The short answer is no — and there are two separate reasons why.
No — do not put a stainless steel water bottle or thermos in the microwave. There are two reasons, and either one is enough: metal reflects microwaves and can spark and damage the oven, and a vacuum-insulated flask is specifically built not to let heat through, so the drink wouldn't warm up anyway.
Microwaves cook by bouncing energy into food. Metal reflects that energy instead of absorbing it. Thin or shaped metal can concentrate the charge enough to throw sparks (arcing), which can damage the magnetron and, in a sealed bottle, build pressure. It is the same reason you don't microwave a fork.
A double-wall vacuum flask works because the vacuum between its two steel walls blocks heat transfer — that is how it keeps coffee hot for 12 hours. Put it in a microwave and that same barrier stops the contents from heating. You'd risk the oven for a drink that stays cold.
We are the factory behind these articles — 500 pieces per design, samples in 7–10 days.
Written by the Jupeng Drinkware team — Yongkang, Zhejiang, China. Manufacturing drinkware since 1998. Contact Beyond: [email protected] | WhatsApp +86 156 5791 8881
Two things, both bad. Metal reflects microwaves instead of absorbing them, so the drink does not heat while the oven works against its own output — that can damage the magnetron. And on a vacuum flask the sealed cavity between the walls has nowhere to vent. Never microwave any of our vacuum bodies; if the end user needs microwave reheating, quote a food-grade PP body instead.