A few years ago I had hand surgery and had a “splint” of sorts made up at the hospital. This splint was made from a flat sheet of colored plastic about 1/8 of an inch thick. They cut out a shape, put it in hot water to make it pliable than molded it to the shape they wanted.
There is a product called Jett Sett that is used by jewelers to hold work in place while they set stones. I usually see it in pellet form. I have seen sheets of similar material but it may be a different brand. Maybe searching for “thermoplastic sheet” would turn up something useful.
In the UK we have Polymorph. This comes as granules (about 2-3mm round) and melts in hot water - about 50-60 degrees C.
You cover the granules in almost boiling water and they turn transparent and mush together. Then form it into any shape you want; it cools and turns opaque white and feels very much like thick polyethelene. You could easily make a sheet from it. Once set you can warm to re-mold.
It’s great stuff!
tim
ETA: When set a piece of about 3-4mm thickness is essentially unbendable.
Generically, the stuff is a thermoplastic splint. The orthopedic department will probably call it a thermoplastic orthosis. Ortho is just weird that way sometimes.
After some poking around, I was able to find a couple of brand names. One constant is that it’s expensive stuff - in the range of $18 - 35 per square foot, and it’s generally sold in 3-foot pieces. Looks like the “Plastic” types are what you’re after, as opposed to “Elastic” The company linked above makes the material. I have no idea how friendly they’d be to a hobbyist wanting to buy one or two sheets, as opposed to a hospital ordering a crate of it.
A couple other names to look up would be Orfit Stiff and Orthoplast.
I just wanted to add that I have had many a splint made from the plastic the OP mentioned. I don’t think the links mentioned are quite it.
All my splints were white/offwhite (can’t recall exactly). Like the OP said, it was a sheet of solid plastic that was put into a vat of heated water. A few minutes later it was removed from the water and the therapist would cut the shape needed. If I recall the water was cool enough that the plastic could be placed on my arm for molding straight out of the vat without burning me. Though I could be mistaken, they might have let it cool a little.
That being said, I have no clue what it was called.
The link I supplied definately wasn’t for the medical grade stuff. It didn’t appear the OP was wanting it for more than hobby work so didn’t need to pay the medical grade premium for it.
I found an actual name for the plastic that’s at the heart of the splinting material that softens in hot water - polycaprolactone, or PCL.
Unfortunately, on the consumer market, it’s pretty much limited to pellet form, as sold under the brand names Shapelock and Polymorph as “feedstock” for rapid-prototyping systems.