Sides are made of metal. I do not know whether it is aluinium or some other kind of alloy...
I have doubts about the top and bottom parts of the frame though.
They look the same compared to the sides, but feels ever so slightly different when you compare consciously.
The difference is more apparent when your hone has been sitting under A/C. Sides get super cool, as u would expect metallic objects to do. But the top and bottom frame doesn't get as cold.