzle. The rest you can imagen yourself. In the end you have a PLA mixer :D you only mix soft PLA with the retraction in there :D 
    No worry, extrude PLA with 245 works, you can turn the heat of while extruding PLA so the hotend can cool down and you dont have degraded PLA left.    One of the 2 problems you may have, I think.
    Thanked by 1cs2000
  • ThingsNSchtuffThingsNSchtuff Posts: 23Member
    I had this happen to me recently after using some glow in the dark filament. The solution i found to work best to step up the nozzle size. The glow in the dark filament is the only one i have done this for so far, but i jumped up to .6 from .4 and vuala. The extruder even stopped "clicking" on the filament. Prints look fairly good, I just adjusted the layer height down to compensate for some of the resolution lost. I won't be printing any fine glow in the dark text like this, but it worked nonetheless.
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