[From Wikipedia]

Due to size and weight, weighted blankets are often difficult to launder (some are also made of materials that cannot be machine-washed). Therefore, many blankets are sold alongside a machine-washable cover. Additionally, the blanket’s weight may exceed the maximum load allowed by a washing machine, and should always be checked in advance. A delicate wash in cold water is generally preferred, as this preserves the properties of the padding material and prevents damage to the weighted pellets/beads. In most cases, how often to wash a weighted blanket depends on how the owner uses it.[18]

Special care should also be exercised when drying a weighted blanket. Most manufacturers recommend flat drying the blankets.[19][20] This can be achieved by using a flat surface, laying down a few towels to soak the excess water, and laying the weighted blanket on top to dry. Hang drying is discouraged because the weighted blanket may lose its shape and reduce its lifespan. Gentle air drying in the machine may be possible, depending on the weighted blanket filling and padding material. Plastic and synthetic fabric or padding materials may lose their properties due to excessive air temperature.[21]