Modifications to QU-BD extruder for reliable ABS printing.

I had been having trouble printing ABS using the QU-BD extruder on my Hadron Ordbot. The extruder runs fine for small models and but when I would do prints of larger models with long print runs, I would consistently get a feed failures after a couple of hours of printing. I tracked this down to hot section heat slowly creaping up the stainless steel feed tube causing melting outside of the hot section melt chamber. Research showed that this was not uncommon for QU-BD styles print heads because