Foam of all kinds seems to be useful in cosplay, and that includes expanding foam. Using the material is sort of like carving something down from a block of foam, but expanding foam offers a little more flexibility. Instead of starting with a large piece of foam you have to carve into the desired shape, you can spray expanding foam over an existing cardboard template. You’ll still have to carve it down, but the basic shape is at least there as a guide.
Kamui Cosplay loves using expanding foam, and she’s created a couple videos explaining how to use the material and how to carve it. She explains that after the piece is carved, she covers it in masking tape, applies a coat of newspaper with wallpaper glue, and covers it in papier mache. After the piece is dry, she sands it to smooth out the surface. I’m not sure that I’m sold on using expanding foam yet because carving it leads to a huge mess, but after watching Kamui’s videos, I can see the potential.
Turn on CC for English subtitles in the videos.
She explains the carving process more in this video: