A couple of breakthroughs -- looks like my problems are being caused by the cooling fan placement & speed.
First off, I noticed I was getting some "cold extrusion prevented" msgs after the cooling fan turned on -- don't know how I missed seeing those before. After jury-rigging some ducting to redirect the airflow, there's still a temperature drop, but not enough to prevent extrusion.
I was printing small hollow tubes, varying the retraction length from 1mm up to 2.5mm -- slight differences between prints, but nothing noteworthy.
I had a lucky accident here -- I was using a 40mm fan from my parts box, probably pulled from an old PC. The fan died during the next test print
, and the rest of the print came out looking great, no threads...
My ToDo list for tonight:
- replace the fan
- fine tune the fan placement & speed
- more test prints
- pictures