How to prevent "pitting" with single-wall prints

[CartesianTheatrics 0]

Hello,

 

I have a persistent issue with "pitting" (underextrusion at start of outer wall) with a lot of my single-wall prints. I'm curious if others have ran into this and what they're solutions might be? I find it almost impossible to control extrusion precisely enough to prevent the occasional "pit" when moving to an outer wall, especially when you have large geometries you're printing with a large nozzle and a stringy filament. In this worse case, sometimes the pits can be pretty bad. My  approach currently is to use "firmware retraction" and simply mess with the firmware implementation in klipper (where I have a little more freedom) until I get a good solution/compromise for the given geometry. Wipe distance / combing / extra prime amount / etc. are not usually helpful in my experience.

 

There are a few strategies that I suspect could be interesting. A simple strategy that I think could help for many geometries is to simply allow the option to print top/bottom segments before infill segments, as sometimes it's easier to stabilize the flow while printing infill segments instead of top/bottom segments (I mention this in a bit more detail in the github issue tracker, but haven't gotten a response).

 

Another potentially interesting feature is to allow starting outer wall segments "early", so basically you have a short inner wall when possible. i.e. you have just enough inner wall to stabilize flow and get the extruder into a "known state" before beginning a visible wall. This might have many edge cases, but for many geometries it might be the most reliable way to stabilize flow prior to beginning the visible outer wall line. 

 

Anyway, just two cents. I'd be interested to hear any other thoughts or approaches people take. Or should I just give up and always print with at least two outer wall lines?

[MariMakes 85]

Hey @CartesianTheatrics,

 

What an interesting topic 😊

Do you have some pictures of the "pitting" that you are encountering?
Perhaps you can share a project file where you are encountering this issue a lot. 

That would really help with the discussion. 

[CartesianTheatrics 0]

Yeah, it happens anytime you have multiple retractions and travels in a short period, resulting in a "undefined" extruder state.

 

For example, here's a particularly bad case. 

 

It's almost perfect until it gets to a layer with several small top/bottom segments that it has to jump to before jumping to the next layer, which is the start of the outer wall lines, as you can see here:

 

 

There are tons of things you can play with to try to mitigate it, but the overall problem is you can't model the extruder state perfectly under arbitrary retraction and travel. Thus we need a strategy to ensure we get the extruder into a known state. 
 

If we could print these small top/bottom segments before the infill, that would often help somewhat, as the extruder state will become predictable while printing these connected infill segments. Generally we can pretty accurately model a single retract/travel/untract sequence, but it quickly becomes intractable as we stack them up, esp. if when dealing with a high-throughput scenario with a volcano nozzle.

 

Again, I'd love to be able to start these outer wall segments early. Basically the logic would be "N+1 wall lines, except the first wall line (if it exists) is trimmed to the last N millimeters of the line".

 

[MariMakes 85]

Thanks for the pictures, they help making the problem a little better.
In Cura 5.0 we introduced a number of new settings with the new slicing engine.
Did you get a chance to try and tune these?

 

Do you have a project file for us? It contains the printer and settings we need for troubleshooting your issue. 
To save a project file go to File -> Save project.