The cardboard inside a microwave is for protecting of the magnetron and distributing of the microwaves and is a microwave waveguide cover that is made of mica, which is a natural mineral that is also heat resistant and insulating and sometimes a durable plastic material may also be used.
The waveguide cover often resembles a piece of grey cardboard and is located inside the microwave cooking chamber.
If you remove the waveguide cover from your microwave the microwave will still function but it can potentially be unsafe.
The waveguide cover for the microwave protects the magnetron as well as other internal components from any food splatters and moisture and prevents damage and potential arcing or fire hazards.
If the waveguide cover is damaged it's best to replace it if you can do so but if you have to remove it temporarily it will be okay as I've done it myself.
Although without the waveguide cover the microwaves won't be distributed evenly although the microwave will still function.
You can use a microwave without a waveguide cover although I've heard you cannot use a microwave without a waveguide cover but I have used my microwave without a waveguide cover and it has worked just fine without issues.
It is not safe to use a microwave with a damaged waveguide cover as it could result in damage to the microwave if you continue to use it and it continues to spark.
While not ideal you can temporarily just remove the damaged waveguide cover and use your microwave until you can replace the waveguide cover.
The microwave will still work without the waveguide cover which I have done myself but when you can replace the waveguide cover you should do so.
You can buy replacement waveguide covers for your microwave online on Amazon.com
A waveguide cover can spark and arc in the microwave because it has become damaged and the metal inside the waveguide cover is exposed and leads to the magnetron causing sparks when it comes into contact with the metal.
Waveguides contain some metal which can be exposed when the waveguide becomes damaged.
Microwaves that spark or arc without any other metal being inside the microwave is often caused by a damaged or a dirty waveguide cover although microwaves sparking can also be caused by metal objects inside or a faulty magnetron.
A waveguide cover can become damaged due to normal wear and tear or even food buildup and they are easy to replace and cheap to purchase new as well.
The waveguide cover in the microwave is a small panel inside your microwave, which directs the microwaves into the cooking chamber of the microwave.
Food splatters or grease buildup on the waveguide cover can become overheated and also cause sparking or a damaged or crack in the waveguide cover can also lead to sparking and arcing.
A common culprit of a sparking microwave is a damaged waveguide cover that is located inside the microwave area where you cook the food and it coves the hole where the magnetron sends the actual microwaves.
When that becomes torn or damaged it can result in sparking of the microwave.
You can use your microwave after it sparks as long as the sparking stops and is not a result of a problem with the microwave itself.
If the microwave sparked due to some metal that was on a dish or something in the food caused the microwave to spark or a bug or something else caused the microwave to spark it's generally safe to continue using the microwave.
In some cases the sparking of the microwave can be caused by a damaged waveguide cover inside of the microwave which is fitted over the over the magnetron on the inside of the microwave on the side.
It's a little cover that is similar to a piece of cardboard and sometimes that can result in sparking if it's damaged.