Why is there a 3 layer gap in support for horizontal hole?

[dodado 0]

I'm new to Cura. I'm slicing a model with a 5mm horizontal hole, and using supports at around 55 degrees.  I tried the default setting for zig-zag for 0.3mm layer height and 0.4mm nozzle, 15% density, but there is a 3 layer gap in the support so that 50% of it is floating in the air.  Is this normal behavior?

I tried switching to line support, and adjusted the density to 20-25% to try to get the line in the center of the hole.  Off to the side, another line is generated which has part of it floating in the air too, by 1 layer.

Edited June 11, 2020 by dodado

[dodado 0]

I did a little more testing.  With zig-zag set to 10% density, I was able to get rid of the floating support layers, but unfortunately it's an enclosed rectangle.  I was expecting at least 1 side to be open to that it's easier to break it loose.

I also tested the line support, changing the density by 1% increments and each time gives really unexpected results of floating supports with missing layers.  Only a few gave satisfactory results without missing support layers, but I could only do it with 1 line.  Whenever there are 2 lines, the second one always has missing layers.

 

[Thegladster 2]

This happens to me a lot too. If this happens, I normally just manually add supports on Tinkercad, or if that's too much work, I just don't do that model. I can't find a way to stop those lines, though, except for increasing the support line distance.

[dodado 0]

Well, I assume these floating supports will risk print failure from dragging loose strands or sagging overhangs if someone isn't careful and paying attention to how Cura is generating them.

Edited June 11, 2020 by dodado

[Framar 38]

Simply block support for this hole, is not necessary.