Writing in the Washington Post, food science writer Robert Wolke once offered a good explanation for why the plastic doesn’t melt: First, the aluminum foil shields the pan (and the plastic) from hot air and infrared radiation in the oven. But because it’s so thin, it can’t absorb enough heat to melt the plastic. It also doesn’t retain heat, so it cools quickly when it leaves the oven.
The second part is the plastic itself. Because it’s preventing steam from escaping, the plastic wrap gets wet. That moisture keeps it from getting any hotter than 212 degrees. And most plastic wraps won’t melt until they get to 220 to 250 degrees. So with moisture on one side and foil on the other, the plastic doesn’t melt.