The inner is made of 304 stainless steel and advanced welding techniques, and the outer can be made of either 304 stainless steel or zinc-coating sheet or plastic aluminium. The heat retention liner is made of 50mm polyurethane which is low is heat loss, good at heat retention. The product is both practical and beautiful in outlook and can be used in the projects of sun energy, heat pump, super-sized bathing beach, hotels etc. The clients can order different materials in different sizes.