MakerBot Industries

Lower you temps. PLA extrudes starting at 165 to 185 C where ABS really needs to be above 200. What happens when you use too high of a temp is the the PLA gets soft in the long metal barrel above the extruder block, expands in width and jams the whole thing. You must modify the temps plugin in skienforge and change all values to 185 for starters. Then, don't forget to change the start.gcode in the alterations folder of your profile to lower the temp there too.

Next is to add one or two of the heatsinks to the tube to help keep it cooler and possibly a small 40mm fan driven constantly by the 12 volts to keep the tube cool. Again, it's lots of hacks but PLA is just odd in how it reacts. The probem is PLA melts different. ABS simply gets softer the warmer it gets, but PLA is pretty solid up to a point and then suddenly turns to liquid in a very narrow temp range. Think of it this way, the metal extruder block is the hottest and the further distance away from that in the metal tube represents a slow temperature drop. The problem is, while warming up, the PLA is heating up in the upper part of the barrel and it get's soft as low as 165 (I've even seen mine melt at 140) When you first start to extrude, most of the material inside the extruder is soft, and when you put the nozzle close to the build platform, we are now blocking the nozzle somewhat so the fresh filament tries to jam through the soft filament but creates a wedge. One trick around this is to not feed any PA into the nozzle until after it is heated to temp. Remove all filament from the system, Warm up the nozzle from the control panel to 185. Once the temp is stabilised, just feed enought PLA so that through the MK6 front plexi, you can see it's not quite in the metal barrel and lock down the filament tensioner and hit print ASAP. It's a pain, but it might work in a pinch and proves the theory if it works.

You may need to open the nozzle (hopefully you kept the little pouch of anti-sieze compound used to put it together) and check for physical blockages as seen here :

http://wiki.makerbot.com/forum/t-374079/stepstruder-mk6-issues

You want the top part of the filament, that one that is inside the tube, to be cool as that is the plunger that is pushing the PLA that is molten down by the die. The length you describe seems OK.

The red PTFE does not usually widen (it is pretty impervious to heat to begin with and slippery as it gets) but it can move up (as it did in my case) and cause other issues (specially if you pull the filament up when it has not properly melt).

PLA is more complicated to handle than ABS and as with ABS not all PLAs are created equally. There are plenty of cases of bad ABS out there ( causing blockages and missed extruder steps).