I had surgery on my left hand to reattach the tendons in my thumb (don't ask:rolleyes) & the support cast that I had to wear afterwards was made from Wonderflex. It held up really well & I didn't notice any softening of the cast in the summer weather. If I could find a local supplier I would use this instead of Sintra.
wonderflex is what some of the Japanese gundam cosplayers use to make their robot costumes. It seems to be able to take a lot of abuse, but they also use thick craft foam, which is very fragile when heat formed and doesn't retain its shape. It is easy to work with, but I wouldn't use it for everything. I have only used it for small things. Here's an example of craft foam and wonderflex.
As I understand it, the helmets are a combination of the two.
The guy in the middle was photoshopped in there...I couldn't resist.
A couple of my Uni friends used this when they were studying. It's used quiet extensively in the UK nursing industry. As BKBT says it can be used to make lightweight and cheap supports etc.