Casters are a general term, including movable casters, fixed casters and movable casters with brakes. Movable casters are also what we call universal wheels, whose structure allows 360-degree rotation; fixed casters are also called directional casters, which have no rotating structure and cannot be rotated. Usually two types of casters are used in combination. For example, the structure of a trolley has two directional wheels at the front, and two universal wheels at the back near the push armrest. Casters are made of various materials, such as pp casters, PVC casters, PU casters, cast iron casters, nylon casters, TPR casters, iron core nylon casters, iron core PU casters, etc.