What is the cardboard plate in my microwave used for?
Prevents short circuits
Suppose you want to heat up a bowl of tomato soup and it splashes a bit. Don't panic. Because of the cover in your microwave, you don't have to worry about a short circuit. The cover covers the waveguide; an electronic component that sends the energy from the microwaves to your food. This way, no dirt or grease gets into the waveguide and your microwave won't short circuit. Thanks to the special coating, this cover can withstand high temperatures.
Replace broken cover
Did the cover get damaged? Don't use the microwave, as this can cause a short circuit and could become a fire hazard. Unplug the power cord and purchase a new cover. You can order this via the manufacturer of your microwave or from a local hardware store. Keep the brand and model number on hand so you can order the correct cover. The model number can be found on the inside of the door or on the back of the microwave.