Are there downsides to using infill combination for cosmetic parts?

[somber patio]

At the moment I'm using aligned rectilinear as it is pretty much the fastest infill to print, and I don't care about strength since it's a cosmetic part.
In this case, the only job the infill needs to do is support the top layers/angled walls like usual, and to provide some extra weight to the part.

Although aligned rectilinear is already basically the fastest infill, I was wondering if I could make it even faster by using infill combination, and would there be any decrease at all in surface finish of the part if I did so?

I've never used infill combination before, but I was imagining that in some cases (such as a slanted surface with an inwards angle that is close to horizontal, like a short cone or pyramid shape), having infill only every other layer could leave some of the top surfaces or walls unsupported on the layers where there is no infill, and so this could potentially cause sagging or some other surface defects.
However since I have never used this setting I wanted to be sure whether there would be any potential issues like that, or if it is "free" to use without a decrease in quality?

P.S. I know I could use lightning infill to maybe save some more time, but generally for cosmetic parts I don't want to use lightning infill, because it makes the part feel too lightweight, and heavier things give the impression of quality (and stops the object from being pushed around/knocked over too easily), even if the strength is not significantly increased.

[real wagon]

I haven't found any downsides to infill combination yet in my usages. Combined is ony faster if you are not flow limited, so check the box and look at your time estimation, depending on your printer it might not even be faster. Also, it isn't skipping infill every other layer, it's printing double height infill every other layer, which saves time if printhead movement is your limiting factor, but it's not using any less plastic or supporting any fewer layers.

[somber patio]

Ah ok thanks for the info 🙂
To clarify, the problem I was talking about was lets say you have a layer height of 0.08mm and you turn on infill combination with the default limit of 100% nozzle width as the max layer height.
So then in that case you would have walls etc being printed with a layer height of 0.08mm but infill would be printed with a layer height of 0.40mm, and so the infill would only be "updated"/get a new layer once every 5 normal layers.

Let's say that there is a feature that needs infill support that occurs in-between that 5 layer interval. For example, a top surface which would need infill support for the internal bridging, etc., or the object has a very close to horizontal slanted surface which would mean the walls would need support internally from the infill to stop them collapsing.

In such a scenario, if the infill is only being printed every 0.40mm, how could it support those features that need support at say Z=0.08, 0.16, 0.24, etc, since they will be printed before the next infill layer is printed?

I did notice some behaviour of the combined infill reducing its layer height sometimes in order to support these features, but I just wanted to make sure that there would be no top surface or wall support issues caused by large layer height of the combined infill?

[real wagon]

I'm not sure about that specific scenario. Have you tried stepping through the preview to see if it shows the issue?

[somber patio]

I had a look at some test cases (see the attached 3MF), and it seems like it shouldn't cause any issues because it does reduce the layer height temporarily if it needs to support some overhanging surface that is in-between two of the 0.40mm infill layers.

For example, for the cube example, you can see that it reduces the height of the very last sparse infill layer from 0.40mm back to 0.08mm in order to make sure that the top layers get support at the correct height.

And for the slanting test case, I cut off just the top section of a sphere, and you can see near the top where the overhang is most steep/horizontal, then there are some sparse infill layers being printed with 0.08mm layer height (blue) in order to provide support of the solid infill and bridging, which then in turn support the top layers and walls.

So if it is fine for cosmetic prints, my only question that remains is are there any possible issues in terms of strength when using infill combination for functional prints?

I read somewhere that it can make the sparse infill weaker due to the large layer height (for example 0.28mm layer height has less layer adhesion than 0.20mm LH, which is weaker than 0.12mm LH), so could this be a reason why it would be a bad idea to use this setting generally in every print profile/ for every type of print?