The phrase "styrofoam plates" refers to disposable plates made from a type of plastic called Styrofoam. These plates are light, inexpensive, and commonly used for serving food at events or parties. Full definition
The children then made boats with Styrofoam plates, plastic straws, wooden sticks, and other supplies before testing how the boats would fare when placed in a plastic tub of water. (aaas.org)
I also have a bunch of styrofoam plates around that I use to hold a little water and to remove some paint from my brushes when I get too much on them. (thepurplepaintedlady.com)
Copper plated steel, copper - plated styrofoam plate, polished minerals, silver, copper and gold dust, 35 x 55 1/2 x 7 inches (steveturner.la)