A cap is an element 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 rotational shells. They can be categorized by the shape of their surface into convex, conical, flat, and combined 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 combined convex-bottom cap with the convex surface inward, which ensures both strength and meets safety requirements for use.