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.
Thanks,
Jeff
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