A cap is a component used to seal the ends of containers, isolating the internal and external media. It is also known as an end cover. The caps for cylindrical containers are typically swaged shells. They can be classified according to the shape of the surface into convex, conical, flat, and combined shapes. A convex cap refers to a cap with a convex outer surface, such as hemispherical, elliptical, dish-shaped, and seamless spherical caps. Some gas cylinders use a combined bottom cap with a convex inner surface, which ensures strength while meeting safety requirements for use.