In the past 8 forges I have gotten chest pieces from using the Polymers. I know it is RNG, but randomize it a bit more. Out of the 15 I have completed I've gotten 11 chests, 3 arms, and 1 helm.
-
Same here, the last piece i need is the helmet & I've used close to 20 polymers. At this point I'm convinced that the helmet doesn't drop from forge polymers.