Do not use any item that is not specifically designed for microwave use.
Answered by Dave, Electrical Safety Expert
You should only ever microwave things which clearly state that they are microwave safe. Do not use any containers, foam trays or any plastic storage bags that are not specifically designed for microwave use. This is because they could melt and cause damage to your appliance and also present a fire risk. It’s also important to be aware that some microwavable plastic containers specify a maximum continuous exposure time, so always check the label or contact the manufacturer for clarification.
If in doubt, you could always use a suitable ceramic container instead.