3d printed PLA is notoriously difficult to glue, but I have discovered an easy-to-handle, non-toxic glue that works really well with it: Polycaprolactone. I get mine in pellet form for about $20/lb — a search on Amazon for polycaprolactone will turn up several different brands, and you can also find it at makershed.com.
Here, you can see a 3d-printed bracket that cracked due to poor layer orientation. No big deal — I just heated up a couple of pellets of PCL in hot water, pressed them into the gap like plumber’s putty, and squeezed the parts together. 2 minute fix, vs a couple of hours to re-print…
The color even matches pretty well! I didn’t bother to clean up the squeeze-out, but that would have been easy enough to scrape off while the glue was still warm.
As another example, I used a single pellet of PCL to carefully glue each gear shaft of this elliptical gear set.
It wasn’t hard to keep the gears from locking up — I just spun the gears for the 30 seconds or so that it takes the glue to cool off.