A cap is an element used to seal the ends of containers, isolating the internal and external media, also known as a cover plate. The caps for cylindrical containers are typically rotating shells. They can be categorized by the shape of their surface into dome-shaped, conical, flat, and composite shapes. A dome-shaped cap refers to a cap with an outward 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 meeting safety requirements for use.