A hot runner system is an assembly of heated components used in plastic injection molds that inject molten plastic into the cavities of the mold. (The cavities are the part of the mold shaped like the parts to be produced.)

By contrast, a cold runner is simply a channel formed between the two halves of the mold, for the purpose of carrying plastic from the injection molding machine nozzle to the cavities. Each time the mold opens to eject the newly formed plastic parts, the material in the runner is ejected as well, resulting in waste. A hot runner system usually includes a heated manifold and a number of heated nozzles. The main task of the manifold is to distribute the plastic entering the mold to the various nozzles which then meter it precisely to the injection points in the cavities.^[1]