Last night I was doing some experimenting with the nozzle in a vice and may have discovered the reason for the filament jamming so frequently. I have a McWire which moves very slow compared to a belt driven RepRap. To compensate for this, I slow my filament extrusion speed (about 1/3) and set the extrusion width in skeinforge up almost twice as high as belt driven machines.
My test setup was simple the nozzle and barrel in a vice and heating with a small torch to ~230-260C. As I was pushing the filament through the barrel at about the speed my extruder operates I noticed the barrel was filling with molten plastic all the way up. Once it was molten near the entrance it started oozing out when I pushed the plastic through, instead of going through the highly resistive nozzle. If I pushed the filament through relatively fast, it wouldn't melt as far up and worked better. Frequently the jams and oozing through the PFTE occurs after a pause or long warm up period. This would cause the filament to melt all the way up the barrel and cause the revers flow I was seeing on the vice. I might try adding another large piece of aluminum bar as a heatsink to try and cool the upper part of the barrel. Now I know why everyone is always saying have a short heat transition zone.
Cleaning the barrel and nozzle with acetone helped reduce the resistance and allow faster extrusion, but I can't operate much faster with my current machine.
If anyone has any suggestions...please help! I'm about 30% done printing parts for a Mendel and would very much like to finish.
This test was with ABS, but PLA has the same problem, perhaps even more due its lower viscosity when at extrusion temps.