A cap is a component used to seal the end of a container, isolating its internal and external media, also known as an end cover. The caps for cylindrical containers are generally rotating shells. Caps can be categorized by the shape of their surface into convex, conical, flat, and composite shapes. A convex cap refers to a cap with an outwardly convex surface, such as hemispherical, elliptical, dish-shaped, and seamless spherical caps. Some gas cylinders use a composite bottom cap with the convex surface inward, which ensures both strength and meets safety requirements for use.