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.