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 rotating shells. Heads can be categorized by the shape of their surface, including convex, conical, flat, and composite. A convex head refers to a head with an outer surface that is convex, 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.