A head is a component used to seal the ends of a container, isolating the internal and external media, also known as an end cap. The heads of cylindrical containers are generally rotating shells. They can be classified according to the shape of their surface into convex, conical, flat, and combined 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 combined bottom head with the convex surface facing in, which ensures strength while meeting safety requirements.