The mica sheet filters the waves produced by the magnetron. It's a component that can tolerate very high temperatures without deforming or degrading as time passes.
However, the interior of your microwave will end up accumulating grease deposits with regular use, and this can lead to blackening of the mica sheet, or even cause it to tear.
If you don't then clean your microwave or replace the mica sheet, sparks may be produced when you turn it on.
Discover our range of mica waveguide cover sheets and get your microwave newly operational again.