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 rotationally symmetrical shells. Caps can be categorized by the shape of their surface, including convex, conical, flat, and composite. 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 composite bottom cap with the convex surface inward, which ensures both strength and meets safety requirements for use.