Hot spots in a microwave come from the common event of food bubbling out or splattering on the inside of the microwave chamber. If these food spills do not get cleaned up when they occur, each time the microwave turns on, the spilled foods heat back up, eventually hardening on the interior surface.
When these hot spots heat up with each use, they can cause a smell much like that of burned plastic. These hot spots may also create a fire hazard, so it’s essential to clean up microwave spills right away.
Though many dish manufacturers label their plastic containers as “microwave-safe,” the safety of those containers is debatable. Despite the heat-resistant coatings added to these thick plastics, many plastic containers will begin to melt at high enough temperatures.
Whether or not a plastic container melts in a microwave depends on a variety of factors, including the food being cooked in the container and how long the container heats in the microwave. If you smell burned plastic while cooking in a plastic container, though, it is likely the container itself.
Styrofoam and Plastic Wrap
Like plastic containers, many plastic wraps have been designed to endure the heat of a microwave oven. These wraps contain labels stating they are "microwave-safe" These plastics can only withstand heat to a certain degree, though, above which they may begin to melt.
Styrofoam, likewise, may be labeled as “microwave-safe,” but nearly all Styrofoam containers begin melting when the food gets too hot inside of them, leading to a smell much like that of the burned plastic.
The majority of microwaves with a turntable for the purpose of evenly heating foods have glass turntables. The rotating piece on which the turntable sits, however, is commonly made of plastic.
Though this plastic is made to endure extreme heat conditions and should hold up under common usage, if you leave the microwave on for an extended period of time and the plastic gets hot enough, it may begin to melt. The best way to avoid this possibility is to never run the microwave with nothing inside of it.