Weird printing issue  

Weird printing issue

Hi,

I have a Prusa MK3.5S, upgraded from a MK3S+ six months ago. It works like a charm, printing all kind of stuff in PLA/PETG,ASA and TPU. I'm using a 0.6mm nozzle.

Yesterday, I wanted to make storage trays for my Drone parts, so I did this in PLA. Result was perfect, especially bed levelling and first layer are top notch. Not a single defect or printing problem. I'm printing with stock PLA 0.32mm SPEED profile.

Then, I mad another one, slightly less tall for smaller objects and with thinner inner walls (1mm instead of 2mm) for saving a bit of filament but same footprint and external walls (2mm).

The bottom printed perfectly, with again very good first layer. But, after the bottom (2mm) was finished, vertical walls printing started, and there quickly was an issue occuring only at the back right wall. On that section, vertical waves started to appear, the nozzle hitting them at the next pass :

Suspecting some random problem occured and snowballing, or some warping and partial detachment from base plate on the corner, I stopped the print and started again. But, it did it again, exactly the same way and at the same place. I then tried to pause the print, cut the "waves" with a blade, and restart, several times, at lower speed (80%, 70%, 50%) and each time the issue came back after 1-2layers.

Various pictures of the issue here:

While all other external walls are great for 0.6mm SPEED :

Then I tried again printing the first part, and it worked fine.

To sum up, the issue :

• Isn't random and is fully reproducible
• Only occurs at a specific place, on about 1/3 length of only one of the four external walls
• Doesn't occur on another very similar print with same footprint and external walls
• No warp at all and first layer issues (can show the bottom surface, it looks perfect)
• When it happens, is likely due to one of the perimeters (the internal one I think, but I'm not sure) causes a kind of extrusion resonance which "snakes" the printed material horizontally and vertically
• Still happens at 50% speed

I have absolutely zero clue why it happens... any idea?