Mystery of the 0.20mm

My parts < 30mm are coming up ~0.20mm taller. That’s beyond the margin of error I want. If I print taller than > 30mm the accuracy is +/- 0.03. The test print is a basic 10mm diameter cylinder of varying heights. The material I’m still working with is PETG.

A basic 10mm diameter test cylinder

I posted my issue here on the Prusa Forums but so far, crickets.

• Could it be layer height? I don’t think so, I would expect that all layers would be additive such that regardless of print height I would see an accumulation.

• Z offset? This one was a close suspect, but my Z calibration looks good and a Z offset error should result in the same amount of error on all print heights. It shouldn’t magically go away after a certain height.

• Extrusion Multiplier? Test cube looks good. Single wall thickness is 0.40mm and all axis are the proper height. Note this is a 20mm cube but single wall. Only when I switch to printing to a solid, I see errors in the Z-axis height.

My current theory is that the material has some moderate thermal expansion in the lower layers which makes the lower N layers just slightly thicker. As the part grows taller, the error from the thermal expansion is averaged out from the height of the piece. I can’t imagine what else could cause this issue.

I need to fix this since I have a part that requires axis accuracy.