Thanks gr5. I checked the fan speed - the profile setting is at 50% on both extruders. I will try to disable the fan.
It looks like under extrusion of PETG in the mixed layers. When looking on the model up close, there are miniature gaps between the layers. Oddly enough, there is no sign of that if I print with only PETG. I tried to print the same model with only PETG. It is super solid.
I am printing with a priming tower. I will try to increase the size of it and see if it makes a difference.
My first thought relates to the fact that:
A) Higher glass temp materials (like ABS, PETG) have trouble more trouble bonding - you really have to melt the layer below a bit as you add the layer above. This is normally solved by less fan (or heated chamber). The symptom is weak layers.
B)My UM3 fans run at 100% speed when you set the fan power to about 8%. So you need fans around 2 or 3% on the UM3 to get the cooling cut in half.
So my first thought is that maybe it's not properly lowering the fan speed when it switches to PLA on the same layer that had some PVA. You could check that in the gcode. PVA doesn't really need any fan anyway.
My second thought is the same as yours - maybe some kind of underextrusion? Maybe add a prime tower. I'm not sure what I'm seeing where the red arrow points - I can't tell if there is any underextrusion or if it's just bad layer adhesion.
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