Hi, I would say that most designers that are reasonably new to having rings made for them, eventual get it done. All 3 methods you mention may get tried depending on who is advising them.
The current method is to rapid prototype/ print into a castable material. Not an inexpensive venture if you are going to set up properly. Programs start at around $550 to excellent, costly programs like 3design in the approx $9000 range for the full manufacturing version and $2000 for the designer version... designer has all the bells, but it can only output files to someone with a master version. Next is a good Jewelry printer, high definition...minimum $4500 to about $80,000. Then there is the learning curve for all that. The best way to get into this kind of stuff is to get the $2000 program with a years tech support, learn to design and make your own files. Then send your files to a company that can print, cast, create models, molds,polish, set etc...essential a manufacturer for designer
If you are using printing to make ring models, this is probably the best way as we can make all the sizes, build in the shrinkage factors and adjust the ring accordingly in each size ... then cast them into metal as permanent models.. or use a molding process directly on the printed models....which I personally don't like as the resin model could get broken and you would have to pay to have another printed.
Ok... let's look at the old method used by US factories well before the advent of Cad, printers and mills....
First , you make a master model.A true master is only used to make sized production models... so... you now make a master rubber mold . This will be used to create all the wax patterns for all the sizes you plan on selling... these wax patterns are sized in the wax, cast into your favorite metal for models.
Then cleaned up to as close to perfection as you can get. From the beginning of the process, there is shrinkage happening and this must be accounted for. This is true in all forms of lost wax casting.
Your Master would usually be 8 to 10 percent bigger, thicker then a final piece. In between that, you have production models and molds.. you get a 3 to 4 % shrinkage at this stage... and this is the stage all the care needs to be taken. You need perfecto production models so the production molds are also as perfect as possible. Shrinkage factors can be different then what I am saying... this depends on the type of rubber you choose for molding purposes.. there are many on the market and they are not all equal.
Why sized metal models? Why not mold the resin prototype from printing.
A metal model is forever... when your product takes off... your production house has to be able to handle your production and quality...the amount of rubber molds you have is directly relevant to that. The more molds that are on the production line, the faster it will go. Vulcanized long life silicone rubber molds are usually the best as wax does not stick to silicone.. and you can vulcanize a silicone mold in any where from 15 minutes for fast curing low temp material.. to high tear strength silicone at around 45 minutes. Having a metal model allows you to make good strong molds quickly.
Another method is to make 1 ring at the first size, mold it. Size the ring to the next size and mold it.
After sizing the same ring multiple times, it's going to show some wear and tear unless you do a perfect job every time. Problem with this method is you can't make molds quickly for a production run since you only have 1 model.