3D Printing

You're not going to be able to do an FDM print without there being layer lines and without there being a plane in which the part is more fragile thanks to layers being easier to delaminate from each other than to break. A resin printer can more or less avoid these problems, but you'll need an expensive engineering resin to get the same strength. None of the problems affect everything you might ever want to make, though, and there are plenty of things you might only want one or two of, so obviously would never machine a mould for.