Hot end blockages

posted 23 Sep 2012, 10:48 by Colin Bell   [ updated 30 Oct 2012, 12:18 ]

I've been having a lot of trouble with the Huxley's hot end blocking up recently, so much so in fact that I've started to get pretty fast at stripping and cleaning it - under 15 minutes! Of course, this is getting quickly boring, and no matter how efficient I get  at cleaning the hot end, I want to fix the issue. After a few days of careful study, I've come up with the following.

The problem is the barrel connector which connects the two aluminium blocks that form the hot-end body. 

The barrel (on the left in this picture (image from when assembled is supposed to butt up against the pneumatic fitting in the top block, and the nozzle in the heater block but it appears that in my top block, the thread is fouled and it doesn't fit up to the pneumatic fitting  when screwed in. 

This is resulting in the a 'cold' area gap, where blobs of plastic are breaking away when the filament is retracted. Under normal operation the printer extrudes just fine, and provided I don't remove or run out of filament it will keep happily printing all day long. As soon as I do need to change filament though, the problems start, and nine times out of ten I get a blockage which needs to be cleared out.

The fix for this, I think will be quite straightforward - I just need to re-tap the top block so that the barrel butts the pneumatic fitting properly - this will I hope reduce blockages here by 70-80%.

Another gotcha I've observed is removing the filament too hot - I generally print between 185c and 200c depending on material, and on a couple of occasions have pulled out the filament as soon as a print job finishes - pulling the material out with the temperature too hot often results in some long thin stringy hairs of plastic following the filament out as it retracts - on dark coloured material this is easy to see, but not always. If it becomes detached from the main body of the filament and gets left in the bowden tube, it's a safe bet that when you load the next lot in, it'll get pushed into the barrel which won't be hot enough to melt it, and form a plug. I seem to get less of this when I let the hot end cool to around 120c before starting the retract - the safe money I guess is using the recommended 78c, but I'm always a bit worried that it will over stress the motors as the plastic will be quite hard at that temperature, particularly if there is a plug forming in the barrel. Given the amount of hassle I've been having though, I will start doing the 78c warm up before retracting to see if this helps me.