Getting rough walls  

Cooling?

Looks like a possible cooling issue. What is the parts orientation on the bed?, also do you have auto cooling enabled?

Regards

Swiss_Cheese

Cooling?

The problem occurs with parts standing upright. 

I forgot to add the info that this is PLA. I wondered why the opposing sides of the print look so different. 

Everything on my MK3s is original – so the cooling should be the same on both sides of the print.

My cooling settings are the standard 100% at layer four.

Sand paper

Your bad and good are so very close to each other I'm thinking you just need to use some sand paper.

When the term part orientation is used it can also mean front /backwards or side/side as well as up down.  The MK3 fan is located at the front, so objects get better cooling from the front and the 'back' of objects can receive less.  If your part was oriented such that your 'bad' side is facing away from the fan then that's quite possibly the logical cause.

With that part (what little we can see of it anyway) then rotating it 90 degrees so that both sloping areas receive equal cooling may result in a better print.  

There are many little 'tricks' that you need to pick up to optimise for printing.  Orientation being a major one.

RE: Further investigation

@dan-rogers It's really hard to show it on a photo. Here's another one. It is very rough. While the "good" side is smooth as it should be, the "bad" feels just like sand paper. And yes I could use sand paper but it wouldn't look nice and shiny and I don't understand why I have this issue just on the one side.

@neophyl Thank you. I already inspected the (original) fan shroud and discovered that it was slightly deformed (I never printed at more than 220 °C / 90 °C). So I reprinted it (also in PETG) and changed the shroud. When I printed the exact same file from my example part from my original posting, I couldn't see any difference – tha bad side was still rough.

I will design a test model to see if and how the orientation matters in terms of cooling.

PS: It's quite annoying that I always have to wait for more than seven hours for my answers to be published here – that really slows down the whole error solving process.

EDIT: Didn't need to wait this time 😀

The only way to get all around cooling is to use a custom different fan shroud.  I havent found a one I liked that worked better as yet for my Mk3.

One other point, the original fan shrouds are printed out of ABS not petg.  If you print it out of petg it wont last very long at all, even at pla printing temps.  Its ok to print one to see if it cools better but you need to print your working one out of at least ABS, or preferably a higher temp material such as PC.  Its also useful to have a printed spare just in case.

RE: Further investigation

That's good to know because Prusa says "RECOMMENDED PRINT SETTINGS:

0.2 mm layer height, infill 20% GRID and material PETG."

https://www.prusa3d.com/prusa-i3-printable-parts/

But when I see the spare parts for purchase their shroud is printed in ASA... but why has it deformed then? I never printed at temperatures higher than 220 °C.

Theres also a link to a detailed article on printing the parts on that same page, if you read that the fact it can't be printed in petg is clearly stated.  To quote "Note that this part can't be printed from PETG, but at least from ABS or Polycarbonate as it has to sustain high temps."

Also on the STL download page https://www.prusaprinters.org/prints/57217-original-prusa-i3-mk3s-printable-parts/files where you download them from it also states it next to the part - "Please print this part with ASA or PC-Blend!!!"

PLA softens at anything from 50-70 depending on the specific blend, petg can soften around 85 on average and ASA can be as low as 95.  The value you are looking for is called VICAT.  It all depends on the specific material blend as not all are identical.  Sit your extruder close to the bed while preheating both and the temp can build up as the parts fan isnt going.  

good to know, thank you. 

Until now the PETG part works fine though. I just hat an eight hour print (195x195 mm, so long time close to the bed) and it looks as good as new. I still have the slightly deformed original part though.

Hmm the post edit function disappeared. I just discovered that the original part in the shop also looks deformed on the same spot:

https://shop.prusa3d.com/de/kunststoffteile/909-fan-shroud-bl-mk3s-mmu2s-mk25s.html

There you go then..

Ship it.

I cant see where that's deformed though.  Looks normal.