I had been having some nagging trouble printing a couple of models recently. Hard to describe, but think of a thin bordered picture frame around the perimeter of the bed, about 2 mm in thickness and 3 mm wide. No matter how carefully I leveled or how precise I tried to make that first layer, I'd keep having various corners of the model start to pull up halfway through the print. I kept messing with bed temp (40-70c) and speed (20-50 extrude/50-70 travel) but I kept having the same failure. I finally decided to try something I'd read either here or on another forum. That being that some people report better results with PLA the faster you print. So on this most recent attempt I increased extruder to 205c and print speed to 75/100, no raft, bed at 60c on blue tape. Also changed infill to 8% with "line" pattern instead of hex. The result was almost perfect with no corners pulling up. My reasoning was that given how thin this thing was I needed to get it down as fast as possible so it didn't have time to peel up on the corners. It seems to have held true although I imagine it wouldn't work the same with all models.