This indicates that there is a quality issue with the thermal mug.
It is undoubtedly a quality defect. After filling the flask with boiling water, touch the outer casing with your hand; if the insulation is effective, you should barely perceive any change in temperature. However, if the exterior feels hot to the touch, it implies rapid heat transfer-a clear indication of poor insulation performance. This is, in fact, the most intuitive and effective method for assessing the quality of a thermal flask's insulation.
Generally speaking, poor insulation in a thermal flask is primarily caused by an insufficient vacuum between the double-layered glass walls. For instance, if the small conical tip at the base-which serves as the vacuum extraction port for the inner liner-becomes damaged, air will seep into the space between the glass layers. This accelerates heat transfer; if the flask contains hot water, the heat from within will rapidly dissipate outward, causing the exterior casing to heat up.
Thermal mugs typically feature an internal vacuum layer designed to provide thermal insulation and heat retention. After pouring in hot water, the outer wall should remain cool to the touch. If the outer wall becomes hot, it signifies that the mug has lost its insulating capability and is effectively broken. If the mug was recently purchased, it is best to contact the retailer for an exchange. If it is an older mug, you will need to replace it with a new one.