How to Keep Freezer Mugs From Cracking
A freezer mug consists of a double-walled mug with a frozen inner core that keeps drinks cold. Typically made of plastic, the mugs become brittle in the freezer, and rapid temperature changes can cause them to crack. Stacking heavy items on top of mugs in the freezer can also trigger breakage. The extreme heat inside a dishwasher eventually weakens the plastic, causing it to split. Pouring a hot beverage directly into a frozen mug shocks the inner surface and may cause it to expand and crack, so allow liquids to cool before you fill the mug.
Wash the freezer mug by hand in warm water with liquid dish soap and a nonabrasive sponge. Dry all the inner and outer surfaces of the mug thoroughly. Place the mug in the refrigerator to cool for one hour.
Move the cooled mug into the freezer. Position the mug so it is upside down and out of the direct path of the cold air inlet. Place other freezer items between the mug and the vent to deflect the air if necessary.
Retrieve the freezer mug immediately before use. Wrap a dry kitchen towel around the outside of the mug to absorb condensation. Pour a room temperature or colder beverage into the mug.