3D printed mold techniques for long and short term usage 3D printing can be used to make injection molds of unimaginable complexity but which kind of 3D printing process is suitable when? 

Suppose that a part is to be made using injection molding in large quantity for an extended period of time, what Additive Manufacturing (AM) process will be the best, such that the mold does not give way too soon? 

Also, suppose that the part to be made is custom and only has to be made in small quantities - that is to say that the injection mold will be used limited number of items and then thrown away - which is the best AM technique then? Best in the sense of economic feasibility, lower cost, lower capital investment etc.?
Yes, this is very broad. That said...

For high detail you want SLA. i.e. jewelry. If you just want a prototype of a mold, you can do a standard FDM style printer (95% of printers are FDM, and that number is a guess)

Really, you should be asking what material you need for your mold, but you can open a second question for that.

Do more research on injection molding. There is a great deal of information on how molds are made, i.e. How It's Made Plastic injection molds.

You will see there is a vast difference between a plastic, or silicon, mold and an injection molding machine. You are thinking that injection molding as a single mold, when it is really it is a system composing of several pieces of heavy duty machinery that can pump out hundreds of items a day automatically. However, it usually starts at 20k USD for the tooling for injection molding. Your costs could be a fraction of that or could be several times that. This is just a generality. So, if you are making 100 units you won't want to go down that route. For 10,000 units, on the other hand, it would be acceptable.