What would be the most effective place to cut someone that drains the most amount of blood without killing them?

Okay i know this sounds absolutely insane, but i swear it's not for psycho reasons (At least not IRL psycho reasons).

I have a group of characters whose blood contains a chemical similar to Manganese Heptoxide (Which is highly explosive and will burst into flames from just being touched), which in turn makes their blood extremely valuable when it comes to making explosives.

So clearly some very messed up people got involved, and i'm trying to figure out how these Firebloods (Not the actual name, but let's roll with it) would have their blood drained as efficiently as possible, while also not being fully bled out so that they can be reused multiple times.

I also need at least 2 of them to survive the entire ordeal for the story to happen. And yes i've tried looking it up on Google, but at this point i feel like i'm gonna get on a watchlist if i keep trying :/