Update: This problem completely went away when I switched to ABS. I am convinced most of the problem is softening of the filament inside the extruder, which ABS would be less prone to do.
I am in the process of printing off parts for a friend's Kossel. The lower frame mounts are the largest parts, and are near the limits of what a Simple can print. It proved rather challenging to print them reliably and without warping.
My printer lives in a basement that stays at about 58 degrees F. I am using Zen Toolworks PLA which seems to work best at around 210 C. To keep warps to a minimum I print inside a plastic enclosure, which raises the temps about 15 degrees, onto blue tape cleaned with alcohol, and I have to print SLOW. 10 mm per second is the travel rate for the perimeters and infill. Even with that, I have to print on a 8 layer raft to get large parts to print without appreciable curling at the edges.
God, I want a heated print bed. But that will be the subject of another post...
So anyway. With these settings, the lower vertex of a Kossel takes 11 hours to print. I was fine with that - I'm in no real hurry and want the best print I can make. However, printing that long introduces it's own issues. I experienced consistent filament stoppages at about 1.5-2 hours, after which the printer would continue to go through the motions and leave nothing behind.
So I tried the obvious stuff. I cleaned and adjusted the extruder. I levelled the bed. Twice. I tore down the extruder and cleaned up the internal edges to reduce friction. Still no go. Jammed at about the same point.
I then read about extruder motor temperature - that a hot motor can transfer enough heat to the filament to soften it before it goes into the extruder. My motor was indeed running hot to the touch. I dialed back the current on the Printrboard until it was just warm to the touch and wasn't skipping. I excitedly started a print and went to bed.
I woke up to another filament jam. There was some swearing, and a bunch of wasted plastic.
What finally fixed it was removing the 40 mm fan from it's normal place blowing on the extruder and tacking it with CA glue to the side of the extruder, blowing into the chamber that the filament passes through on it's way to the hot end. After all, I was not using the fan anyway, since my layers were so slow.
I turned on the fan 100% full blast in software, and started the print in the middle of the day. Five hours in, the extruder motor was cool to the touch, below body temperature, and no jams. I have since completed two back to back 11 hour prints. It's fixed.
If you have a Printrbot Simple that is regularly jamming the filament, I encourage you to put a cooling fan on the extruder. The difference was immediate and remarkable. If your extruder motor even feels a little warm, it can start to soften PLA on long prints.
After the Kossel parts are done, I intend to make this mount for a more dignified fix:
40mm Diameter Fan mount for Printrbot siple with a wooden extruder