Hot end jam with retractions after heat brake change?  

Hot end jam with retractions after heat brake change?

Hey guys,

I got a mk3s+ last year and it was working pretty good, however, when I tried to print ninjaflex I found that no matter how slow I went on the print speed (and im talking reeaaaallllll slow here) the printer would randomly stop extruding, with the gears seemingly not able to push the filament through the hot end and just spinning on it, grinding it away.  I found this thread:

https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-hardware-firmware-and-software-help/prusa-edition-heat-break-easily-jam-clog-with-pla/

And with various other research, decided it would be a good idea to swap the heat brake for the regular V6 one without the 2.2mm step to try and allow me to print ninjaflex.  I fitted the new heat brake a few weeks ago but now I'm having issues that I never had before.

It feels like whenever i print something that has quite a few retractions in short succession, the hot end jams completely - even with PLA.  The gears cannot push the filament through the hot end and they just spin away.

When I remove the nozzle after this occurs, the jam still seems to remain. I generally clear it by undoing the drive gear tension screw, heating up the hot end, and manually pushing the filament down through the extruder with no nozzle installed.  It often takes a few "Jabs" of the filament to clear whatever blockage is in there.  It seems to me like some filament is melting in some place it shouldn't in the path, and solidifying enough on retractions that it blocks the whole path.

My retraction distance is standard at 1.9mm ish. Literally the only thing I changed was the heat brake, and now I'm having an issue I didn't have before.  However, I'm not sure what to do to try and fix it.

I'm looking for advice and guidance on troubleshooting, as I've already reassembled the heatbrake / block assembly a couple of times with fresh thermal paste and I'm not sure what I'm doing wrong.  It feels to me like I'm perhaps getting some heat creep up the heat brake which is making the filament melt enough to jam at a position above the heater block, but I just don't know what to do about it.

I read something a while back about issues with the PTFE tube causing something like this, but I'm not really sure if thats relevant.  Has anyone had behaviour like this before?  I've printed a fair few parts just fine, so long as they don't have a lot of retractions.  As soon as I print something retraction-heavy, i get jams.  I attach a picture of a print that caused the issue - you can see there are lots of small islands of material that would cause a lot of retractions.  The hot end blocked less than 20 mins into this print; But I did an 18 hour lithophane print before this just fine.

Thanks for any help!

Thanks!