I couldn't get other toothpaste caps to actually fit my toothpaste tube. So I found this library and fiddled with the settings to produce this simple toothpaste cap that is easy to print.

it uses: buttress_thread(16.3, pitch=2.1, length=8, internal=true,n_starts=1); 16.3 is the outer (largest) diameter of the thread (measured and then tweaked) pitch=2.1 is how many mm there are between peaks on the threads (measured) length=8 is how many mm long the threads should be.

I am not sure what internal and n_starts does.

The threads.scad and helix.scad I did not modify (except commenting out the demo)