Cura making strange dots on thin walls

[znib 2]

Hi,

I made an object with 0.8mm thin walls. I thought 0.8 should be easy to print because the nozzle is 0.4, so it simply has to do two passes.

But strangely, in one wall, it did some strange dots. You can see them in the pictures bellow (the big blop on the left is not one of the dots, it's where I stopped the print). They can even be seen in cura's layer view as a blue line with dots. And not just on the straight wall, it also does three dots on the circle, where this leads to stringing when it moves from one dot to the next..

When it makes them, it goes from dot to dot, and stops at each dot for a moment. Between the dots it seems to retract. In the video you can see how it moves and hear the retractions.

Why does it do this, and how can I prevent it from doing it?

 

[gr5 2,005]

I'm guessing that's infill. You should know that if your wall is .8 then a skin of .4 will do two passes and setting skin to .8 doesn't really make a difference.

I don't know the answer but I have some things that will take 2 seconds to try as you can see the results in slice view.

Try turning off infill just to see what happens.

Try leaving infill on but setting infill overlap to 0% or even -50% (yes, negative). If there is no overlap then there shouldn't be room for any infill. I think the overlap is enough that Cura tries to connect the walls together with infill.

If the above two approaches convince you that the issue isn't infill related, then I guess lie to cura and tell it your nozzle is .39 and then .41 mm just to see what happens. You can lie to Cura about your nozzle width - it will put out more or less plastic than normal accordingly. People have said you can lie by about 25% - much more than that and it the results aren't so good. I've never tried it.

 

[illuminarti 18]

I think that it's some sort of rounding error, possibly compounded by infill overlap considerations, causing Cura to think that there's room to try and fill it in. As George said, try setting the skin thickness to 0.4, and turning off infill, and see if that helps.

 

[nick-foley 5]

Yeah, this has been a nuisance for me as well in the latest versions of Cura.

Changing skin thickness or part wall thickness can help, but for me, there are plenty of cases where I have smoothly transitioning wall thicknesses (say, a long, thin wall that goes from 1mm to 2mm) and those areas inevitably have some crazy dotting behavior on them, regardless of the number of skins or actual wall thickness.

 

[znib 2]

It really seems to try to make some infill where it makes the dots. When I set Fill Density to 0%, it doesn't do the dots anymore. But this is not a solution, because in other (wider) areas I need the infill.

Changing the nozzle size doesn't help. If I make it smaller, it still does the dots. If I make it bigger, it doesn't do all the walls anymore (which is strange, because they are all the same width).

What seems to help (at least in the layer view, I haven't really printed it yet), is to increase the Shell thikness. If I put it to 0.81 the dots on the straight wall are gone. If I put it to 0.84 the dots on the circles are gone too.

Could that cause any problems? With a nozzle of 0.4 it's not really possible to do a 0.84 thick wall, so how does Cura handle this?

 

[gr5 2,005]

It handles it by extruding slightly more plastic than normal. This sounds like a great solution. Don't forget to put the settings back for a future "normal" print (whatever that is).

 

[illuminarti 18]

Yes, when the shell thickness and nozzle size aren't exact multiples, Cura basically fudges the effective nozzle size. It will behave as if the nozzle is up to 50% bigger, or 33% smaller, in order to fit a whole number of ' (pretend) nozzle-width' passes into the requested skin thickness.

 

[3dcase 37]

Hi,

In some thin walls I had the same problem, try to use no retraction,

In my case the extruder retracted in the infill pass, that gave the dots.

Switching of retraction will not harm your print, because it does'not retract anyhow where i want it too, and that's on the blue lines in your screenprint, crossing the print from one wall to the other :(

In the old 13.04 version you can adjust the minimum travel, so you have no retraction on infill.