Cap plastic is big....and it's not just for bald caps.
Cap plastic is so called as it was originally pretty much used only for making bald caps, so those with hair could temporarily be without it.
Over the last ten years or so, it's used much more widely as an encapsulant or barrier in a mould so that when silicone gel appliances are cast into it, they come out of the mould with that cap plastic as a surface. This allows glue and makeup to remain attached to it - after all, silicone is at its best a mould material because almost nothing sticks to it!
Chek out the blog post that supports this episode of the podcast.
-Stuart & Todd.