Commercially made folding knives make bad throwers. They are terribly unbalanced and break immediately.
Funny this thread exists. I have been pondering for at least two years now on how to tackle the folding hinge problem for a 'folding thrower'.
To have any thrower worth its weight in scrap steel it should be at least 12 inches long, more like 14 inches would be better. That means that a folding thrower would be 7 or 8 inches when folded - not your usual folder size but much better than a 14 inch hunk of pointed steel slapping at your side.
Now back to specifically designing a pivot joint/locking mechanism that would standup to throwing. I have considered the following:
1. pin-type joint with notch/pin cam locking device,
2. pin-type joint with a side-lock device,
3. pin-type joint with a lever lock-back device, and
4. a slip blade (from handle) with a spring lock.
One of these methods should work, but getting the details correct is the difficult part.
Any comments?