[chuck x goren]

I figured it was some sort of flexible metal alloy with a very tight fit. Almost like wearing an Under Armour ski-mask... but made of thin, flexible metal.

Someone mentioned the good ol' "unstable molecules". Another (Marvel) comic explanation, but in the mid-70's, Iron Man altered his face mask with some sort of experimental alloy, so that it could be more expressive and show his facial features more. I think it was mostly an artist's request, so they could convey emotion with Iron Man screaming, narrowing his eyes, etc.