We've been printing pretty successfully with our UM3+ for the last few months, but this week the BB core (printing PVA support) has blocked twice. The first time we had to dismantle the print core (I know this isn't 'authorised' - but we found that out afterwards! :-) ) and managed to unblock it with a very thin drill bit. The second time it stopped printing support midway through a print and seems to have blocked up again. I tried to clear the blockage by heating the print core to 220C and pushing fresh material through by hand, but it wouldn't budge. Disassembling it again we managed to get the blockage out, however the bit we removed wasn't really burnt up so I'm at a loss as to what went wrong.
Our best guesses so far:
1. The PVA has 'taken on' humidity and it's burning this off in the core, causing debris
2. The temperature of the PVA core is too high (but I am using the Cura defaults)
3. Some dust/dirt/plastic got in to the print core and has now damaged it
4. The first blockage was a fluke, and we've damaged the print core now by disassembling it
5. The BB core has run it's course and worn, and just needs replacing (though we've not used even a whole PVA reel yet)
6. The blockages are happening because the filament is grinding, and then the PVA burns up in the print core as it can't extrude.
In both cases there has been grinding on the filament, but as we've not been there to witness the moment the blockage occurred we can't tell if the grinding was caused by the blockage or the blockage caused by the grinding, if that makes sense. I loosened the tension on the feeder a while ago to resolve another grinding issue.
Can anyone shed any light on this? What's the most common cause of blockages on the BB core?
Thanks for your help,