Step 5: Mold!
It will take about two days to cure, but if you're using a mold where it cannot easily breathe, it will take longer.
One thing to watch out for is that the casein will warp when it's drying, especially if you have it rolled out into a sheet. It's best to put a weight on it. Watch out, though, because it will seep a milky-vinegary scented liquid into whatever is holding it in place. I used a heavy programming book...and it now has a funny smell to it. Awesome.
For my project, I just need to roll the casein out and let it dry.
Lay another sheet of wax paper over the casein. Use a rolling pin to spread it out. If you didn't dry it too much in the previous step, it should roll nicely, without many cracks or chunks.
Finally, I pressed it between two sheets of aluminum foil, and curved it across the bevel I'm using.
Give it a few days to dry, and it'll be ready to go. This is critical. Depending on how you wrap/mold your casein, you may find it takes more than a week to dry. Wrapping mine in foil took two weeks to get remotely dry. As is expected, the thicker it is, the longer it will take to dry.
I should also note that if you're making a flat piece like I am, the plastic will curl if you remove it from its mold before its dry. I made this mistake with this project, and ended up with a piece that was unusable because it curled.
The final product is quite rigid when it's thick (1/8 inch or thicker), moderately pliable when it's a little thinner, and brittle if it's paper thin. It's also sandable and paintable.
Go forth and have homemade plasticy fun!