A cap is an element used to seal the ends of containers, isolating the internal and external media, also known as an end cover. The caps for cylindrical containers are typically rotating shells. They can be classified 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 strength while also meeting safety requirements.