Help with Wall Separation issue

[d12 0]

I've printed a few hundred of these parts, and they have developed an issue with the outer skin layer breaking free of the rest of the print.

 

The problem started near - but seemingly unrelated to - noticing one of my Y-axis blocks had worn out. (no longer holding the rod securely, getting a little rattle/shimmy in the printhead)

 

I was printing 4 at a time, and the first one would be good, and random numbers of the other 3 on the build plate would have this issue too. Sometimes 1 would work, 2 fail, and 3 and 4 work. sometimes 1 works and the other 3 fail. etc.

 

 

Steps taken so far to diagnose:

 

Slicer

• Adjustments to speed & temp
• Skin Overlap / SO%
• Experiments with 1st layer speed, # of slower layers
• Fan (normally 0, tried 20% and 50%
• Infill overlap
• Print orientation (flipping upside down)

 

With a bunch of calibration cubes in between for reality checks. Still getting the issue.

 

 

Machine

• Different print heads, (2x AA 0.4, and AA 0.8)
• Replaced Y-sliding blocks
• Calibrated axes using this method
• Manual bed level

 

No progress. 

 

Other

• Changed the design to have no radius or taper for the bottom 2mm of the print, thinking that doing immediate radii in the first few layers might be responsible. 

 

 

Any thoughts would be appreciated, I'm stuck here. Pics attached include failed parts before the wall is pulled off and a good print collage.