A head is an element used to seal the ends of containers to isolate the internal and external media, also known as a cap. The heads of cylindrical containers are typically rotational shells. They can be classified by the shape of the surface into convex, conical, flat, and composite shapes. 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.