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 categorized by the shape of their surface into convex, conical, flat, and composite shapes. A convex cap refers to a cap with an outer surface that is convex, such as hemispherical, elliptical, dish-shaped, and seamless spherical caps. Some gas cylinders use a composite bottom cap with an inward-facing convex surface, which ensures strength while also meeting safety requirements.