Cura slicing bug?

[jonnybischof 60]

Hi community,

It seems I've found a bug in Cura. The issue is not fatal, but it's just something that's wrong so I thought I'd report it:

This is the first time I noticed the issue, so it might be because of my "unusual" model. But all in all there is nothing wrong with the model - it's a very simple Sketchup model with very low polygon count and simple, clean, straigt faces.

So, I've started putting a "raft" into my models, meaning I always fill out the whole base layer, covering holes or any kind of internal structure. This helps me a lot with base layer adhesion to the platform. It needs to be cut away after printing, but that's a small tradeoff for getting almost perfect bottom layers.

Now, first of all - the pictures:

What you see there:

This is always the very same view, going upwards in layers. If I jump from layer 2 to layer 11, it's because layers 3 - 10 are identical to layer 2. The same goes for the other gaps (it's really a pretty simple model...).

So, as you can see, I start with a clean and easy to print first layer. Then there's two holes. The lower one goes all the way through the model, whereas the other one will stop at 1mm height (layer11), making this one a completely inside structure of the model (because of the covered first layer).

Now, from layer11 onwards it's supposed to cover the top of that second hole, which it does for layer12. But then there's layer 13, which is where the 23% infill starts for the whole model. As you can see, the covering of the second hole is disrupted for one layer. After that, it continues covering the hole until layer 23, making a 1.1mm top thickness but with the disruption at the third layer.

This is no big deal, but it's a bug... I don't know what causes this, and where else it might occur, but there's a possibility where this may be more of an issue than in my case.

 

[Daid 306]

This is no big deal, but it's a bug... I don't know what causes this, and where else it might occur, but there's a possibility where this may be more of an issue than in my case.

 

I'm not 100% sure if I understand it correctly. But I do think I know the cause.

What the Cura code does is when it need to check if somewhere needs solid infill it looks down X (usually 6) layers, and checks if there is a solid or open space there. If it's open, then it needs solid infill, else sparse infill. In this case, the filled bottom layer causes the higher layer at layer 13 to be "empty".

 

[jonnybischof 60]

You are right!

I checked that by adding another 0.1mm (1 layer) to my "bottom raft". This resulted in 2 "wrong" covering layers.

Adding another 0.1mm resulted in 3 "wrong" covering layers.

So it's exactly how you described it.

I can't really think of any actual models where this could be an issue, except mine which is really unusual. It could affect vertical engravings, but only if they go deeper than the wall thickness. Other than that, I don't see where else this could occur.

If you were to check every layer instead of just the one "X layers below", it would probably need much more time to slice models. Probably not worth making everything worse just to get one very rare bug fixed...