https://www.youtube.com/watch?v=uKN0VOuul0o
I ran across this great explanation of how it can get nasty in the hotend with a great 3d printed visual aid. I didn't have to do everything he mentions, but the gap he describes is the same issue I faced and fixed.
Recently while printing with some brittle white PLA, I got some nasty jams in my bowden tube and in the hotend.
While clearing out the bowden tube and reassembling, I think I miss assembled the bowden tube back into the top of the hotend pneumatic coupler.
With a pneumatic coupler, you have to push a black ring as you push in the tube. And I don't think I pushed it in far enough.
Well, a little bit into my next print, after the first retraction or two, the filament got stuck in a bad way.
T F T
T F T
XXXXX T F T XXXXX
XXXXX T F T XXXXX
XXXXX T F T XXXXX
XXXXX T F T XXXXX
XXXXX T F T XXXXX
BBBBB T F T BBBBB
BBBB HH F HH BBBB
BBBB HH F HH BBBB
HHHHFHHHH
HHHFHHH
HHFHH
HFH
F
In the diagram above the T is for tubing aka the bowden tube. F is for filament, like PLA filament. The X's is the finned cooling area of the extruder. The B's is the hot end block that the nozzle screws into, and the H is the hotend nozzle.
Note how the tube butts up against the threaded nozzle. If you don't get the tube all the way in, you can have it have a weird gap between the tube and the top of the nozzle input. Like this:
T F T
T F T
XXXXX T F T XXXXX
XXXXX T F T XXXXX
XXXXX T F T XXXXX
XXXXX T F T XXXXX < end of tubing
XXXXX F XXXXX Gap
BBBBB F BBBBB Gap
BBBB HH F HH BBBB < top of nozzle
BBBB HH F HH BBBB
HHHHFHHHH
HHHFHHH
HHFHH
HFH
F
Now when the filament is getting melted it instead of only filling the nozzle, it also fills the gap and makes fat cylindrical block in the colder part of the extruder. It sticks against the wall, and doesn't reheat back up very well, and you end up with an awful plug.
T F T
T F T
XXXXX T F T XXXXX
XXXXX T F T XXXXX
XXXXX T F T XXXXX
XXXXX T F T XXXXX
XXXXX FFFFF XXXXX
BBBBB FFFFF BBBBB < Plug filled in the gap
BBBB HH F HH BBBB
BBBB HH F HH BBBB
HHHHFHHHH
HHHFHHH
HHFHH
HFH
F
So after you remove the tubing and remove the hotend nozzle, you are left with the nasty plug basically glued in there.
F
F
XXXXX F XXXXX
XXXXX F XXXXX
XXXXX F XXXXX
XXXXX F XXXXX
XXXXX FFFFF XXXXX
BBBBB FFFFF BBBBB < Plug doesn't come out easily
BBBB F BBBB
BBBB F BBBB
F
F
F
F
F
So my printer came with a replacement heater block and the threaded insert for adjusting the nozzle height. I examined it a lot and compared the hole to some drill bits I had. I took one of my smallest drill bits (like 1/16") and drilled out the middle of the plug while I had the extruder on.
Now I could push out the plug.
I ended up cleaning the hole with a larger drill bit that closely matched the tube diameter. I used melted plastic to clean up any stray bits of metal shavings out of the hole.
After all of that, I reassembled. I noted the need to bottom out the bowden tube and I pre marked it at the lowest point I could get it to while the nozzle was in place.
Then I put the pneumatic coupler back on the top, and slid the tube in for final assembly, this time able to see if I got it in deep enough instead of guessing while pushing on three places.
I'm printing again. Life is good. I now know the cross section of my hotend a lot better, and I am more confident about repairing it if needed.
FLSUN QQ-S Pro, Delta style