Cura uses lines instead of concentric infill

Question

I'm printing a ring that's a replacement for the non-slip base of a mixing bowl. The ring is about 130mm in diameter, with a rectangular cross section, like this:

I'm using Cura as the slicer, and I've set the infill to 100% and concentric, but after slicing it looks like Cura used lines instead; the ring is filled with parallel straight lines:

Is this a problem with Cura? Is there something I can do to encourage it to use concentric infill? I don't really care what the infill pattern is, but I think concentric would print a lot faster since the head wouldn't have to switch directions all the time.

Answer 1

This is a known issue. Cura's profile variable logic sets the number of bottom layers to 999999 if infill is set to 100%, overriding infill by replacing it with additional bottom layers. If you go find the setting for number of bottom layers and set it back to the number you actually want, overriding this, infill should work as expected.

Alternatively, setting top/bottom pattern to concentric should also fix it, and you probably want that anyway so that you don't have distinct bottom layers that are printed as lines.

Answer 2

Is this a problem with Cura?

I don't know if the good folks at Ultimaker consider this a bug or a feature, but it appears that Cura uses the Lines option regardless of the Infill Pattern setting when Infill Density is set to 100%. There may well be some reason for that; for example, the fact that the Lines option alternates the direction of the lines from one layer to the next probably makes for stronger parts.

Is there something I can do to encourage it to use concentric infill?

Yes! It turns out that setting Infill Density to anything less than 100% gives the expected concentric infill (provided Infill Pattern is set to Concentric, of course). When I changed the setting to 99.99% and re-sliced, I got concentric infill in the Preview panel. I haven't tried printing yet, but I have no doubt that I'll get the same thing in the actual print.

I don't really care what the infill pattern is, but I think concentric would print a lot faster

With Infill Density set to 99.99% and Infill Pattern set to Concentric, the estimated time to print my part drops from 4 hours 50 minutes to 2 hours 44 minutes, a 44% time savings compared to 100% infill. That's probably a lot more savings than you'd get on a part that wasn't so narrow, but it's worth knowing that at least some parts can print much faster with concentric infill.