A head, also known as an end cover, is an element used to seal the ends of containers, isolating the internal and external media. The heads of cylindrical containers are typically rotatable shells. They can be classified by the shape of their surface into convex, conical, flat, and composite types. A convex head refers to a head with a convex outer surface, such as hemispherical, elliptical, dish-shaped, and seamless spherical heads. Some gas cylinders use a composite bottom head with the convex surface facing inward, which ensures both strength and meets safety requirements for use.