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