What is the cardboard cover inside my microwave for?

The mica cover protects the magnetron's antennae and waveguide. When grease is heated by the microwaves it will turn to carbon and start arcing. This arcing will eventually damage the magnetron. The tape may start to smell or burn, I've not tried it. You should be able to find a piece of mica online and cut to size, or order the OEM part.

Hi, a mica cover is inside the oven cavity, usually on the right hand side or in the roof you will usually see a small cover that looks like it is made of cardboard. It is often held in by one or more screws. It is called the waveguide cover. It is usually made of a material called Mica. Its purpose is to cover the aperture through which the microwaves enter the oven cavity from where they are produced by the Magnetron behind the control panel. It stops moisture and food particles getting into the "workings" of the microwave.

That's the waveguide cover you're talking about. It covers the opening in the side of the cooking cavity where the microwaves enter. The oven will work without it, but if you don't have the cover in place, food particles can get into the waveguide and cause problems. You can buy a replacement here.

That sheet is called the Waveguide Cover and its function is to stop debris and food entering the aerial and magnetron.
Replacement is essential and must be made of mica.
It can be purchased from a dealer at a reasonable cost.
Google the manufacturer and find a dealer.
Hope that helps.

http://www.repairclinic.com/PartDetail/Waveguide-Cover/1085082 it called a wave guide

It sounds like what you are describing is called a "waveguide cover". You can purchase a replacement from a local parts supplier, or on-line at www.repairclinic.com. It is very easy to replace.

you need to replace the cavity cover "the cardboard like",the antenna bearing set "the gun barrel like" and the gear motor.If the "gun barrel like" is stuck up because the gear motor is not rotating excessive heat will occur inside the microwave thus burning the cavity cover.

This is the mica or plastic waveguide cover. It protects the waveguide and magnetron form grease and food particles.

Assuming you're going to scrap the unit, carefully clean the area around the outside of opening (watch for sharp edges) with a lightly damp soft cloth.

Then use alcohol or a hair dryer, etc. to be sure the area is dry.

Then you can use a couple of strips of clear packing tape to cover the opening and trim it with a razor blade so only about 1/4" or so extends out from the opening.

Note that I only recommend this procedure on an oven that is going to be scrapped. If the oven will see continued use, a new waveguide cover should be purchased.

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Quite often this is due to grease or food particles stuck to the inside of the oven or stuck on or behind the waveguide cover.

The waveguide cover is usually about the size of a playing card and often held in place by a plastic rivet.

It can be fragile, and the metal edges of the oven behind it can be sharp, so be careful!

The cover and the metal behind it should be cleaned with a mild, non-ammonia cleaner.

If the cavity metal of the oven is chipped or charred, it will have to be sanded smooth and clean and dried thoroughly.

Then you can apply some high-temperature appliance paint, available from places like Wal-Mart, Lowe's or Builder's Square.

If your waveguide cover is burned, it will need to be replaced. It provides important protection for the magnetron.

You can usually find helpful exploded view diagrams and order parts by entering your full model number here.

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Try replacing the screws with larger ones. Maybe you ordered the wrong piece, if your microwave comes with a mica cover (A fine, square piece of mica) and you received a plastic one, you will need to get the right piece or adapt the one you received.
You can get the mica cover on any microwave service.