I have one just like that on my scrotum and was told it's normal so I wouldn't worry about it too much.

Randomize Z seam and make sure your flow/retraction settings are as good as you can get them.

Messing around with your nozzle temperature might help too: Keep in mind that the thermistor in the heat block might be off by a few degrees so if you can measure the temperature directly as a comparison, it can help you figure out how much it's usually off by and adjust accordingly.

Most probably PA. You should try adjusting it while printing using some g-code console. This way you get immediate feedback. What they often don't tell you is that pressure advance is not something you can calibrate once and forget, it's dependent on material, temperature, nozzle diameter and also a lot on feed rate and extrusion rate.

The need for PA and precise tuning is more pronounced the higher changes in extrusion rate you have eg, you print very fast or with wide or high layers.

Just play around with it a bit and I'm sure that you'll find a better value after some layers.

There are even macros that try to remove the feed rate dependency by effectively replacing every g-code that adjusts feed-rate by a combination of the original g-code pre-ceded by an adjustment of PA. I haven't tried them out yet though.

What's weird is the differences of the effect between the layers. Is the every layer the same or is the path different for different layers? Maybe post some images of the inside of the model.

_ I’m not using .... “retract on layer change”_

There's your problem.

Disclaimer: I'm not into 3d printing. I'm just a wannabe with not enough time on my hands, so take this post with a bucket of salt. I have, on the other hand, worked a lot with automated machinery.

But if I were to make a guess, I would suspect some slack in either the X or Y direction, resulting in inaccuracy when the nozzle changes direction. This should in theory be less visible when the nozzle is not making sharp turns.

Are your belts tight? Any gears with poorly meshed teeth?

Also, are you able to turn the print (and the seam) 45 and 90 degrees? Do they show up just as ugly? If not, the issue is mechanically limited to one axis.

In what can only be described as "perfect timing", the 3d printing channel teaching tech uploaded a video about just this problem literally a day ago: https://youtu.be/vl0FT339jfc

So I stared at this for a while, particularly the pattern of the seam, I have a few questions. What type of infill are you using? how many outer wall loops? How much infill overlap (or whatever it is called)?

Also what happens if you rotate the part on the bed 90 degrees so the seam is on a different axis? Or maybe 45 degrees? This will make sure it’s not some sort of mechanical issue in one direct that shows up in this specific situation.

I know that all seems random but those are what I would check hah.

That retract distance seems a bit low to me. My (direct drive) printer is happier around 1mm to 1.5mm. I'm not convinced that has anything to do with your particular problem, though.

Anyhow, to my untrained eye that looks like underextrusion at layer change and/or the perimeter lines not actually hitting each other. Are you sure your pressure advance/linear advance values are correct for that filament?

Also, does Orca Slicer allow you to specify a negative value for the seam gap? If so, that might be worth fooling with to see if it has any impact. That's probably a band-aid, though. I think your issue lies in pressure advance tuning still.

There is a setting in super slicer called Seam gap, do you see a setting like that you can play with?

Have you tried checking the belts and all the bearings/rails for tightness and smoothness? It almost seems like layer shift at the seam, and if you compare both seams side by side, layer by layer, it almost seems like the print head shifts equally on both prints for any given layer (e.g. left print looks over-extruded while right print is under-extruded, next layer up, it flips and right print looks over-extruded while left print looks under-extruded).

If not the belts or other hardware, perhaps it's a flow issue. Do you know if the gross looking seams are the start or the end of the layer?

That's a really weird looking z-seam, something is going on there that I'm not quite sure the cause of.

However, in this case I think you could try randomizing z-seam position. It's not gonna fix the issue completely but it might help you with this particular part.

Duct tape.

Is the bad side of the seam where it stops or where it starts printing the outer wall? I assume it's where it stops and then it cross the wall to form the infill?

To add to the PA questions, are you sure that your PA setting actually are changing anything?

What printer is this and what firmware?

Does a spiral mode print work fine?

What if you print the part significantly slower (to rule out rigidity/acceleration issues)