I'm printing some trays that are thin on the bottom maybe 2mm. Not enough for there to be infill. I'm having the same issue on two completely different printers with different filament and different files. I'm using the same Orca profile for both of them though so it has to be a setting, maybe bed temperature? The sides of the trays are perfect.

I've had this happen if there a bit too much squish on the first layer. The level may look good and work for parts with smaller areas, but it compounds for large bottom areas and caused similar effects for me. Increasing distance to bed by 0.01-0.02mm or so fixed it for me.

Thanks for the advice I will give it a shot.

Increase number of layers so its solid. 2mm is too thin for infill.

I had a similar issue (much milder though) happen after I wrongly set the internal bridge flow to the same value as the external bridge flow. This caused extra thick internal bridges ruining the layers on top with a bumpy texture.

Exactly this, the first layer is smooshing and tearing up and that is being carried through the subsequent layers.

Since the bottom is so thin there's no infill layers to reset the density to normal. If there was infill it would build over that damage and then lay fresh new layers on top effectively hiding it unless you're looking at the bottom.

I would also check your hotend, while cold, to make sure it's not loose. It can cause similar issues by dragging the nozzle.

I had a somewhat similar issue on large layers because of insufficient temperature for the speed. Large layers allowed the toolhead to accelerate to higher speeds than normal, and print temp became insufficient for layer adhesion. On regular walls this wasn't an issue because the printer couldn't go that fast. Slowing down or increasing the temperature helped.

How old are your printer, filament and slicer? Zits are common in adolescents.

And now serously, if you look closely at how inconsistent your lines are, I would definitely look into the extruder. Maybe you have power issues, that could account for both printers having the same problem. If you suspect it's the slicer, simply install another one and try.

• How's the environment? I mean do you print in a heated enclosure or free air? Or something else?
• The bottom layers do they look like this from layer 1 or does this only occur in the top layer?

Whenever I see bubbles I immediately think moisture. Since the sides are ok, I don't think it's moisture in the filament, but what about the air? That's why I asked about the environment. It could be moisture or contamination (dust) causing localized layer adhesion issues.

My environmental hypothesis can also explain the acceptable sides. Since the layers with only sides are faster to print, those layers don't collect as much of whatever it is between layers.

If it is environmental I'd expect to see the issue with every layer. Perhaps only noticable while printing because the nozzle will iron out the bumps.

And just to rule out the moist filament, do you dry your filament before printing? I kinda guess you do, because you have multiple printers. But asking about drying is akin to IT support asking me if I have rebooted my computer.

Could it be warping related? As in the ones that usually happens by lifting corners.

Another candidate or a partial subject is an uneven print bed. It does happend over time. If your printer has a layer test print you could check if the print result is even.

Anyway, both of the above can be alleviated with a slower print speed to give the extra time for print bed adhesion. Not really a guaranteed fix, but the results might be good enough.