When changing materials make sure to stay at the higher of the two materials for a sufficient length of time  to clear the higher temperature material is cleared from your extruder. Trying to extrude remains of a material at a lower temperature than it melts at can break your extruder. When switching from ABS to PLA we extruded for 10 + minutes before lowering the temperature.

For new or experiamental materials like switching to PLA we recommend setting your temperature by hand, meaning: after we sufficiently cleared all the old material releasing  the drive and tested how much force we needed to push the filament through by hand. We started low at about 150C for PLA 4042D and worked our way up until we could easily push it by hand.

We are very successfully printing PLA 4042D at 163C - 178C. (: We have yet to verify the accuracy of our thermistor, waiting on proper thermocouple to arrive, but hope it is fairly close ;) We have actually been printing at 168C.

We print on the blue painters tape as recommended by Vik. If we have trouble getting the first layer outline to stick we wipe down the build surface with rubbing alcohol. This can clean the oil from the tape a little too good and can make the part very hard to remove from the build surface.

We set the bottom altitude down (ours is -0.1, combined with our other settings results in a first level height of 0.27mm, handle with care) in skeinforge to push the PLA firmly onto the build platform. The side effects of this is the plastic being pushed a little wider than the object should be on the first layer and reducing the height of the part. For what we are printing neither of these are deal breakers, but this needs resolved in the future. We also have trouble some times keeping that first layer from getting peeled back up by the extruder passing back over, but we are mostly blaming that on a combination of our fat faced nozzle and a warped acrylic platform from printing over a Mendel worth of parts on it. We have been manually modifying the first part of our gcode files to drop to the platform outside of the part's perimiter - starting extrusion - then entering the item. It just takes modifying two lines and leaves the bugger that kept messing up our outline outside the part and is being much less painful than dropping in the middle then going to the outline like we were doing.

We get really good prints with PLA 4042D with our current settings. We are using all factory MakerBot extruder parts except my thermal barrier and heater/nozzle which is .5mm on our MakerBot.

PLA is stiffer and more brittle so avoiding extreme bends while entering the extruder is necessary. I use a wire clothes hanger poked into the extra little holes on top of my MakeBot as a guide. It looks pretty ghetto, but you know.

All that benefit from the ease of printing and all other PLA greatness that its use for RepRap and related was pioneered by the good Mr. Vik Oliver. The PLA page on RepRap.org shows his first post there recommending its use on November 17, 2006 and slightly documents his pioneering it being extruded into 3mm filament. We are catching on now Vik, thanks for all your hard work, help, and vision.

What knowledge contained in this post is learned from or inspired by the woldchanging RepRap community.