Can anyone give me tips on reducing the layer separation happening here? The piece seems solid (the internal plastic bonded) but I get a bit of this…
Could be a number of things, but if the temperature is stable and your layer thickness is below 80% of the nozzle diameter, you could try decreasing the outer perimeter speed.
Possibly z calibration where it is moving up more than the thickness of the layer by a fraction thus not getting the adhesion necessary. Also what temp, layer height, material? I’ve had this happen due to under extrusion and temp being too low. How accurate is you z with a 40mm test cube?
Or perhaps it is under -extrusion since it is manifested mostly where the arc is longer (top and bottom)
When i really need a good layer bond or water tight print, I usually over extruded by 5%
is it just this print (long arcs) or is it all prints showing separation like this?
What type of plastic? Temperatures? Speeds? Cooling? I had those issues with ABS and a fan, but bumping up the temperature and turning off the fan helped.
Under extrusion or over retraction
This looks a lot like the under-extrusion I was getting a while back, which was from the thermistor reading fluctuating. I ended up cleaning the hotend and extruder wheel, then adjusting/securing the thermistor to provide a consistent reading… I hope this helps you! 
Looks like either it’s ABS and it’s cooling too quickly, causing the perimeters to bond weakly and warp apart, or you have too much resistance on your spool and it is pulling up on your extruder.
It’s ABS, 3mm. printing at 215c. Two fans pointed at the nozzle, in a heated chamber. The Spool does offer resistance, but for this particular print I was manually rotating the spool, it shouldn’t have experienced any resistance.
All my prints are showing something like this. The thermistor is very solidly placed - it’s screwed into position. I do have a bit of temperature flux from the fans blowing at the nozzle.
You should never have fans pointed at the nozzle (down, toward the print) for ABS. Fans cooling the top part of thermal barrier are ok, as long as that air does not go down and cool the ABS.
I only use cooling fans on ABS on very small layers (small parts or details), and even then at maybe 50% power (and my fans are tiny). Also 215 C would be too cool for ABS with my rig anyway. I print ABS at 230 C with a 110 C bed with good results, no heated build chamber but mostly no fans either.
Yeah, I print ABS at 230-250C. I print fast and have a pretty short hot zone, but I would never print ABS below 230C.
Agree with @Whosa_whatsis about the temp, 215 seems quite low and definitely could be a primary culprit. I also only use a cooling fan on very small prints with fine detail, and definitely never on any structural part. I do have one fan cooling my nozzle, but it is ducted so that it does not blow on the part being printed.
Slow down your peremeter speed and turn off the fan. Also check know your z-axis on the printer for imperfections that would effect the print.
Lastly, make sure the bed is level…