A cap is an element used to seal the ends of containers, isolating the internal and external media, also known as a cover plate. 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 outer surface that is convex, 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 usage requirements.