I’ve only just started using cookiecad but when I slice an imprint cutter, the imprint part is given double shell lines - and when printed there are gaps between these shell lines. I’m slicing using the latest version of cura and can’t see how to fill the gaps.
Ideally I want the imprint lines to be single lines but I do not know how to do that.
This could be a tricky problem to resolve depending on whether the lines in your original image are consistent. The first thing to try is to use Experimental Settings → Change imprint thickness. Then you can use a negative value to make the imprint thinner. You may also have to turn on the thin walls setting in Cura. That should reduce the thickness, and the slicer should use a single line. You can also try Prusa Slicer to see if it handles this better.
Handling small details like this is tough with current day slicers. If a thicker imprint is acceptable, that will actually make it a lot easier for the slicers to fill it in. To make it thicker use the change imprint thickness setting with a positive number.