Casters are a general term, including movable casters, fixed casters and movable casters with brakes. Movable casters are what we call universal wheels, and their structure allows 360-degree rotation; fixed casters are also called directional casters, which have no rotating structure and cannot rotate. Usually two kinds of casters are usually used together. 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. There are casters 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.