Sometimes, as a silversmith/giolsmith/jeweler, you need a generic ring. In order to get to such item, you may either try to order one from a supplier - which might be of high quality but costly and not absolutely to your requirement - or you may carve one from wax and cast the ring using the lost wax casting process.

Doing this process in plastic and cast the ring with sand casting will of course not be as fine as the lost wax process, but it will probably still save some time as you will not have to carve your model by hand in wax (which will be lost after this).

Now some suppliers also offer a couple of metal moulds for ring casting that will (or will not) do the same. Those can be configured to different sizes and thicknesses as well, but they cannot be configures for D-shape rings. And those moulds are extremely costly as well (Ok - if you happen to sell some rings from those moulds, you're in the clear anyway, but as a pure hobbyist... no go).

Casting in sand is cheap and you can combine whatever size and thickness and shape you like. After that, you can add other things as well - like ornamenting with punches or adding bezels for holding jewels.