A customized thermos' purpose is to maintain heat by insulating the liquid with vacuum, preventing loss of temperature due to conduction. Microwave, on the other hand, is designed to consistently heat things through radiation. This could result in the thermos melting, and worse still, the insulation on its body will prevent the contents from actually getting heated. All you're going to get is a melted, deformed thermos with lukewarm water inside.
Custom Thermos products have a sterling reputation for being virtually indestructible. Having once thought that stories about their durability and toughness are just marketing hype or a bunch of exaggerated retellings, I decided to get to the bottom of things and do some experimenting of my own.
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